Patent Publication Number: US-2022233316-A1

Title: Fluoroscopic visualization of heart valve anatomy

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is the US National Phase of International Patent Application PCT/IL2020/050807 to Sheps et al., filed Jul. 22, 2020, and entitled “Fluoroscopic visualization of heart valve anatomy,” which published as WO 2021/014439, and which claims the benefit of:
         U.S. Provisional Patent Application No. 62/988,322, filed Mar. 11, 2020 and entitled “Fluoroscopic visualization of heart valve anatomy;” and   U.S. Provisional Patent Application No. 62/877,785, filed Jul. 23, 2019 and entitled “Fluoroscopic visualization of heart valve anatomy,”
 
each of which is incorporated herein by reference in its entirety for all purposes.
       

    
    
     BACKGROUND 
     Implantation of medical devices can be aided by fluoroscopy, for example, in catheter-based procedures involving cardiac valve repair and replacement. Patient exposure to fluoroscopy is ideally kept at a minimum. 
     SUMMARY OF THE INVENTION 
     This summary is meant to provide some examples and is not intended to be limiting of the scope of the invention in any way. For example, any feature included in an example of this summary is not required by the claims, unless the claims explicitly recite the features. Also, the features described can be combined in a variety of ways. Various features and steps as described elsewhere in this disclosure can be included in the examples summarized here. 
     In some applications, systems and methods are provided for aiding implantation of cardiac devices under the guidance of fluoroscopy, using radiopaque devices which act as guides in order to facilitate enhanced imaging of the cardiac space during implantation of the cardiac implant, thereby minimizing patient exposure to fluoroscopy over a given period. 
     There is therefore provided, in accordance with some applications, a system and/or an apparatus for use with a subject, the system/apparatus including a visualization device or anatomy-marking device (e.g., an annulus-marking device, etc.) including a radiopaque material, and an implant for implantation along the native heart valve annulus of the subject. In some implementations, the visualization device, anatomy-marking device, or annulus-marking device is configured to provide a guide for implantation of the implant along the annulus during implantation, and retrievable following the implantation of the implant. 
     Throughout this application, the term annulus-marking device is often used for illustration, but the terms anatomy-marking device, heart valve-marking device, and visualization device can be substituted in place of the term “annulus-marking device” and, in any case, the devices can be used to mark or visualize other regions inside a patient&#39;s heart and/or other organs. 
     In some applications, the annulus-marking device and/or the radiopaque material are configured and shaped to define a base frame having a shape such that it tracks a circumference of a native heart valve annulus, and/or one or more struts projecting away from a plane defined by the base frame. The one or more struts can be configured to provide an indicator of one or more commissures of a native heart valve. 
     In some applications, the annulus-marking device is compressible during delivery toward the native heart valve, and expandable from a compressed state for positioning along the native heart valve annulus. 
     In an application, the annulus-marking device includes a superelastic material. In an application, the base frame and the one or more struts are fabricated from a single piece. 
     In an application, the one or more struts are sized so as to provide an indication as to a height of the native heart valve annulus. 
     In an application, the base frame is circular. In an application, the base frame is substantially D-shaped. 
     In an application, the base frame includes a wire. 
     In an application, the base frame includes an adjustment mechanism which expands and contracts a perimeter of the base frame. 
     In an application, the adjustment mechanism includes a wire that runs at least partially within a lumen of the base frame, and the wire is pullable to adjust the perimeter of the base frame. 
     In an application, the adjustment mechanism includes a wire that runs at least partially within a lumen of the base frame, and the wire is twistable to adjust the perimeter of the base frame. 
     In an application, the adjustment mechanism includes a wire that runs at least partially within a lumen of the base frame, and at least a portion of the base frame collapses telescopically in response to pulling of the wire. 
     In an application, the annulus-marking device includes a plurality of radiopaque filaments coupled at least to the base frame, each one of the plurality of filaments projecting radially away from the base frame and configured to mark the native heart valve annulus and tissue coupled thereto. 
     In an application, each one of the plurality of radiopaque filaments includes a material that is flexible. 
     There is further provided, in accordance with some applications, a method, including placing at a native heart valve annulus of a subject an annulus-marking device including a radiopaque material, implanting an implant along the native heart valve annulus of the subject using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. The method can further include retrieving the annulus-marking device following the implanting. 
     The annulus-marking device and/or the radiopaque material can be the same as or similar to any annulus-marking device and/or the radiopaque material described herein. In some implementations, the annulus-marking device and/or the radiopaque material are shaped to define a base frame having a shape such that it tracks a circumference of the native heart valve annulus, and/or one or more struts projecting away from a plane defined by the base frame, the one or more struts providing an indicator of one or more commissures of a native heart valve. 
     The annulus-marking device can be compressible during delivery toward the native heart valve, and expandable from a compressed state for positioning along the native heart valve annulus. 
     In an application, placing the annulus-marking device includes measuring a height of the annulus using the annulus-marking device. 
     In an application, the method further includes adjusting a perimeter of the base frame. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, retrieving the annulus-marking device following the implanting includes constraining the annulus-marking device within a tool and extracting the annulus-marking device from the subject. 
     In an application, placing includes placing the annulus-marking device along an annulus of a mitral valve. 
     In an application, placing includes placing the annulus-marking device along an annulus of a tricuspid valve. 
     In an application, the annulus-marking device includes a plurality of radiopaque filaments coupled at least to the base frame, each one of the plurality of radiopaque filaments projecting radially away from the base frame, and the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the plurality of radiopaque filaments. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the plurality of radiopaque filaments against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the plurality of radiopaque filaments responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a system and/or apparatus for use with a subject, the system/apparatus including an implant configured for placement along a native heart valve annulus. The implant including a body portion including flexible material, the body portion having a longitudinal axis that runs along a length of the body portion (e.g., when the implant and/or body portion is straightened), and an annulus-marking device, structure, or assembly including a plurality of radiopaque projections that project away from the longitudinal axis. 
     The implant can further include a contracting member coupled to the body portion. The contracting member can be coupled to and/or extend along or through the plurality of radiopaque projections in a manner in which during application of tension to the contracting member, the contracting member is configured to change a structural configuration of the plurality of radiopaque projections. 
     In an application, the contracting member is configured to compress the plurality of radiopaque projections in a radial direction toward the longitudinal axis of the body portion. 
     In an application, the contracting member is configured to contract the body portion during the application of tension to the contracting member. 
     In an application, the apparatus further includes an additional contracting member extending along the body portion, the additional contracting member being configured to contract the body portion. 
     In an application, the body portion includes a plurality of radiopaque markings configured to indicate placement of anchors along the body portion. 
     In an application, the contracting member extends along a perimeter of each one of the plurality of radiopaque projections. 
     In an application, the plurality of radiopaque projections are flexible and include a fabric. 
     In an application, the body portion and the plurality of radiopaque projections are flexible and include a fabric. 
     In an application, the each one of the plurality of radiopaque projections is shaped so as to define respective flat and planar element. 
     In an application, each flat and planar element has a longest dimension that is measured a long an axis that is at a nonzero angle with respect to the longitudinal axis of the body portion. 
     In an application, each one of the plurality of radiopaque projections is shaped so as to define a plurality of tubular elements. 
     In an application, the contracting member extends along a perimeter of each opening of each of the plurality of tubular elements. 
     In an application, each one of the plurality of tubular elements tapers away from the longitudinal axis of the body portion. 
     There is further provided, in accordance with some applications, a method, including placing at a native heart valve annulus of a subject an implant configured for placement along a native heart valve annulus and comprising a body portion and an annulus-marking device; deploying a plurality of tissue anchors through the body portion of the implant and into tissue of the native heart valve annulus under imaging and using the annulus-marking device as guidance; and changing a structural configuration of the implant. 
     The implant can be the same as or similar to other implants described herein. For example, in some implementations, the implant includes a body portion including flexible material, the body portion having a longitudinal axis that runs along a length of the body portion (e.g., when the implant and/or body portion is straightened), and an annulus-marking device including a plurality of radiopaque projections that project away from the longitudinal axis. The implant can include a contracting member coupled to the body portion. The contracting member can be coupled to and/or extend along or through the plurality of radiopaque projections in a manner in which during application of tension to the contracting member, the contracting member is configured to change a structural configuration of the plurality of radiopaque projections. 
     In some applications, changing a structural configuration of the implant comprises changing a structural configuration of the plurality of radiopaque projections by applying tension to the contracting member plurality of radiopaque projections by applying tension to the contracting member. 
     In an application, changing the structural configuration of the plurality of radiopaque projections includes compressing the plurality of radiopaque projections in the radial direction toward the longitudinal axis of the body portion. 
     In an application, applying the tension to the contracting member includes adjusting a perimeter of the implant by contracting the body portion using the contracting member. 
     In an application, the implant includes an additional contracting member extending along the body portion, and the method further includes adjusting a perimeter of the implant by contracting the body portion using the additional contracting member. 
     In an application, placing includes placing the implant along an annulus of a mitral valve. 
     In an application, placing includes placing the implant along an annulus of a tricuspid valve. 
     In an application, changing the structural configuration of the plurality of radiopaque projections includes sequentially changing the structural configuration of the plurality of radiopaque projections. 
     In an application, the body portion includes a plurality of radiopaque markings configured to indicate placement of anchors along the body portion and deploying the plurality of tissue anchors includes deploying each one of the plurality of tissue anchors in accordance with a respective radiopaque marking. 
     In an application, the contracting member extends along a perimeter of each one of the plurality of radiopaque projections, and changing the structural configuration of the plurality of radiopaque projections includes compressing the plurality of radiopaque projections by drawing each one of the plurality of radiopaque projections toward the longitudinal axis by contracting the contracting member along the perimeter of each one of the plurality of radiopaque projections. 
     In an application, the plurality of radiopaque projections are flexible and include a fabric. 
     In an application, the each one of the plurality of radiopaque projections is shaped so as to define a plurality of flat and planar elements and changing the structural configuration of the plurality of radiopaque projections includes drawing each one of the plurality of radiopaque projections toward the longitudinal axis by folding each one of the plurality of flat and planar elements. 
     In an application, each one of the plurality of radiopaque projections is shaped so as to define a plurality of tubular elements. 
     In an application, the contracting member extends along a perimeter of each opening of each of the plurality of tubular elements, and changing the structural configuration of the plurality of radiopaque projections includes closing each opening of each of the plurality of tubular elements. 
     In an application, each one of the plurality of tubular elements tapers away from the longitudinal axis of the body portion. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto under imaging the plurality of radiopaque projections. 
     In an application, viewing tissue includes imaging using fluoroscopy. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the plurality of radiopaque projections with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the plurality of radiopaque projections against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus includes imaging the plurality of radiopaque projections with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the plurality of radiopaque projections responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a system and/or an apparatus, including a tissue anchor including a distal tissue-coupling element having a longitudinal axis measured from a distal end to a proximal end of the distal tissue-coupling element, the distal tissue-coupling element configured for anchoring into and/or securing to tissue of a native heart valve annulus; and an annulus-marking device coupled to the tissue anchor. In some implementations, the annulus-marking device includes a radiopaque material and/or is configured to project away from the longitudinal axis of the distal tissue-coupling element. 
     In an application, the distal tissue-coupling element is hollow, and the annulus-marking device extends through a lumen of the distal tissue-coupling element. 
     In an application, the tissue anchor includes a proximal head coupled to the proximal end of the distal tissue-coupling element, the annulus-marking device being coupled to the proximal head. 
     In an application, the apparatus further includes an annuloplasty structure including a tubular body portion, and the proximal head is configured to be disposed within the tubular body portion while the distal tissue-coupling element is configured to be anchored within the tissue of the native heart valve annulus. 
     In an application, the annulus-marking device is coupled to the distal tissue-coupling element. 
     In an application, the annulus-marking device is coupled to the distal end of the distal tissue-coupling element. 
     In an application, the apparatus further includes an annuloplasty structure including a fabric, and the annulus-marking device is configured to pass through the fabric of the annuloplasty structure. 
     In an application, the annulus-marking device includes one or more radiopaque filaments configured to mark the native heart valve annulus and tissue coupled thereto. 
     In an application, each one of the one or more radiopaque filaments includes a material that is flexible. 
     There is further provided, in accordance with some applications, a method, including marking a location of a native heart valve annulus of a subject by implanting in tissue of the native heart valve annulus a tissue anchor including a distal tissue-coupling element having a longitudinal axis measured from a distal end to a proximal end of the distal tissue-coupling element, the distal tissue-coupling element configured for anchoring into tissue of the native heart valve annulus. 
     An annulus-marking device can be coupled to the tissue anchor, the annulus-marking device including a radiopaque material. The annulus-marking device can be configured to project away from the longitudinal axis of the distal tissue-coupling element. 
     The method further includes imaging the location, and during the imaging, viewing the annulus-marking device with respect to tissue of the native heart valve annulus. 
     In an application, imaging tissue includes imaging using fluoroscopy. 
     In an application, marking the location includes marking the location along an annulus of a mitral valve. 
     In an application, marking the location includes marking the location along an annulus of a tricuspid valve. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto under imaging the annulus-marking device. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the annulus-marking device against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the annulus-marking device responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     In an application, the tissue anchor includes a proximal head coupled to the proximal end of the distal tissue-coupling element, the annulus-marking device being coupled to the proximal head. 
     In an application, the method further includes implanting along the native heart valve annulus an annuloplasty structure including a tubular body portion, and implanting the tissue anchor includes positioning the proximal head is within the tubular body portion while implanting the distal tissue-coupling element within the tissue of the native heart valve annulus. 
     In an application, the annulus-marking device is coupled to the distal tissue-coupling element. 
     In an application, the annulus-marking device is coupled to the distal end of the distal tissue-coupling element. 
     In an application, the method further includes implanting along the native heart valve annulus an annuloplasty structure including a fabric, and implanting the tissue anchor includes passing the annulus-marking device through the fabric of the annuloplasty structure. 
     In an application, the passing the annulus-marking device through the fabric of the annuloplasty structure includes passing the annulus-marking device through the fabric of a portion of the annuloplasty structure before the portion of the annuloplasty structure is positioned along the native heart valve annulus, and the method further includes imaging the native heart valve annulus before the portion of the annuloplasty structure is positioned along the native heart valve annulus. 
     In an application, the annulus-marking device includes one or more radiopaque filaments configured to mark the native heart valve annulus and tissue coupled thereto. 
     In an application, each one of the one or more radiopaque filaments includes a material that is flexible. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a system and/or an apparatus for use with a subject, the system/apparatus including an implant configured for placement along a native heart valve annulus. The implant can include a body portion including flexible material, the body portion having a longitudinal axis that runs along a length of the body portion (e.g., when the implant and/or body portion is straightened). The implant can also include an annulus-marking device including one or more planar radiopaque fins that extends along at least a portion of the body portion ad projects away from the longitudinal axis, each one of the one or more planar radiopaque fins has a longest dimension that is measured along the longitudinal axis. 
     In an application, the body portion includes a plurality of radiopaque markings configured to indicate placement of anchors along the body portion. 
     In an application, the one or more planar radiopaque fins includes a flexible fabric. 
     In an application, the one or more planar radiopaque fins includes two or more planar radiopaque fins. 
     In an application, the apparatus further includes at least one tissue anchor deployable through the body portion of the implant in-between the two or more planar radiopaque fins. 
     There is further provided, in accordance with some applications, a method, including placing at a native heart valve annulus of a subject an implant configured for placement along a native heart valve annulus, the implant including a body portion including flexible material; and an annulus-marking device. The method further including deploying at least one tissue anchor through the body portion of the implant and into tissue of the native heart valve annulus under imaging and using the annulus-marking device as guidance. 
     In some implementations, the body portion has a longitudinal axis that runs along a length of the body portion (e.g., when the implant and/or body portion is straightened), and the annulus-marking devices includes one or more planar radiopaque fins that extends along at least a portion of the body portion and projects away from the longitudinal axis. In some implementations, each one of the one or more planar radiopaque fins has a longest dimension that is measured along the longitudinal axis. 
     In an application, deploying under imaging includes imaging using fluoroscopy. 
     In an application, placing includes placing the implant along an annulus of a mitral valve. 
     In an application, placing includes placing the implant along an annulus of a tricuspid valve. 
     In an application, the body portion includes a plurality of radiopaque markings configured to indicate placement of anchors along the body portion, and deploying the plurality of tissue anchors includes deploying each one of the plurality of tissue anchors in accordance with a respective radiopaque marking. 
     In an application, the one or more planar radiopaque fins are flexible and include a fabric. 
     In an application, the one or more planar radiopaque fins includes two or more planar radiopaque fins. 
     In an application, deploying the at least one tissue anchor through the body portion of the implant includes deploying the at least one tissue anchor through the body portion of the implant in-between the two or more planar radiopaque fins. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto under imaging the one or more planar radiopaque fins. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the one or more planar radiopaque fins with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the one or more planar radiopaque fins against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus includes imaging the one or more planar radiopaque fins with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the one or more planar radiopaque fins responsively to movement of the tissue. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a system and/or an apparatus for use with a subject, the system/apparatus including an implant configured for placement along a native heart valve annulus of the subject, the implant including a body portion including a flexible material. The system/apparatus also including an annulus-marking device. 
     The annulus-marking device can be the same as or similar to other annulus-marking devices herein. For example, in some implementations, the annulus-marking device includes a scaffolding including radiopaque material. The scaffolding can be collapsible and expandable and configured, when expanded, to run alongside at least one side of the body portion of the implant. In some implementations, a plurality of radiopaque filaments are coupled to the scaffolding at at least a distal end of the scaffolding, the plurality of filaments being configured to mark the native heart valve annulus and/or tissue coupled thereto. 
     In an application, the body portion includes a plurality of radiopaque markings configured to indicate placement of anchors along the body portion. 
     In an application, each one of the plurality of radiopaque filaments includes a material that is flexible. 
     In an application, the scaffolding includes a plurality of struts collectively arranged in a triangular shape, and the scaffolding is generally planar and runs alongside a lateral wall of the body portion when the scaffolding is expanded. 
     In an application, the annulus-marking device is coupled to a delivery tool which is configured to deliver the implant to the native heart valve annulus, and the annulus-marking device is retrievable upon removal of the delivery tool from the subject. 
     In an application, the delivery tool is configured to surround a portion of the body portion of the implant, and the annulus-marking device is configured to surround the body portion of the implant at least in part. 
     In an application, the delivery tool includes a fin that is coupled to a distal portion of the delivery tool and to a portion of the scaffolding in a manner in which movement of the fin responsively to blood flow rotationally orients the scaffolding with respect to the body portion of the implant. 
     In an application, the scaffolding is coupled to a ring at a proximal end of the scaffolding, the ring surrounding at least a portion of the body portion of the implant and moveable proximally and distally with respect to the body portion of the implant in a manner in which the scaffolding is moveable to multiple locations along the body portion of the implant. 
     In an application, the scaffolding is shaped so as to partially surround a given portion of the body portion of the implant, the plurality of radiopaque filaments includes a first subset of radiopaque filaments having a first length and a second subset of filaments having a second length that is greater than the first length, and the first and second subsets are configured to rotationally orient the scaffolding with respect to the implant. 
     In an application, the scaffolding is semitubular. In an application, the scaffolding is planar and generally triangular. In an application, the scaffolding is frustoconical. However, other shapes are also possible. 
     In an application, the scaffolding is shaped so as to partially surround a given portion of the body portion of the implant, the plurality of radiopaque filaments includes a first subset of radiopaque filaments having a first rigidity and a second subset of filaments having a second rigidity that is greater than the first rigidity, and the first and second subsets are configured to rotationally orient the scaffolding with respect to the implant. 
     In an application, the scaffolding is semitubular. In an application, the scaffolding is planar and generally triangular. In an application, the scaffolding is frustoconical. However, other shapes are also possible. 
     In an application, the scaffolding includes a plurality of struts collectively arranged in a frustoconical shape, and the scaffolding surrounds at least a portion of the body portion of the implant. 
     In an application, the scaffolding is moveable proximally and distally with respect to the body portion of the implant in a manner in which the scaffolding is moveable to multiple locations along the body portion of the implant. 
     There is further provided, in accordance with some applications, a method, including placing at a native heart valve annulus of a subject an implant including a body portion including flexible material; and viewing the placing under imaging by imaging an annulus-marking device. 
     In some applications, the annulus-marking device comprises a scaffolding including radiopaque material, the scaffolding being collapsible and expandable and configured, when expanded, to run alongside at least one side of the body portion of the implant. In some implementations, a plurality of radiopaque filaments are coupled to the scaffolding at at least a distal end of the scaffolding, the plurality of filaments being configured to mark the native heart valve annulus and tissue coupled thereto. 
     In an application, viewing the placing further includes imaging a plurality of radiopaque markings of the body portion of the implant, and deploying anchors along the body portion in accordance with the imaging the plurality of radiopaque markings of the body portion of the implant. 
     In an application, each one of the plurality of radiopaque filaments includes a material that is flexible. 
     In an application, the scaffolding includes a plurality of struts collectively arranged in a triangular shape, the scaffolding is generally planar and runs alongside a lateral wall of the body portion when the scaffolding is expanded, and imaging the annulus-marking device includes imaging the triangular shape of the scaffolding with respect to the tissue and the body portion of the implant. 
     In an application, placing the implant includes delivering the implant using a delivery tool that is coupled to the annulus-marking device, and the method further includes retrieving the annulus-marking device during removing of the delivery tool from the subject. 
     In an application, the delivery tool is configured to surround a portion of the body portion of the implant, and the annulus-marking device is configured to surround the body portion of the implant at least in part. 
     In an application, the delivery tool includes a fin that is coupled to a distal portion of the delivery tool and to a portion of the scaffolding, and the method further includes rotationally orienting the scaffolding with respect to the body portion of the implant responsively to movement of the fin responsively to blood flow. 
     In an application, the scaffolding is coupled to a ring at a proximal end of the scaffolding, the ring surrounding at least a portion of the body portion of the implant and moveable proximally and distally with respect to the body portion of the implant, and the method further includes facilitating moving of the scaffolding to multiple locations along the body portion of the implant. 
     In an application, the scaffolding is shaped so as to partially surround a given portion of the body portion of the implant, the plurality of radiopaque filaments includes a first subset of radiopaque filaments having a first length and a second subset of filaments having a second length that is greater than the first length, and the method further includes rotationally orienting the scaffolding with respect to the implant using the first and second subsets. 
     In an application, the scaffolding is semitubular. In an application, the scaffolding is planar and generally triangular. In an application, the scaffolding is frustoconical. Other shapes are also possible. 
     In an application, the scaffolding is shaped so as to partially surround a given portion of the body portion of the implant, the plurality of radiopaque filaments includes a first subset of radiopaque filaments having a first rigidity and a second subset of filaments having a second rigidity that is greater than the first rigidity, and the method further includes rotationally orienting the scaffolding with respect to the implant using the first and second subsets. 
     In an application, the scaffolding is semitubular. In an application, the scaffolding is planar and generally triangular. In an application, the scaffolding is frustoconical. Other shapes are also possible. 
     In an application, the scaffolding includes a plurality of struts collectively arranged in a frustoconical shape, and the scaffolding surrounds at least a portion of the body portion of the implant. 
     In an application, the scaffolding is moveable proximally and distally with respect to the body portion of the implant, and the method further includes facilitating moving of the scaffolding to multiple locations along the body portion of the implant. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a system and/or an apparatus for use with a subject, the system/apparatus including an annulus-marking device including a radiopaque material and an implant for implantation along the annulus of the valve of the subject. 
     The annulus-marking device and/or radiopaque material can be the same as or similar to other annulus-marking devices and/or radiopaque materials described elsewhere herein. For example, in some implementations, the annulus-marking device and/or radiopaque material is shaped to define a tubular stent body having a central longitudinal axis and configured for placement within a native heart valve of the subject; and/or a plurality of extensions coupled to a proximal end of the tubular stent body and projecting away from the longitudinal axis of the stent body, the plurality of extensions configured for placement along a circumference of an annulus of the native heart valve. 
     The annulus-marking device can be compressible during delivery toward the native heart valve, and expandable from a compressed state for positioning in the native heart valve. 
     In an application, the annulus-marking device is configured to provide a guide for implantation of the implant along the annulus during implantation and is retrievable following the implantation of the implant. 
     In an application, the annulus-marking device includes a superelastic material. 
     In an application, the stent body and the plurality of extensions are fabricated from a single piece. 
     In an application, the tubular stent body includes two or more prosthetic leaflets. 
     In an application, the apparatus further includes a plurality of anchors, each anchor of the plurality of anchors being configured to anchor the implant to the annulus of the native valve, and each one of the anchors is configured for implantation between adjacent extensions of the plurality of extensions. 
     There is further provided, in accordance with some applications, a method, including placing at a native heart valve of a subject an annulus-marking device including a radiopaque material shaped to define: (1) a tubular stent body having a central longitudinal axis and configured for placement within the native heart valve of the subject; and (2) a plurality of extensions coupled to a proximal end of the tubular stent body and projecting away from the longitudinal axis of the stent body, the plurality of extensions configured for placement along a circumference of an annulus of the native heart valve. The method further includes implanting an implant along the annulus using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. The method can include retrieving the annulus-marking device following the implanting. 
     In some applications, the annulus-marking device is compressible during delivery toward the native heart valve, and expandable from a compressed state for positioning in the native heart valve. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, retrieving the annulus-marking device following the implanting includes constraining the annulus-marking device within a tool and extracting the annulus-marking device from the subject. 
     In an application, placing includes placing the annulus-marking device within a mitral valve. In an application, placing includes placing the annulus-marking device within a tricuspid valve. 
     In an application, implanting the implant includes anchoring the implant to the annulus of the native valve by deploying a respective anchor of a plurality of anchors between adjacent extensions of the plurality of extensions. 
     In an application, retrieving the annulus-marking device following the implanting includes sliding the plurality of extensions from under the implant. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the plurality of extensions. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the plurality of extensions against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the plurality of extensions responsively to movement of the tissue. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including placing at a native heart valve annulus of a subject an annulus-marking device including a mapping catheter, using the mapping catheter, generating a map of the native heart valve annulus under imaging, and responsively to generating the map, implanting an implant at the native heart valve annulus under imaging. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, the method further includes retrieving the annulus-marking device following the generating of the map, and subsequently, extracting the annulus-marking device from the subject. 
     In an application, placing includes placing the annulus-marking device along an annulus of a mitral valve. In an application, placing includes placing the annulus-marking device along an annulus of a tricuspid valve. 
     In an application, the mapping catheter includes radiopaque material, and generating the map includes imaging the mapping catheter under fluoroscopy. 
     In an application, the mapping catheter includes magnetic subunits, and generating the map includes generating a magnetic field and mapping the valve under magnetic imaging. 
     In an application, the mapping catheter includes electrodes, and generating the map includes generating the using the electrodes. 
     In an application, the method further includes retrieving the mapping catheter. In an application, retrieving the mapping catheter includes retrieving the mapping catheter prior to the implanting, and the implanting includes implanting under the guidance of the map generated by the mapping catheter. In an application, retrieving the mapping catheter includes retrieving the mapping catheter subsequently to the implanting. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the mapping catheter. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the mapping catheter against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the mapping catheter responsively to movement of the tissue. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including placing within at least an atrium of a heart of a subject an annulus-marking device including a radiopaque material shaped to define a plurality of expandable elements which expand radially within the atrium such that the plurality of expandable elements provides an indication as to a location of a native heart valve annulus of a native heart valve of the subject. In some applications, the annulus-marking device is compressible during delivery toward the native heart valve, and expandable from a compressed state for positioning at least within the atrium. The method further includes implanting an implant along the native heart valve annulus of the subject using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. In some applications, the method includes retrieving the annulus-marking device following the implanting. 
     In an application, implanting under imaging includes implanting using fluoroscopy. In an application, retrieving the annulus-marking device following the implanting includes constraining the annulus-marking device within a tool and extracting the annulus-marking device from the subject. 
     In an application, placing includes placing the annulus-marking device in a left atrium. In an application, placing includes placing the annulus-marking device in a right atrium. 
     In an application, the plurality of expandable elements collectively form the annulus-marking device into a generally spherical shape, and implanting the implant includes positioning the implant between the annulus-marking device and tissue of an atrial wall. 
     In an application, the plurality of expandable elements include a plurality of woven radiopaque fibers assuming a mesh. 
     In an application, the plurality of expandable elements include a plurality of curved wires. In an application, implanting the implant includes positioning the implant between the annulus-marking device and tissue of an atrial wall and deploying a tissue anchor at a site along the annulus marked between successive curved wires. 
     In an application, each one of the plurality of curved wires has a proximal end and a distal end and a middle section between the proximal and distal ends. 
     In an application, a collective proximal diameter of the proximal ends of the plurality of expandable elements is equal to a collective distal diameter of the distal ends of the plurality of expandable elements, and a collective middle diameter of the plurality of expandable elements is greater than the collective proximal diameter and greater than the collective distal diameter. 
     In an application, the plurality of expandable elements collectively form the annulus-marking device into a partially-spherical shape, and implanting an implant includes positioning the implant between the annulus-marking device and tissue of an atrial wall and deploying a tissue anchor at a site along the annulus marked between successive expandable elements. 
     In an application, the plurality of expandable elements include a plurality of struts collectively forming a partially-spherical stent. In an application, the partially-spherical stent includes a plurality of radiopaque filaments coupled at distal end of the partially-spherical stent, and the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the plurality of radiopaque filaments. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the plurality of radiopaque filaments against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the plurality of radiopaque filaments responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     In an application, the plurality of expandable elements include a plurality of curved wires. 
     In an application, placing the annulus-marking device including expanding the annulus-marking device in a manner in which a distal end of each one of the plurality of expandable elements is disposed within the atrium. 
     In an application, placing the annulus-marking device including expanding the annulus-marking device in a manner in which a distal end of each one of the plurality of expandable elements is disposed within a ventricle of the heart. 
     In an application, the plurality of expandable elements collectively form the annulus-marking device into a partially-bulbous shape, and implanting the implant includes positioning the implant between the annulus-marking device and tissue of an atrial wall and deploying a tissue anchor at a site along the annulus marked between successive expandable elements. 
     In an application, the method further includes delivering a radiopaque helical stent between the plurality of expandable elements. In an application, delivering the helical stent includes delivering the helical stent between native leaflets of the native heart valve. In an application, delivering the helical stent between native leaflets of the native heart valve includes positioning a distal end of the helical stent in a ventricle of the heart of the subject. 
     In an application, the plurality of expandable elements include a plurality of curved wires each having proximal and distal ends and a middle section between the proximal and distal ends. 
     In an application, a collective proximal diameter of the proximal ends of the plurality of expandable elements is smaller than a collective distal diameter of the distal ends of the plurality of expandable elements, and a collective middle diameter of the plurality of expandable elements is greater than the collective proximal diameter and greater than the collective distal diameter. 
     In an application, placing the annulus-marking device including expanding the annulus-marking device in a manner in which the distal end of each one of the plurality of expandable elements is disposed within the atrium. 
     In an application, placing the annulus-marking device including expanding the annulus-marking device in a manner in which the distal end of each one of the plurality of expandable elements is disposed within a ventricle of the heart. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto by viewing the plurality of expandable elements. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the plurality of expandable elements with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the plurality of expandable elements against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the plurality of expandable elements with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the plurality of expandable elements responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the plurality of expandable elements with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     In an application, the annulus-marking device includes a plurality of radiopaque filaments coupled at least to a distal end of the annulus-marking device, and the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the plurality of radiopaque filaments. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the plurality of radiopaque filaments against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the plurality of radiopaque filaments responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a system and/or an apparatus for use with a subject, the system/apparatus including: 
     an annulus-marking device including a radiopaque material shaped to define:
         (1) a tubular stent body having a central longitudinal axis and configured for placement within a native heart valve of the subject; and   (2) a frame coupled to a proximal end of the tubular stent body and projecting away from the longitudinal axis of the stent body, the frame configured for placement along at least a part of a circumference of an annulus of the native heart valve, the annulus-marking device being:
           compressible during delivery toward the native heart valve, and   expandable from a compressed state for positioning in the native heart valve; and   an implant for implantation along the annulus of the valve of the subject,   and the annulus-marking device is:   configured to provide a guide for implantation of the implant along the annulus and within a space defined by the frame, and   retrievable following the implantation of the implant.   
               

     In an application, the annulus-marking device includes a superelastic material. 
     In an application, the stent body and the frame are fabricated from a single piece. 
     In an application, the tubular stent body includes two or more prosthetic leaflets. 
     There is further provided, in accordance with some applications, a method, including placing at a native heart valve of a subject an annulus-marking device including a radiopaque material shaped to define: (1) a tubular stent body having a central longitudinal axis and configured for placement within the native heart valve of the subject; and (2) a frame coupled to a proximal end of the tubular stent body and projecting away from the longitudinal axis of the stent body, the frame configured for placement along at least a part of a circumference of an annulus of the native heart valve. In some applications, the annulus-marking device is compressible during delivery toward the native heart valve, and expandable from a compressed state for positioning in the native heart valve. The method further includes implanting an implant along the annulus using the annulus-marking device as a guide for implantation of the implant along the annulus and within a space defined by the frame under imaging. Some methods include retrieving the annulus-marking device following the implanting. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, retrieving the annulus-marking device following the implanting includes constraining the annulus-marking device within a tool and extracting the annulus-marking device from the subject. 
     In an application, placing includes placing the annulus-marking device within a mitral valve. In an application, placing includes placing the annulus-marking device within a tricuspid valve. 
     In an application, retrieving the annulus-marking device following the implanting includes sliding the frame around the implant and proximally away from the annulus. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the frame. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the frame against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of frame responsively to movement of the tissue. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including: placing within at least an atrium of a heart of a subject an annulus-marking device including a radiopaque material shaped to define a plurality of expandable elements which include respective curved sections at distal ends thereof, plurality of expandable elements being configured to expand radially within the atrium such that the plurality of expandable elements provides an indication as to a location of a native heart valve annulus of a native heart valve of the subject. In some applications, the annulus-marking device is compressible during delivery toward the native heart valve, and expandable from a compressed state for positioning at least within the atrium. 
     The method can further include implanting an implant along the native heart valve annulus of the subject and within a concave section of each one of the plurality of expandable elements using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. The method can also include retrieving the annulus-marking device following the implanting. 
     In an application, placing the annulus-marking device includes placing the annulus-marking device while the implant is disposed within the concave sections of the plurality of expandable elements. 
     In an application, implanting the implant includes placing the implant within the concave sections of the plurality of expandable elements subsequently to the placing of the annulus-marking device. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, retrieving the annulus-marking device following the implanting includes constraining the annulus-marking device within a tool and extracting the annulus-marking device from the subject. 
     In an application, retrieving the annulus-marking device following the implanting includes sliding the curved sections of the plurality of expandable elements from under the implant. 
     In an application, placing includes placing the annulus-marking device in a left atrium. In an application, placing includes placing the annulus-marking device in a right atrium. 
     In an application, placing the annulus-marking device including expanding the annulus-marking device in a manner in which a distal end of each one of the plurality of expandable elements is disposed within the atrium. 
     In an application, the plurality of expandable elements collectively form the annulus-marking device into a partially-pear shape, and implanting an implant includes deploying a tissue anchor at a site along the annulus marked between successive expandable elements. 
     In an application, the plurality of expandable elements collectively form the annulus-marking device into a partially-bulbous shape, and implanting the implant includes deploying a tissue anchor at a site along the annulus marked between successive expandable elements. 
     In an application, the plurality of expandable elements include a plurality of curved wires each having proximal and distal ends and a middle section between the proximal and distal ends. 
     In an application, a collective proximal diameter of the proximal ends of the plurality of expandable elements is smaller than a collective distal diameter of the distal ends of the plurality of expandable elements, and a collective middle diameter of the plurality of expandable elements is greater than the collective proximal diameter and greater than the collective distal diameter. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto by viewing the plurality of expandable elements. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the plurality of expandable elements with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the plurality of expandable elements against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the plurality of expandable elements with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the plurality of expandable elements responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the plurality of expandable elements with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     In an application, the annulus-marking device includes a plurality of radiopaque filaments coupled at least to a distal end of the annulus-marking device, and the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the plurality of radiopaque filaments. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the plurality of radiopaque filaments against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the plurality of radiopaque filaments responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including placing at a ventricular surface of a native heart valve annulus of a subject a distal end portion of an annulus-marking device including a radiopaque material, the distal end portion being shaped to define a curved section that curves upward toward the ventricular surface; facilitating imaging of the heart valve annulus by imaging movement of the distal end portion of the annulus-marking device along a perimeter of the ventricular surface of the native heart valve annulus; and implanting an implant along the native heart valve annulus of the subject using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. The method can also include retrieving the annulus-marking device following the implanting. 
     In an application, placing the distal end portion of the annulus-marking device at the ventricular surface includes placing the distal end portion of the annulus-marking device at the ventricular surface of a native mitral valve. 
     In an application, placing the distal end portion of the annulus-marking device at the ventricular surface includes placing the distal end portion of the annulus-marking device at the ventricular surface of a native tricuspid valve. 
     In an application, implanting includes implanting in conjunction with the imaging movement of the distal end portion of the annulus-marking device. 
     In an application, the method further includes generating a map of the native heart valve annulus by imaging movement of the distal end of the annulus-marking device along the perimeter of the ventricular surface of the native heart valve annulus. In an application, generating the map includes generating the map prior to the implanting. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including placing at a surface of a native heart valve annulus of a subject an annulus-marking device including a toroidal stent including a radiopaque material, facilitating imaging of the heart valve annulus by imaging movement of the distal end portion of the annulus-marking device along a perimeter of the ventricular surface of the native heart valve annulus, and implanting an implant along the native heart valve annulus of the subject using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. The method can also include retrieving the annulus-marking device following the implanting. 
     In an application, implanting the implant includes implanting the implant between an external surface of the toroidal stent and tissue of an atrial wall. 
     In an application, placing at the surface includes placing the annulus-marking device at an atrial surface of the native heart valve annulus. 
     In an application, placing includes placing the annulus-marking device at a surface of a native mitral valve. In an application, placing includes placing the annulus-marking device at a surface of a native tricuspid valve. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto under imaging the annulus-marking device. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the annulus-marking device against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the annulus-marking device responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     In an application, toroidal stent includes a plurality of radiopaque filaments coupled at an inner surface of the toroidal stent, and placing the annulus-marking device includes placing the toroidal stent along the annulus in a manner in which the plurality of radiopaque filaments project toward an orifice of the valve, and the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the plurality of radiopaque filaments. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the plurality of radiopaque filaments against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the plurality of radiopaque filaments responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including placing at a surface of a native heart valve annulus of a subject an annulus-marking device including an implant-leading device including a radiopaque material, facilitating imaging of the heart valve annulus by imaging movement of the implant-leading device along a perimeter of a surface of the native heart valve annulus, and in conjunction with the placing, implanting an implant along the native heart valve annulus of the subject using the implant-leading device as a guide for implantation of the implant along the annulus under imaging. The method can also include retrieving the annulus-marking device following the implanting. 
     In an application, placing the annulus-marking device includes advancing the annulus-marking device along an implantation path upstream of the implant. 
     In an application, placing includes placing the annulus-marking device at a surface of a native mitral valve. In an application, placing includes placing the annulus-marking device at a surface of a native tricuspid valve. 
     In an application, placing includes placing the annulus-marking device in a manner in which a portion of the annulus-marking device spans a portion of an orifice of the valve. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto under imaging the annulus-marking device. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the annulus-marking device against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the annulus-marking device responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     In an application, implant-leading device includes a plurality of radiopaque filaments coupled thereto, and the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the plurality of radiopaque filaments. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the plurality of radiopaque filaments against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the plurality of radiopaque filaments responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including placing at a surface of a native heart valve annulus of a subject an annulus-marking device including a loop-shaped wire including a radiopaque material, facilitating imaging of the heart valve annulus by imaging movement of the wire along at least a portion of a perimeter of a surface of the native heart valve annulus, in conjunction with the placing, implanting an implant along the native heart valve annulus of the subject using the wire as a guide for implantation of the implant along the annulus under imaging. The method can also include retrieving the annulus-marking device following the implanting. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, placing the annulus-marking device includes advancing the annulus-marking device along an implantation path upstream of the implant. 
     In an application, placing includes placing the annulus-marking device at a surface of a native mitral valve. In an application, placing includes placing the annulus-marking device at a surface of a native tricuspid valve. 
     In an application, placing includes pushing a first portion of the annulus-marking device against a first portion of the annulus of the valve and thereby, pushing a second portion of the annulus-marking device that is opposite the first portion of the annulus-marking device against a second portion of the annulus of the valve. 
     In an application, implanting the implant includes implanting the implant at an external perimeter of the annulus-marking device responsively to the pushing. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto under imaging the annulus-marking device. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the annulus-marking device against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the annulus-marking device responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     In an application, annulus-marking device includes a plurality of radiopaque filaments coupled thereto, and the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the plurality of radiopaque filaments. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the plurality of radiopaque filaments against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the plurality of radiopaque filaments responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including deploying within tissue of a native heart valve annulus of a subject an annulus-marking device including a plurality of radiopaque pins including a radiopaque material, facilitating imaging of the heart valve annulus by imaging the plurality of pins, and subsequently, implanting an implant along the native heart valve annulus of the subject using the plurality of pins as a guide for implantation of the implant along the annulus under imaging. 
     In an application, each one of the plurality of pins has a barb configured for anchoring to tissue of the annulus. 
     In an application, each one of the plurality of pins has a longest width of 0.5-3.0 mm. 
     In an application, facilitating imaging of the heart valve annulus by imaging the plurality of pins includes facilitating imaging movement of the plurality of pins responsively to movement of the annulus. 
     In an application, implanting the implant includes deploying a plurality of tissue anchors to fasten the implant to tissue of the annulus. 
     In an application, deploying the plurality of tissue anchors includes deploying a larger number of tissue anchors than a number of pins. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including positioning within tissue of a native heart valve annulus of a native heart valve of a subject an annulus-marking device including a plurality of radiopaque pins that are moveable proximally and distally in response to variations in a topography of tissue of the valve, facilitating imaging of the heart valve annulus and tissue coupled thereto by moving the plurality of pins along the native heart valve and imaging the plurality of pins in order to generate an image of the topography of the heart valve, and implanting an implant along the native heart valve annulus of the subject using the image as a guide for implantation of the implant along the annulus under imaging. The method can further include retrieving the plurality of radiopaque pins from the subject. 
     In an application, retrieving includes retrieving subsequently to the implanting. In an application, retrieving includes retrieving prior to the implanting. 
     In an application, facilitating imaging of the heart valve annulus by imaging the plurality of pins includes facilitating imaging movement of the plurality of pins responsively to movement of the annulus. In an application, implanting includes implanting during the facilitating of the imaging. In an application, facilitating imaging includes viewing movement of the plurality of pins proximally in response to movement of the plurality of pins over a peak in tissue of the annulus. 
     In an application, facilitating imaging includes viewing movement of at least a first portion of the plurality of pins proximally in response to movement of the plurality of pins over tissue of the annulus, and viewing at least a second portion of the plurality of pins not moving in response to movement of the plurality of pins over tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a system and/or an apparatus for use with a subject, the system/apparatus including a multilumen tube, at least a first annulus-marking device expandable from within the multilumen tube. 
     In some applications, the at least a first annulus-marking device includes a distal frame wire, the distal frame wire, when the annulus-marking device is in an expanded state, having an expanded shape in which it assumes a generally linear configuration. In some applications, the at least a first annulus-marking device includes a plurality of radiopaque filaments coupled to the distal frame wire, the plurality of radiopaque filaments including radiopaque material and projecting away from the distal frame wire in the expanded state of the annulus-marking device. 
     In some applications, at least one central rod is coupled to a middle portion of the distal frame wire and disposed primarily and slidable within a primary sublumen of the multilumen tube, the central rod being configured to constrain the distal frame wire and the plurality of radiopaque filaments from the expanded state of the annulus-marking device and pull the distal frame wire and the plurality of radiopaque filaments within the primary sublumen of the multilumen tube. 
     In some applications, at least two peripheral wires are coupled to the distal frame wire at opposite end portions thereof, the at least two peripheral wires being disposed primarily and slidable within respective secondary sublumens of the multilumen tube, the at least two peripheral wires being configured to stabilize the distal frame wire in the expanded state of the annulus-marking device. 
     In some applications, the annulus-marking device is compressible during delivery toward the native heart valve, and expandable from a compressed state for positioning along the native heart valve annulus. 
     In some applications, the system/apparatus further comprises an implant for implantation along the native heart valve annulus of the subject, and the annulus-marking device is configured to provide a guide for implantation of the implant along the annulus during implantation. 
     In some applications, the annulus-marking device is retrievable following the implantation of the implant. 
     In an application, the multilumen tube is shaped so as to define a central lumen, and the implant is configured for delivery to the heart valve annulus via the central lumen. 
     In an application, the peripheral wires are configured to trail behind the distal frame wire as the central rod pulls the distal frame wire the and the plurality of radiopaque filaments within the primary sublumen of the multilumen tube. 
     In an application, each one of the plurality of radiopaque filaments and the distal frame wire include a material that is flexible. 
     In an application, the at least the first annulus-marking device includes at least first and second annulus-marking devices, the multilumen tube is shaped so as to define first and second primary sublumens, the multilumen tube is shaped to as to define four secondary sublumens. 
     In some applications, the apparatus includes first and second central rods configured to respectively constrain the first and second annulus-marking devices within the respective first and second primary sublumens. In some applications, the apparatus includes four peripheral wires configured to respectively stabilize the distal frame wires of the respective first and second annulus-marking devices, the four peripheral wires being slidable within the four secondary sublumens. 
     In an application, the first and second annulus-marking devices are independently controllable by the respective first and second control rods. 
     In an application, the at least the first annulus-marking device includes first, second, third, and fourth annulus-marking devices, the multilumen tube is shaped so as to define first, second, third, and fourth primary sublumens, the multilumen tube is shaped to as to define eight secondary sublumens, the apparatus includes first, second, third, and fourth central rods configured to respectively constrain the first, second, third, and fourth annulus-marking devices within the respective first, second, third, and fourth primary sublumens, and the apparatus includes eight peripheral wires configured to respectively stabilize the distal frame wires of the respective first, second, third, and fourth annulus-marking devices, the eight peripheral wires being slidable within the eight secondary sublumens. 
     In an application, the first, second, third, and fourth annulus-marking devices are independently controllable by the respective first, second, third, and fourth control rods. 
     There is further provided, in accordance with some applications, a method, including delivering within a heart chamber of a subject a distal end portion of a central multilumen tube, expanding from within the multilumen tube at least a first annulus-marking device, and implanting an implant along the native heart valve annulus of the subject using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. The method can include retrieving the annulus-marking device following the implanting. 
     In some applications, the at least a first annulus-marking device includes a distal frame wire, the distal frame wire, when the annulus-marking device is in an expanded state, having an expanded shape in which it assumes a generally linear configuration. In some applications, a plurality of radiopaque filaments are coupled to the distal frame wire, the plurality of radiopaque filaments including radiopaque material and projecting away from the distal wire in the expanded state of the annulus-marking device. 
     In some applications, the method includes controlling a position of the at least first annulus-marking device by sliding primary sublumen of the multilumen tube at least one central rod coupled to a middle portion of the distal frame wire and disposed primarily within the primary sublumen of the multilumen tube. 
     In some applications, the method includes stabilizing the distal frame wire by at least two peripheral wires coupled to the distal frame wire at opposite end portions thereof, the at least two peripheral wires being disposed primarily and slidable within respective secondary sublumens of the multilumen tube, the at least two peripheral wires being configured to stabilize the distal frame wire in the expanded state of the annulus-marking device. 
     In some applications, the method includes constraining the annulus-marking device by pulling on the central rod to constrain the distal frame wire and the plurality of radiopaque filaments from the expanded state of the annulus-marking device, and by the pulling, pulling the distal frame wire and the plurality of radiopaque filaments within the primary sublumen of the multilumen tube. 
     In some applications, the annulus-marking device is compressible during delivery toward the native heart valve, and expandable from a compressed state for positioning along the native heart valve annulus; 
     In an application, the multilumen tube is shaped so as to define a central lumen, and the method further includes delivering the implant to the heart valve annulus via the central lumen. 
     In an application, the constraining the annulus-marking device by pulling on the central rod includes allowing the peripheral wires to trail behind the distal frame wire as the central rod pulls the distal frame wire the and the plurality of radiopaque filaments within the primary sublumen of the multilumen tube. 
     In an application, each one of the plurality of radiopaque filaments and the distal frame wire include a material that is flexible. 
     In an application, controlling the position of the at least first annulus-marking device includes placing the at least first annulus-marking device along an annulus of a mitral valve. 
     In an application, controlling the position of the at least first annulus-marking device includes placing the at least first annulus-marking device along an annulus of a tricuspid valve. 
     In an application, the at least the first annulus-marking device includes at least first and second annulus-marking devices, the multilumen tube is shaped so as to define first and second primary sublumens, the multilumen tube is shaped to as to define four secondary sublumens, and the method further includes respectively constraining the first and second annulus-marking devices within the respective first and second primary sublumens by pulling respective first and second control rods. In some applications, the method further includes respectively stabilizing the wires of the respective first and second annulus-marking devices using four peripheral wires that are slidable within the four secondary sublumens. 
     In an application, the method further includes independently controlling the first and second annulus-marking devices using the respective first and second control rods. 
     In an application, the at least the first annulus-marking device includes first, second, third, and fourth annulus marking devices, the multilumen tube is shaped so as to define first, second, third, and fourth primary sublumens, the multilumen tube is shaped to as to define eight secondary sublumens, and the method further includes respectively constraining the first, second, third, and fourth annulus-marking devices within the respective second, third, and fourth primary sublumens by pulling respective first, second, third, and fourth control rods. In some applications, the method further includes respectively stabilizing the distal frame wires of the respective second, third, and fourth annulus-marking devices using eight peripheral wires that are slidable within the eight secondary sublumens. 
     The method can further include independently controlling the first, second, third, and fourth annulus-marking devices using the respective first and second control rods. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the plurality of radiopaque filaments. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the plurality of radiopaque filaments against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the plurality of radiopaque filaments responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including placing at a native heart valve annulus of a subject an annulus-marking device including a radiopaque material shaped to define a plurality of inflatable fingers, the annulus-marking device being in a compressed state during delivery toward the native heart valve, and expandable from the compressed state for positioning along the native heart valve annulus, and implanting an implant along the native heart valve annulus of the subject using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. The method can include retrieving the annulus-marking device following the implanting. 
     In an application, placing the annulus-marking device includes delivering the annulus-marking device using a delivery tool, the annulus-marking device surrounds the tool, and implanting the implant includes delivering the implant through a lumen of the tool around which the annulus-marking device surrounds. 
     In an application, placing the annulus-marking device includes measuring a height of the annulus using the annulus-marking device. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, retrieving the annulus-marking device following the implanting includes deflating the annulus-marking device and constraining the annulus-marking device within a tool and extracting the annulus-marking device from the subject. 
     In an application, placing includes placing the annulus-marking device along an annulus of a mitral valve. In an application, placing includes placing the annulus-marking device along an annulus of a tricuspid valve. 
     In an application, implanting using the annulus-marking device as the guide includes viewing a shape of each one of the plurality of fingers. 
     In an application, viewing the shape includes determining that the annulus-marking device is at the annulus responsively to viewing a bend in at least one of the plurality of fingers. 
     In an application, viewing the shape includes determining that the annulus-marking device is at at least a portion of a leaflet responsively to viewing a movement of at least one of the plurality of fingers responsively to movement of the at least one of the plurality of fingers. 
     In an application, the method further includes inflating the plurality of fingers prior to the placing. In an application, inflating includes inflating the plurality of fingers with a radiopaque fluid. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto under imaging the plurality of fingers. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the plurality of fingers with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the plurality of fingers against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus includes imaging the plurality of fingers with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the plurality of fingers responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a system and/or an apparatus for use with a subject, the system/apparatus including an annulus-marking device including a radiopaque material shaped to define: (1) a plurality of concentric wire loops connected by a scaffolding configured for placement at an orifice of a native heart valve of the subject; and (2) a wire loop frame coupled to the scaffolding and concentric with respect to the plurality of concentric wire loops, the wire loop frame configured for placement along at least a part of a circumference of an annulus of the native heart valve. 
     In some applications, the annulus-marking device is compressible during delivery toward the native heart valve, and expandable from a compressed state for positioning in the native heart valve. 
     In some applications, the system/apparatus further includes an implant for implantation along the annulus of the valve of the subject. 
     In some applications, the annulus-marking device is configured to provide a guide for implantation of the implant along the annulus and within a space defined by the frame. In some applications, the annulus-marking device is retrievable following the implantation of the implant. 
     In an application, the annulus-marking device includes a superelastic material. In an application, the plurality of concentric wire loops, the scaffolding, and the wire loop frame are fabricated from a single piece. 
     In an application, the annulus-marking device includes a plurality of radiopaque filaments coupled at least to the plurality of concentric wire loops, each one of the plurality of filaments being configured to sway responsively to movement of blood through the orifice of the valve to provide an indication of a location of leaflets of the valve. 
     In an application, the annulus-marking device includes a locking ring in a center of the plurality of concentric wire loops, the locking ring being pushable distally in order to lock the annulus-marking device in the expanded state. 
     In an application, the apparatus further includes a plurality of radiopaque filaments coupled to the plurality of concentric wire loops, the plurality of radiopaque filaments including radiopaque material. 
     In an application, the plurality of radiopaque filaments are configured to provide an indication of a location of leaflets of the valve by moving responsively to movement of the native heart valve. 
     There is further provided, in accordance with some applications, a method, including placing at a native heart valve of a subject an annulus-marking device including a radiopaque material shaped to define: (1) a plurality of concentric wire loops connected by a scaffolding; and (2) a wire loop frame coupled to the scaffolding and concentric with respect to the plurality of concentric wire loops, the wire loop frame configured for placement along at least a part of a circumference of an annulus of the native heart valve. 
     In some applications, the annulus-marking device is compressible to a compressed state during delivery toward the native heart valve, and expandable from a compressed state for positioning in the native heart valve to an expanded state. 
     In some applications, the method includes, under imaging, implanting an implant along the annulus using the annulus-marking device as a guide for implantation of the implant along the annulus and within a space defined by the frame. In some applications, the method includes retrieving the annulus-marking device following the implanting. 
     In an application, the method further includes locking the annulus-marking device in the expanded state by pushing distally a locking ring that is disposed in a center of the plurality of concentric wire loops. 
     In an application, the method further includes transitioning the annulus-marking device from the compressed state to the expanded state by pushing distally a locking ring that is disposed in a center of the plurality of concentric wire loops. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, placing includes placing the annulus-marking device at a mitral valve, placing the plurality of concentric wire loops at an orifice of the valve, and placing the wire loop frame along at least a part of a circumference of the annulus of the mitral valve. 
     In an application, placing includes placing the annulus-marking device at a tricuspid valve, placing the plurality of concentric wire loops at an orifice of the valve, and placing the wire loop frame along at least a part of a circumference of the annulus of the tricuspid valve. 
     In an application, retrieving the annulus-marking device following the implanting includes sliding the frame around the implant and proximally away from the annulus. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the annulus-marking device. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the annulus-marking device against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the annulus-marking device responsively to movement of the tissue. 
     In an application, the annulus-marking device includes a plurality of radiopaque filaments coupled to the plurality of concentric wire loops, and the method further includes determining that the annulus-marking device is at at least a portion of a leaflet responsively to viewing a movement of at least some of the plurality of radiopaque filaments responsively to movement of the valve. 
     In an application, the method further includes determining that the annulus-marking device is at at least a portion of the annulus responsively to viewing a lack of movement of at least a first of the plurality of radiopaque filaments while a second portion of the plurality of radiopaque filaments move with responsively to movement of the valve. 
     In an application, retrieving the annulus-marking device following the implanting includes constraining the annulus-marking device within a tool and extracting the annulus-marking device from the subject. 
     In an application, retrieving the annulus-marking device includes transitioning the annulus-marking device from the expanded state to the compressed state by pulling proximally a locking ring that is disposed in a center of the plurality of concentric wire loops. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including placing at a native heart valve annulus of a subject an annulus-marking device including a radiopaque material shaped to define a plurality of radiopaque petals or loops, the annulus-marking device being in a compressed state during delivery toward the native heart valve, and expandable from the compressed state for positioning along the native heart valve annulus, and implanting an implant along the native heart valve annulus of the subject using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. The method can include retrieving the annulus-marking device following the implanting. 
     In an application, placing the annulus-marking device includes delivering the annulus-marking device using a delivery tool, the annulus-marking device surrounds the tool, and implanting the implant includes delivering the implant through a lumen of the tool around which the annulus-marking device surrounds. 
     In an application, placing the annulus-marking device includes measuring a height of the annulus using the annulus-marking device. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, retrieving the annulus-marking device following the implanting includes constraining the annulus-marking device within a tool and extracting the annulus-marking device from the subject. 
     In an application, placing includes placing the annulus-marking device along an annulus of a mitral valve. 
     In an application, placing includes placing the annulus-marking device along an annulus of a tricuspid valve. 
     In an application, at least one of the plurality of petals or loops is a larger petal or loop than the other petals or loops, and placing includes placing the annulus-marking device in the valve in manner which the larger petal or loop is positioned between leaflets of the valve. 
     In an application, implanting using the annulus-marking device as the guide includes viewing a shape of each one of the plurality of petals or loops. 
     In an application, viewing the shape includes determining that the annulus-marking device is at the annulus responsively to viewing a bend in at least one of the plurality of petals or loops. 
     In an application, viewing the shape includes determining that the annulus-marking device is at at least a portion of a leaflet responsively to viewing a movement of at least one of the plurality of petals or loops responsively to movement of the at least one of the plurality of petals or loops. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto under imaging the plurality of petals or loops. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the plurality of petals or loops with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the plurality of petals or loops against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus includes imaging the plurality of petals or loops with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the plurality of petals or loops responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a system and/or an apparatus, including a guidewire having a distal end portion that is configured to assume a shape in an expanded state of the guidewire; and an annulus-marking device including a plurality of radiopaque filaments coupled to the distal end portion of the guidewire. 
     In an application, each one of the plurality of radiopaque filaments includes a material that is flexible. 
     In an application, the apparatus further includes a tube, and the distal end portion of the guidewire surrounds a portion of the tube at least in part in the expanded state of the guidewire. 
     In an application, the apparatus further includes an implant deliverable through a lumen of the tube, and the plurality of radiopaque filaments are configured to guide implantation of the implant. 
     There is further provided, in accordance with some applications, a method, including positioning a distal end portion of a guidewire within a chamber of a heart of a subject, the guidewire being configured to assume a shape in an expanded state of the guidewire, and the distal end portion of the guidewire being coupled to an annulus-marking device including a plurality of radiopaque filaments; and moving the distal end portion of the guidewire along tissue surrounding the chamber of the heart; and imaging the tissue surrounding the chamber of the heart by viewing the moving of the distal end portion of the guidewire and by viewing the plurality of radiopaque filaments. 
     In an application, moving the distal end portion of the guidewire along tissue includes measuring a height of a native annulus of a valve of the heart using the annulus-marking device. 
     In an application, imaging includes imaging using fluoroscopy. 
     In an application, positioning the distal end portion of the guidewire includes positioning the distal end portion of the guidewire along an annulus of a mitral valve. In an application, positioning the distal end portion of the guidewire includes positioning the distal end portion of the guidewire along an annulus of a tricuspid valve. 
     In an application, positioning the distal end portion of the guidewire includes positioning the distal end portion of the guidewire in a subannular space of a native heart valve of the subject. 
     In an application, imaging the tissue surrounding the chamber includes viewing tissue of a native heart valve annulus and tissue coupled thereto using the plurality of radiopaque filaments. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the plurality of radiopaque filaments against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     In an application, the method further includes implanting an implant along a native heart valve annulus of the subject using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. In an application, the method includes retrieving the annulus-marking device following the implanting. 
     In an application, the implant is delivered through a tube, the distal end portion of the guidewire surrounds a portion of the tube, and implanting an implant along a native heart valve annulus of the subject includes guiding the portion of the tube along the annulus using the annulus-marking device. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including expanding within an atrium of a heart of a subject an annulus-marking device including a radiopaque material shaped to define: (1) a first radiopaque loop, and (2) a second radiopaque loop configured to pivot and tilt with respect to the first radiopaque loop. The method can include tilting the second radiopaque loop with respect to the first radiopaque loop and allowing the second radiopaque loop to pivot along a plane that is at a non-zero angle with respect to a plane of the first radiopaque loop. 
     In some applications, the method includes positioning the annulus-marking device in its fully expanded state at least in part within a native heart valve of the heart in a manner in which (1) the first radiopaque loop is disposed between leaflets of the native heart valve, an upper portion of the first radiopaque loop is disposed within the atrium and a lower portion of the first radiopaque is disposed within a ventricle of the heart, and (2) the second radiopaque loop is disposed along an atrial surface of an annulus of the valve. 
     In some applications, the annulus-marking device is compressible to a compressed state during delivery toward the native heart valve, and expandable from a compressed state for positioning in the native heart valve to an expanded state, and implanting an implant along the annulus using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. In some applications, the method includes retrieving the annulus-marking device following the implanting. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, positioning includes positioning the annulus-marking device at a mitral valve. In an application, positioning includes positioning the annulus-marking device at a tricuspid valve. 
     In an application, retrieving the annulus-marking device following the implanting includes pivoting and tilting the second radiopaque loop with respect to the first radiopaque loop. 
     In an application, positioning includes positioning the first radiopaque loop between the leaflets and by the positioning, applying a force to commissures of the valve by the first radiopaque loop. 
     In an application, the first and second radiopaque loops each include wire frames surrounded at least in part by a respective radiopaque spring, and positioning the annulus-marking device includes allowing the springs to compress and expand. 
     In an application, retrieving the annulus-marking device following the implanting includes constraining the annulus-marking device within a tool and extracting the annulus-marking device from the subject. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the annulus-marking device. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the annulus-marking device against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the annulus-marking device responsively to movement of the tissue. 
     In an application, the method further includes, in the fully expanded state of the annulus-marking device, moving the second radiopaque loop vertically along a portion of the first radiopaque loop. 
     In an application, moving the second radiopaque loop vertically along a portion of the first radiopaque loop includes measuring a height of the annulus. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a system and/or an apparatus including an annulus-marking device, the annulus-marking device including: a central pole; two or more expandable wires connected at their respective proximal and distal ends to the central pole, the two or more expandable wires each shaped to define an indented section to fit a native heart valve annulus of a valve of a subject; and at least one ultrasound transducer slidable along and rotational with respect to the central pole. In some applications, the annulus-marking device is compressible to a compressed state during delivery toward the native heart valve, and expandable from a compressed state for positioning in the native heart valve to an expanded state. 
     In an application, the apparatus further includes an implant implantable at the annulus under guidance from imaging using the annulus-marking device. 
     In an application, the central pole is hollow, and the at least one ultrasound transducer is disposed within the central pole. 
     In an application, the apparatus further includes at least one radiopaque marker slidable along the two or more expandable elements until the radiopaque marker abuts the annulus. 
     In an application, the at least one radiopaque marker includes a wire ring. 
     In an application, the at least one radiopaque marker includes a plurality of radiopaque filaments coupled to the wire ring. 
     There is further provided, in accordance with some applications, a method, including expanding within a native heart valve of a subject an annulus-marking device shaped to define two or more expandable wires connected at their respective proximal and distal ends to a central pole, the two or more expandable wires each shaped to define an indented section to fit a native heart valve annulus of the valve. 
     In some applications, the annulus-marking device is compressible to a compressed state during delivery toward the native heart valve, and expandable from a compressed state for positioning in the native heart valve to an expanded state. 
     In some applications, the method includes sliding at least one ultrasound transducer along and rotationally with respect to the central pole, imaging the annulus of the valve using the ultrasound transducer, and implanting an implant along the annulus using the annulus-marking device as a guide for implantation of the implant along the annulus under the imaging. In some applications, the method includes retrieving the annulus-marking device following the implanting. 
     In an application, imaging includes measuring a height of the annulus. 
     In an application, expanding includes expanding the annulus-marking device at a mitral valve. 
     In an application, expanding includes expanding the annulus-marking device at a tricuspid valve. 
     In an application, expanding includes expanding the two or more expandable wires between leaflets of the valve and by the expanding, applying a force to commissures of the valve by the two or more expandable wires. 
     In an application, retrieving the annulus-marking device following the implanting includes constraining the annulus-marking device within a tool and extracting the annulus-marking device from the subject. 
     In an application, expanding includes positioning the annulus-marking device within the native heart valve of the heart in a manner in which the two or more expandable wires are disposed between leaflets of the native heart valve, an upper portion of each expandable wire being disposed within an atrium, and a lower portion of each expandable wire being disposed within a ventricle. 
     In an application, the method further includes: 
     sliding a radiopaque marker vertically along the two or more expandable elements until the radiopaque marker abuts the annulus; and 
     imaging the annulus under fluoroscopy. 
     In an application, sliding the radiopaque marker includes measuring a height of the annulus. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the annulus-marking device. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the annulus-marking device against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the annulus-marking device responsively to movement of the tissue. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including positioning an annulus-marking device including a clip within a ventricle of a heart of a subject, the clip including: radiopaque material, first and second jaws coupled together at a hinge point, each one of the first and second jaws having an end, and first and second filaments extending from the respective ends of the first and second jaws. 
     The method can include clipping together first and second leaflets of a heart valve of the subject using the clip, and by the clipping, allowing the first filament to abut an atrial surface of at least one of the first and second leaflets in a manner in which an end of the first filament is positioned in a vicinity of a hinge of an annulus of the valve in a vicinity of an atrial wall, and the second filament to abut a ventricular surface of at the least one of the first and second leaflets in a manner in which an end of the second filament is positioned in a subannular groove of the valve in a vicinity of a ventricular wall. 
     The method can further include implanting an implant along the native heart valve annulus of the subject using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. 
     In an application, the method further includes retrieving the annulus-marking device following the implanting. 
     In an application, the first and second filaments include material that is superelastic. 
     In an application, clipping includes implanting the annulus-marking device. 
     In an application, implanting includes affixing at least one of the first and second filaments to the valve. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, positioning the annulus-marking device in the ventricle includes positioning the annulus-marking device in a right ventricle, and clipping together the leaflets includes clipping the leaflets of a tricuspid valve. In an application, positioning the annulus-marking device in the ventricle includes positioning the annulus-marking device in a left ventricle, and clipping together the leaflets includes clipping the leaflets of a mitral valve. 
     In an application, clipping includes creating a double orifice of the mitral valve. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the first and second filaments. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the first and second filaments against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the first and second filaments responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including positioning an annulus-marking device including a clamp within a ventricle of a heart of a subject, the clamp including radiopaque material, first and second arms coupled together at a hinge point, each one of the first and second arms having an end, and first and second curved elements coupled to the respective ends of the first and second arms. 
     The method can further comprise clamping a leaflet of a heart valve of the subject between the first and second arms using the clamp, and by the clamping, allowing: the first curved element to abut an atrial surface of the leaflet in a vicinity of a hinge of an annulus of the valve in a vicinity of an atrial wall; and the second curved element to abut a ventricular surface of at the leaflet in a subannular groove of the valve in a vicinity of a ventricular wall. 
     The method can further include implanting an implant along the native heart valve annulus of the subject using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. 
     In an application, the method further includes retrieving the annulus-marking device following the implanting. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, positioning the annulus-marking device in the ventricle includes positioning the annulus-marking device in a right ventricle, and clamping includes clamping the leaflet of a tricuspid valve. 
     In an application, positioning the annulus-marking device in the ventricle includes positioning the annulus-marking device in a left ventricle, and clamping includes clamping the leaflet of a mitral valve. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the first and second curved elements. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the first and second curved elements against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the first and second curved elements responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including delivering an annulus-marking device including a balloon within a native heart valve of a heart of a subject and implanting an implant along a native heart valve annulus of the subject using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. 
     The balloon can be the same as or similar to other inflatable elements and/or balloons described herein. In some applications, the balloon includes an upper inflatable section inflatable to assume a generally paddle shape, a lower inflatable section inflatable to assume a spherical shape, and a central waist between the upper and lower inflatable sections. 
     In some applications, the method includes positioning the balloon such that the upper inflatable section is disposed within an atrium of the heart, the lower inflatable section is disposed within a ventricle of the heart, and the central waist is disposed between leaflets of the valve. 
     In some applications, the method includes inflating the balloon such that the upper inflatable section expands to assume the generally paddle shape, and the lower inflatable section expands to assume the spherical shape. 
     In an application, implanting includes implant the implant between an external surface of the upper inflatable element and an atrial wall of the heart. 
     In an application, the method further includes retrieving the annulus-marking device following the implanting. 
     In an application, an upper surface of the upper inflatable section is slanted. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, the balloon is shaped so as to define an hourglass shape at at least one cross-section thereof. 
     In an application, positioning the annulus-marking device in the valve includes positioning the annulus-marking device in a mitral valve. In an application, positioning the annulus-marking device in the valve includes positioning the annulus-marking device in a tricuspid valve. 
     In an application, the upper inflatable section is less compliant than the lower inflatable section. In an application, the upper inflatable section is noncompliant. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the balloon. 
     In an application, the balloon includes radiopaque material. In an application, inflating the balloon includes inflating the balloon with radiopaque fluid. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the upper and lower inflatable elements against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including delivering an annulus-marking device including a balloon within a ventricle of a native heart valve of a heart of a subject, inflating the balloon within the ventricle, and implanting an implant along a native heart valve annulus of the subject using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. In some applications, the method includes retrieving the annulus-marking device following the implanting. 
     In an application, inflating includes inflating the balloon such that it assumes a spherical shape. In an application, inflating includes inflating the balloon such that it assumes a toroidal shape. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, delivering the annulus-marking device within the ventricle includes positioning the annulus-marking device in a left ventricle. In an application, delivering the annulus-marking device within the ventricle includes positioning the annulus-marking device in a right ventricle. 
     In an application, the balloon includes a magnetic substance within a space defined by the balloon and implanting under imaging includes drawing the magnetic substance to an upper surface of the balloon and marking the annulus of the valve from a ventricular surface of the valve. 
     In an application, delivering the annulus-marking device includes delivering the annulus-marking device using a delivery tool including a magnet, and drawing the magnetic substance to the upper surface of the balloon includes using the magnet of the delivery tool. 
     In an application, the magnet includes a circular magnet, delivering the annulus-marking device using the delivery tool includes positioning the magnet at an atrial surface of the valve, and implanting the implant includes implanting the implant between an external surface of the magnet and an atrial wall. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the balloon. 
     In an application, the balloon includes radiopaque material. In an application, inflating the balloon includes inflating the balloon with radiopaque fluid. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the balloon against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including delivering an annulus-marking device within a native heart valve of a heart of a subject and implanting an implant along a native heart valve annulus of the subject using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. The annulus-marking device can be the same as or similar to other annulus-marking devices described herein. 
     In some applications, the annulus-marking device includes an upper inflatable element inflatable to assume a first toroidal shape and a lower inflatable element inflatable to assume a second toroidal shape. In some applications, the method includes positioning the annulus-marking device such that the upper inflatable element is disposed within an atrium of the heart and the lower inflatable element is disposed within a ventricle of the heart, and inflating the upper and lower inflatable elements such that the upper inflatable element expands to assume the first toroidal shape, and the lower inflatable element expands to assume the second toroidal shape. 
     In an application, implanting includes implant the implant between an external surface of the upper inflatable element and an atrial wall of the heart. 
     In an application, the method further includes retrieving the annulus-marking device following the implanting. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, positioning the annulus-marking device in the valve includes positioning the annulus-marking device in a mitral valve. In an application, positioning the annulus-marking device in the valve includes positioning the annulus-marking device in a tricuspid valve. 
     In an application, the upper and lower inflatable elements include compliant material. In an application, the upper and lower inflatable elements include noncompliant material. 
     In an application, the upper and lower inflatable elements are discrete. 
     In an application, the annulus-marking device includes a single balloon including the upper and lower inflatable elements coupled together. In an application, the balloon includes a central waist between the upper and lower inflatable elements. In an application, the balloon is shaped so as to define an hourglass shape at at least one cross-element thereof. 
     In an application, positioning the annulus-marking device includes positioning the central waist between leaflets of the valve. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the annulus-marking device. 
     In an application, the annulus-marking device includes radiopaque material. 
     In an application, inflating the annulus-marking device includes inflating the annulus-marking device with radiopaque fluid. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the upper and lower inflatable elements against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including delivering an annulus-marking device including at least a first magnetic element to one or more surfaces of a native heart valve of a heart of a subject, the one or more surfaces selected from the group consisting of: an atrial surface and a ventricular surface, generating a magnetic field around the at least the first magnetic element, and implanting an implant along a native heart valve annulus of the subject using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. 
     The method can also include retrieving the annulus-marking device following the implanting. 
     In an application, the at least the first magnetic element includes a circular wire. In an application, the at least the first magnetic element includes a flat disc. In an application, the at least the first magnetic element includes a toroid. 
     In an application, generating the magnetic field includes preventing movement of the magnetic element with respect to tissue of the valve. 
     In an application, generating the magnetic field includes positioning the magnetic element at a suitable position with respect to tissue of the valve. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, delivering the annulus-marking device includes delivering the annulus-marking device to a mitral valve. In an application, delivering the annulus-marking device includes delivering the annulus-marking device to a tricuspid valve. 
     In an application, generating the magnetic field around the at least the first magnetic element includes providing an external magnetic field. 
     In an application, delivering the annulus-marking device includes positioning the at least the first magnetic element at the atrial surface, and generating the magnetic field includes generating the magnetic field from within a ventricle of the heart. 
     In an application, delivering the annulus-marking device includes positioning the at least the first magnetic element at the ventricular surface, and generating the magnetic field includes generating the magnetic field from within an atrium of the heart. 
     In an application, delivering the at least the first magnetic element includes delivering the first magnetic element to the atrial surface of the valve. In an application, the method further includes delivering a second magnetic element to the ventricular surface of the valve, and generating the magnetic field includes generating the magnetic field responsively to the delivering the second magnetic element. 
     In an application, implanting the implant includes implanting the implant between an external surface of the first magnetic element and an atrial wall. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the at least the first magnetic element. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the first magnetic element against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including placing at a surface of a native heart valve annulus of a subject an annulus-marking device including a coil-shaped wire including a radiopaque material, facilitating imaging of the heart valve annulus by imaging movement of the coil-shaped wire along at least a portion of a perimeter of a surface of the native heart valve annulus, and in conjunction with the placing, implanting an implant along the native heart valve annulus of the subject using the coil-shaped wire as a guide for implantation of the implant along the annulus under imaging. The method can also include retrieving the annulus-marking device following the implanting. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, placing includes placing the annulus-marking device at a surface of a native mitral valve. In an application, placing includes placing the annulus-marking device at a surface of a native tricuspid valve. 
     In an application, placing includes (1) anchoring a first end portion of the coil-shaped wire to a first commissure of the valve, (2) allowing the coil-shaped wire to expand along a portion of the circumference of the valve, and (3) anchoring a second end portion of the coil-shaped wire to a second commissure of the valve. 
     In an application, allowing the coil-shaped wire to expand along the portion of the circumference of the valve includes applying a pushing force to a portion of the annulus at the portion of the circumference of the valve. 
     In an application, placing includes placing the annulus-marking device along an atrial surface of the valve, and (1) anchoring the first end portion of the coil-shaped wire to the first commissure of the valve includes anchoring the first end portion to the first commissure using a first anchor that locks in place at the first commissure in a ventricle of the heart of the subject, and (2) anchoring the second end portion of the coil-shaped wire to the second commissure of the valve includes anchoring the second end portion to the second commissure using a second anchor that locks in place at the second commissure in the ventricle of the heart of the subject. 
     In an application, the valve includes a mitral valve, and (1) anchoring the first end portion of the coil-shaped wire to the first commissure of the valve includes anchoring the first end portion to an anterolateral commissure of the valve, (2) allowing the coil-shaped wire to expand along the portion of the circumference of the valve includes allowing the coil-shaped wire to expand along the posterior circumference of the valve, and (3) anchoring the second end portion of the coil-shaped wire to the second commissure of the valve includes anchoring the second end portion of the coil-shaped wire to a posteromedial commissure of the valve. 
     In an application, implanting the implant includes implanting the implant at an external perimeter of the annulus-marking device responsively to the pushing. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto under imaging the annulus-marking device. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the annulus-marking device against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the annulus-marking device responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with some applications, a method, including placing within a native heart valve of a heart of a subject an annulus-marking device including a radiopaque material shaped to define an expandable element which expands within the heart valve that the expandable element provides an indication as to a location of a native heart valve annulus of the native heart valve of the subject. 
     In some applications, the annulus-marking device is compressible during delivery toward the native heart valve, and expandable from a compressed state for positioning at least within the heart valve. The method can further include expanding the annulus-marking device to an expanded state. 
     The method can further include implanting an implant along the native heart valve annulus of the subject using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. 
     The method can also include retrieving the annulus-marking device following the implanting. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, retrieving the annulus-marking device following the implanting includes constraining the annulus-marking device within a tool and extracting the annulus-marking device from the subject. 
     In an application, placing includes placing the annulus-marking device in a mitral valve. In an application, placing includes placing the annulus-marking device in a tricuspid valve. 
     In an application, the expanding the expandable device includes expanding the expandable device to assume a generally spherical shape, and implanting the implant includes positioning the implant between the annulus-marking device and tissue of an atrial wall. 
     In an application, the expandable element includes a plurality of expandable elements including a plurality of woven radiopaque fabric fibers assuming a mesh. 
     In an application, the expandable element includes a plurality of expandable elements including a plurality of woven radiopaque metal fibers assuming a mesh. 
     In an application, the expandable element includes a balloon. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto by viewing the annulus-marking device. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the expandable element with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the expandable element against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the expandable element with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the expandable element responsively to movement of the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the expandable element with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with an application of the present invention, a system for use with a subject, the system including an implant configured for placement along a native heart valve annulus of a native heart valve of the subject and an annulus-marking device. 
     In some applications, the implant includes a body portion including flexible material, the body portion having a longitudinal axis that runs along a length of the body portion 
     The annulus-marking device can be the same as or similar to any of the annulus-marking devices described herein. In some applications, the annulus-marking device includes a scaffolding including radiopaque material, the scaffolding being collapsible and expandable and configured, when expanded, to laterally push against tissue of the heart valve. A plurality of radiopaque elements can be coupled to the scaffolding, the plurality of radiopaque elements being configured to mark the native heart valve annulus and tissue coupled thereto. 
     In an application, the body portion includes a plurality of radiopaque markings configured to indicate placement of anchors along the body portion. In an application, each one of the plurality of radiopaque elements includes a material that is flexible. In an application, each one of the plurality of radiopaque elements includes a radiopaque filament. 
     In an application, when the scaffolding is expanded, the scaffolding is configured to push against tissue of a leaflet of the valve in a manner in which the leaflet assumes two subcusps. 
     In an application, the annulus-marking device is coupled to a delivery tool, and the annulus-marking device is retrievable upon removal of the delivery tool from the subject. 
     In an application, the scaffolding includes at least one a rod having a vertical orientation when the scaffolding is expanded. In an application, when the scaffolding is expanded, the rod extends from an atrial surface of the heart valve toward a ventricular surface of the heart valve. 
     In an application, the plurality of radiopaque elements includes a plurality of radiopaque filaments and the rod is coupled to the plurality of radiopaque filaments such that, when the scaffolding is expanded, the plurality of radiopaque filaments are configured to be pressed against tissue of the native heart valve annulus and tissue coupled thereto in a manner in which the plurality of radiopaque filaments provide an indication of the native heart valve annulus and tissue coupled thereto. 
     In an application, the at least one rod includes a plurality of rods and the scaffolding includes an expandable basket coupled to the plurality of rods such that the scaffolding expands circumferentially with respect to the native heart valve in a manner in which the plurality of rods are disposed circumferentially with respect to the native heart valve. 
     In an application, the scaffolding includes a central rod, an upper laterally-expandable element configured to expand laterally away from the central rod, a lower laterally-expandable element configured to expand laterally away from the central rod; and at least one flexible wire coupled to and extending between the upper and lower laterally-expandable elements, and when the scaffolding is expanded, the at least one flexible wire is configured to push against the tissue of the heart valve. 
     In an application, the upper and lower laterally-expandable elements are moveable longitudinally with respect to the central rod to control a tension of the at least one flexible wire. 
     In an application, when the scaffolding is expanded, the upper laterally-expandable element is configured to be disposed in an atrium of a heart of the subject and the lower laterally-expandable element is configured to be disposed in a ventricle of the heart of the subject. 
     In an application, the upper laterally-expandable element includes a first expandable and collapsible ring, the lower laterally-expandable element includes a second expandable and collapsible ring, the at least one wire includes at least two wires coupled at corresponding locations circumferentially along the first and second rings, and when the scaffolding is expanded, the first and second rings are in an expanded state. 
     In an application, the upper laterally-expandable element includes a first expandable and collapsible cross-beam that extends laterally away from the central rod, the lower laterally-expandable element includes a second expandable and collapsible cross-beam that extends laterally away from the central rod, the at least one wire includes at least two wires coupled at corresponding locations along the first and second cross-beams, and when the scaffolding is expanded, the first and second cross-beams are in an expanded state. 
     In an application, the scaffolding includes a central rod, a first loop element configured to expand laterally away from the central rod, a second loop element configured to expand laterally away from the central rod, at least one curved, flexible wire coupled to and extending from the rod at least within a space defined by the first and second loop elements; and a first magnet coupled to an end of the flexible wire, the first magnet being moveable by a second magnet that is not coupled to the scaffolding. When the scaffolding is expanded, the first and second loop elements are configured to push against the tissue of the heart valve. 
     In an application, the first and second loop elements are moveable longitudinally with respect to the central rod to control a tension of the first and second loop elements. 
     In an application, a delivery tool is configured to deliver the implant, the system includes the second magnet, and the delivery tool is coupled to the second magnet. 
     In an application, when the scaffolding is expanded, a first half of each of the first and second loop elements is configured to be disposed in an atrium of a heart of the subject and a second half of each of the first and second loop elements is configured to be disposed in a ventricle of the heart of the subject. 
     In an application, the first and second loop elements include radiopaque material. In an application, the first and second loop elements are coupled to radiopaque material. 
     In an application, the scaffolding includes a central rod, at least one curved, flexible wire coupled to and extending from the rod, and a first magnet coupled to an end of the flexible wire, the first magnetic element being moveable by a second magnetic element that is not coupled to the scaffolding. 
     In an application, when the scaffolding is expanded, (1) the at least one curved, flexible wire is configured to be disposed within a ventricle of a heart of the subject, and (2) the first magnetic element is configured to be disposed within a subannular space of the heart. 
     In an application, the at least one curved, flexible wire is moveable longitudinally with respect to the central rod. 
     In an application, the second magnetic element is configured to be positioned within vasculature surrounding the native heart valve. 
     In an application, the at least one curved, flexible wire is coupled to radiopaque material. In an application, the at least one curved, flexible wire includes radiopaque material. 
     In an application, the scaffolding includes a central rod, at least one cross-beam coupled to and extending laterally from the rod, and a first magnetic element coupled to an end of the least one cross-beam, the first magnetic element being moveable by a second magnetic element that is not coupled to the scaffolding. 
     In an application, when the scaffolding is expanded, the at least one least one cross-beam is configured to be disposed within an atrium of a heart of the subject. 
     In an application, the at least one least one cross-beam is moveable longitudinally with respect to the central rod. 
     In an application, the second magnetic element is configured to be positioned within vasculature surrounding the native heart valve. 
     In an application, the at least one least one cross-beam is coupled to radiopaque material. In an application, the at least one least one cross-beam includes radiopaque material. 
     There is further provided, in accordance with an application of the present invention, a system for use with a subject, the system including an implant configured for placement along a native heart valve annulus of a native heart valve of the subject and an annulus-marking device discrete from the implant and removable from within the subject following implantation of the implant. The annulus-marking device can be the same as or similar to any annulus-marking devices described herein. 
     In some applications, the annulus-marking device includes a plurality of radiopaque markers juxtaposing each other at a given distance from each other, the plurality of radiopaque markers each being deformable by tissue at different intervals indicating proximity of tissue to the implant. 
     In some applications, the implant includes a body portion including flexible material, the body portion having a longitudinal axis that runs along a length of the body portion. 
     In an application, the plurality of radiopaque markers are sized differently from each other. In an application, the plurality of radiopaque markers include concentric loops. In an application, the plurality of radiopaque markers include concentric petals or loops. In an application, the plurality of radiopaque markers include a plurality of radiopaque strips. 
     In an application, the plurality of radiopaque markers include wire. 
     In an application, each one of the plurality of radiopaque markers includes a radiopaque sail extending therefrom. 
     In an application, each one of the plurality of radiopaque markers includes a radiopaque filament extending therefrom. 
     There is further provided, in accordance with an application of the present invention, a system for use with a subject, the system including an implant configured for placement along a native heart valve annulus of a native heart valve of the subject and an annulus-marking device including an elongate radiopaque element and a plurality of flexible radiopaque filaments coupled to the elongate radiopaque element configured to mark the native heart valve annulus and tissue coupled thereto. 
     In some applications, the elongate radiopaque element is slidable along the body portion of the implant and along the longitudinal axis, and the plurality of radiopaque filaments. 
     In some applications, the implant includes a body portion including flexible material, the body portion having a longitudinal axis that runs along a length of the body portion. 
     In an application, the annulus-marking device is removable from the subject following implantation of the implant. 
     In an application, the body portion includes a plurality of radiopaque markings configured to indicate placement of anchors along the body portion. In an application, each one of the plurality of radiopaque filaments includes a material that is flexible. In an application, the elongate radiopaque element includes a wire. In an application, the elongate radiopaque element includes a rod. 
     In an application, the plurality of radiopaque filaments are disposed at a distal end of the elongate radiopaque element, the annulus-marking device includes a tube coupled to a proximal end of the elongate radiopaque element, and the tube surrounds the body portion and slides with respect to the body portion to move the plurality of radiopaque filaments with respect to the implant. 
     In an application, the plurality of radiopaque filaments are disposed at a distal end of the elongate radiopaque element, the body portion includes a plurality of eyelets, and the elongate radiopaque element is slidable with respect to the plurality of eyelets to move the plurality of radiopaque filaments with respect to the implant. 
     In an application, the plurality of radiopaque filaments are collapsible as they pass through each one of the plurality of eyelets. 
     There is further provided, in accordance with an application of the present invention, a method, including placing at a native heart valve annulus of a subject, at a first angle of delivery with respect to a planar surface of a leaflet of the valve, an annulus-marking device including a radiopaque material, implanting an implant along the native heart valve annulus of the subject using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. In some applications, the method further includes retrieving the annulus-marking device following the implanting. 
     The annulus-marking device can be the same as or similar to any of the annulus-marking devices described herein. 
     In some applications, the annulus-marking device comprises (1) a wire radiopaque extension, and (2) at least one radiopaque distal curved tip disposed at a nonzero angle with respect to the wire extension. 
     In some applications, the method includes, subsequently to the placing, enabling the annulus marking device to move incrementally along the leaflet, and by the moving, changing the angle of delivery of the annulus-marking device with respect to the planar surface of the leaflet of the valve. 
     The method can further include, by the changing the angle, determining a position of an annulus of the valve by visualizing the changing of the angle. 
     In an application, determining the position includes determining that the curved distal tip is disposed along the leaflet responsively to visualizing beating of the annulus-marking device. 
     In an application, determining the position includes determining that the curved distal tip is disposed at the annulus responsively to visualizing that the annulus-marking device does not move. 
     This method can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     There is further provided, in accordance with an application of the present invention, a system for use with a subject, the system including an implant configured for placement along a native heart valve annulus of a native heart valve of the subject and an annulus-marking device including an expandable radiopaque braided mesh that is expandable from a collapsed state to an expanded state. 
     In some applications, the expanded state comprises a frustoconical shape. In some applications, two or more pull wires are coupled to the expandable braided mesh, the two or more pull wires being configured to be pulled in order to transition the braided mesh from the frustoconical shape to a shape in which the mesh assumes (1) a sloped upper portion configured for positioning within an atrium of a heart of the subject, (2) a bulging ledge portion configured for positioning above the heart valve, (3) a narrow portion for positioning within the heart valve, and (4) a trumpet portion configured for expanding within a ventricle of the heart of the subject. 
     In some applications, the implant including a body portion including flexible material, the body portion having a longitudinal axis that runs along a length of the body portion. 
     In an application, the annulus-marking device is removable from the subject following implantation of the implant. 
     In an application, the body portion includes a plurality of radiopaque markings configured to indicate placement of anchors along the body portion. 
     In an application, the one two or more pull wires includes three pull wires. 
     In an application, the bulging portion has a greater diameter than the other portion of the annulus-marking device. In an application, the implant is slidable along the sloped upper portion toward the annulus. 
     There is further provided, in accordance with an application of the present invention, a system for use with a subject, the system including an implant configured for placement along a native heart valve annulus of a native heart valve of the subject and an annulus-marking device. The annulus-marking device can be the same as or similar to other annulus-marking devices described herein. 
     In some applications, the annulus-marking device comprises an expandable radiopaque braided mesh that is expandable from a collapsed state to an expanded state, in the expanded state, the mesh assumes (1) a sloped upper portion configured for positioning within an atrium of a heart of the subject, and (2) an asymmetrical portion for positioning within the heart valve. 
     In an application, the annulus-marking device is removable from the subject following implantation of the implant. 
     In some applications, the implant including a body portion including flexible material, the body portion having a longitudinal axis that runs along a length of the body portion. 
     In an application, the body portion includes a plurality of radiopaque markings configured to indicate placement of anchors along the body portion. 
     In an application, the implant is slidable along the sloped upper portion toward the annulus. 
     In an application, the system includes a stabilizing rod and a tissue anchor coupled to an end of the stabilizing rod and configured to be reversibly coupled to tissue of the heart of the subject, the annulus-marking device is slidably coupled to the stabilizing rod, and the stabilizing rod is configured to stabilize and guide positioning of the annulus-marking device. 
     In an application, in the expanded state, the mesh assumes a trumpet portion configured for expanding within a ventricle of the heart of the subject. 
     In an application, the trumpet portion has a greater diameter than the other portions of the annulus-marking device. 
     In an application, the system includes a plurality of expandable snares coupled to a distal end portion of the expandable radiopaque braided mesh, the plurality of expandable radiopaque snares being configured to ensnare one or more native leaflets of the native valve of the subject. 
     In an application, the plurality of expandable snares includes a rigid material. In an application, the plurality of expandable snares includes a flexible material. In an application, the plurality of expandable snares includes a radiopaque material. 
     In an application, the plurality of expandable snares extend distally from a distal end of the expandable radiopaque braided mesh and then curve proximally. 
     In an application, the system includes a plurality of expandable radiopaque elements which are coupled to a distal end portion of the expandable radiopaque braided mesh and configured to expand radially such that the plurality of expandable elements provides an indication as to a location of the native heart valve annulus of the native heart valve of the subject. 
     In an application, the plurality of radiopaque expandable elements collectively form the annulus-marking device into a generally spherical shape. 
     In an application, the plurality of expandable radiopaque elements include a plurality of woven radiopaque fibers assuming a mesh. In an application, the plurality of expandable radiopaque elements include a plurality of curved wires. 
     In an application, the system includes an inflatable annular element coupled to a distal end portion of the expandable radiopaque braided mesh, the inflatable annular element being configured to position the expandable radiopaque braided mesh within the native valve of the subject. 
     In an application, the inflatable annular element includes a radiopaque material. 
     In an application, the inflatable annular element includes a prosthetic valve. 
     In an application, the expandable radiopaque braided mesh is positionable within the native heart valve, and the inflatable annular element is positionable below the native heart valve. 
     There is further provided, in accordance with an application of the present invention, a system for use with a subject, the system including an implant configured for placement along a native heart valve annulus of a native heart valve of the subject and an annulus-marking device including a temporary valve. 
     In some applications, the temporary valve is an inflatable temporary valve that is inflatable from a collapsed state to an inflated state or expanded state. In some applications, in the expanded state, the inflatable temporary valve includes (1) a proximal non-compliant balloon configured for positioning within the native heart valve and partially within an atrium of a heart of the subject, and (2) a distal compliant balloon configured for positioning in a subannular space of the native heart valve. 
     In an application, two or more prosthetic leaflets are coupled to the temporary valve. 
     In some applications, the implant includes a body portion including flexible material, the body portion having a longitudinal axis that runs along a length of the body portion. 
     There is further provided, in accordance with an application of the present invention, a method, including delivering an annulus-marking device including at least a first magnetic element to one or more surfaces of a native heart valve of a heart of a subject, the one or more surfaces selected from the group consisting of: an atrial surface and a ventricular surface, delivering a second magnetic element to vasculature surrounding the heart valve, generating a magnetic field around the at least the first magnetic element; and implanting an implant along a native heart valve annulus of the subject using the annulus-marking device as a guide for implantation of the implant along the annulus under imaging. 
     In some applications, the method further includes retrieving the annulus-marking device following the implanting. 
     In an application, the at least the first magnetic element is coupled to a curved wire and delivering the annulus-marking device includes delivering the first magnetic element to the ventricular surface. 
     In an application, the at least the first magnetic element is coupled to an end of at least one cross-beam, and delivering the annulus-marking device includes delivering the first magnetic element to the atrial surface. 
     In an application, generating the magnetic field includes positioning the magnetic elements at a suitable position with respect to tissue of the valve. 
     In an application, implanting under imaging includes implanting using fluoroscopy. 
     In an application, delivering the annulus-marking device includes delivering the annulus-marking device to a mitral valve. In an application, delivering the annulus-marking device includes delivering the annulus-marking device to a tricuspid valve. 
     In an application, the method further includes viewing tissue of the native heart valve annulus and tissue coupled thereto using the at least the first magnetic element. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the first magnetic element against the tissue. 
     In an application, viewing the tissue of the native heart valve annulus and tissue coupled thereto includes imaging the annulus-marking device with respect to the tissue of the native heart valve annulus, tissue of at least one leaflet, and tissue of an atrial wall. 
     This and other methods herein can be performed on a living animal or on a simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     The present invention will be more fully understood from the following detailed description of embodiments thereof, taken together with the drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1A-C  are schematic illustrations of examples of respective annulus-marking devices for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 2A-F  are schematic illustrations of a method for implanting the annulus-marking devices of  FIGS. 1A-C , in accordance with some applications; 
         FIGS. 3A-C  are schematic illustrations of examples of respective annulus-marking devices, in accordance with some applications; 
         FIGS. 4A-B  are schematic illustrations of an implant comprising an annulus-marking device, in accordance with some applications; 
         FIGS. 5A-B  are schematic illustrations of an implant comprising an annulus-marking device, in accordance with some applications; 
         FIGS. 6A-B  are schematic illustrations of respective tissue anchors comprising an annulus-marking device, in accordance with some applications; 
         FIGS. 7A-C  are schematic illustrations of an implant comprising an annulus-marking device, in accordance with some applications; 
         FIGS. 8A-B  are schematic illustrations of an annulus-marking device for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 9A-B  are schematic illustrations of a navigational-based guidance system, which employs one or more longitudinal guides configured to facilitate guidance of an implant to specific portions of the mitral valve by the guides contacting a surface of the mitral valve, in accordance with some applications; 
         FIGS. 10A-B  are schematic illustrations of an annulus-marking device for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 11A-C  are schematic illustrations of respective annulus-marking devices for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 12A-B  are schematic illustrations of a system for facilitating imaging of cardiac tissue during implantation of a cardiac implant, the system comprising a mapping catheter, in accordance with some applications; 
         FIG. 13  is a schematic illustration of an annulus-marking device comprising a generally spherical expandable element for facilitating imaging of cardiac tissue during implantation of a cardiac implant, in accordance with some applications; 
         FIG. 14  is a schematic illustration of an annulus-marking device comprising a generally spherical expandable mesh for facilitating imaging of cardiac tissue during implantation of a cardiac implant, in accordance with some; 
         FIG. 15  is a schematic illustration of a system comprising an annulus-marking device comprising a guidewire that runs alongside an implant aiding implantation of the implant under the guidance of imaging, in accordance with some applications; 
         FIGS. 16A-C  are schematic illustrations of respective annulus-marking devices for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 17A-C  are schematic illustrations of respective annulus-marking devices comprising expandable elements for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIG. 18  is a schematic illustration of an annulus-marking device for use in a ventricle for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 19A-C  are schematic illustrations of respective annulus-marking devices comprising expandable elements for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 20A-B  are schematic illustrations of an annulus-marking device comprising a toroidal stent for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 21-27  are schematic illustrations of respective annulus marking devices comprising implant-leading devices for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 28A-B  are schematic illustrations of an annulus marking device comprising a plurality of implantable radiopaque pins for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIG. 29  is a schematic illustration of an annulus-marking device comprising a plurality of radiopaque pins which move proximally and distally for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 30A-B  and  31  are schematic illustrations of respective annulus marking devices each comprising a plurality of radiopaque filaments deliverable through a multilumen tube and aid implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIG. 32  is a schematic illustration of an annulus-marking devices comprising expandable elements for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 33A-B  are schematic illustrations of annulus-marking devices comprising inflatable elements for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 34A-C  are schematic illustrations of annulus-marking devices comprising concentric wire loops for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIG. 35  is a schematic illustration of annulus-marking devices comprising a plurality of petals for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIG. 36  is a schematic illustration of an annulus-marking device comprising a plurality of radiopaque filaments coupled to a distal end portion of a guidewire, in accordance with some applications; 
         FIGS. 37A-G  are schematic illustrations of an annulus-marking device comprising first and second radiopaque loops for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIG. 38  is a schematic illustration of an annulus-marking device comprising two or more expandable wires and an ultrasound transducer for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIG. 39  is a schematic illustration of an annulus-marking device comprising a clip for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIG. 40  is a schematic illustration of an annulus-marking device comprising a clamp for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIG. 41  is a schematic illustration of an annulus-marking device comprises a balloon having upper and lower inflatable sections for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIG. 42  is a schematic illustration of an annulus-marking device comprises a balloon for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIG. 43  is a schematic illustration of an annulus-marking device comprises a balloon having upper and lower inflatable sections for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some other applications; 
         FIGS. 44-46  are schematic illustrations of an annulus-marking device comprises magnetic elements for aiding implantation of cardiac devices under the guidance of imaging, in accordance with respective applications; 
         FIG. 47  is a schematic illustration of an annulus-marking device comprising a spring for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIG. 48  is a schematic illustration of an annulus-marking device comprising an expandable element for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 49A-B  are schematic illustrations of an annulus-marking device comprising a scaffolding for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 50A-B  are schematic illustrations of an annulus-marking device comprising a scaffolding comprising a rod for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 51A-C  are schematic illustrations of an annulus-marking device comprising a scaffolding comprising an expandable basket for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 52A-B  are schematic illustrations of an annulus-marking device comprising a plurality of radiopaque markers for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 53A-B  and  54 A-B are schematic illustrations of an annulus-marking device coupled to an implant, for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 55A-C  are schematic illustrations of an annulus-marking device comprising a radiopaque guide for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 56A-C  are schematic illustrations of an annulus-marking device comprising a radiopaque expandable mesh for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 57A-B ,  58 A-B,  59 A-B,  60 A-B, and  61 A-B are schematic illustrations of respective annulus-marking device comprising a radiopaque asymmetrical mesh for aiding implantation of cardiac devices under the guidance of imaging, in accordance with respective applications; 
         FIGS. 62A-B  are schematic illustrations of an annulus-marking device comprising an inflatable prosthetic valve for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; 
         FIGS. 63A-B  are schematic illustrations of an annulus-marking device comprising a scaffolding and a magnet for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications; and 
         FIGS. 64-65  are schematic illustrations of an annulus-marking device comprises magnetic elements for aiding implantation of cardiac devices under the guidance of imaging, in accordance with respective applications. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Reference is now made to  FIGS. 1A-C , which are schematic illustrations of respective annulus-marking devices for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications of the present invention. The steering procedure can be performed with the aid of imaging, such as fluoroscopy, transesophageal echo, and/or echocardiography. 
       FIG. 1A  shows a system  20  comprising an annulus-marking device  22  comprising a radiopaque material shaped so as to define a base frame  24  having a shape such that it tracks a circumference of a native heart valve annulus and approximates the shape of the annulus. That is, frame  24  has a circular shape that tracks the circumference of the native heart valve. Device  22  comprises one or more struts  26  (e.g., three as shown by way of illustration and not limitation). Struts  26  project away from a plane define by base frame  24  and are shaped so as to be placed in the commissures of the native valve. Struts  26  thereby provide an indicator of the location, height, and orientation of the commissures under imaging. Struts  26  are desirably sized and configured to contact tissue near or within the heart valve annulus to brace base frame  24  against migration within the annulus. Struts  26  are spaced apart to rest in engagement with tissue at or near the leaflet commissures (or wherever tissue contact with the struts  26  is intended to occur). For some applications, frame  24  and struts  26  are fabricated from a single piece. Optionally, frame  24  and struts  26  can be fabricated as separate pieces and coupled together by welding, clamping, etc., for example. Struts  26  can provide an indication as to the height of the annulus of the valve, so that when placing device  22 , a height of the annulus can be measured, for example by imaging the struts when in contact with the annulus. 
     Device  22  can be delivered percutaneously, thoracoscopically through the chest, or using open heart surgical techniques. To help with percutaneous delivery and/or for other reasons, the device  22  can be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter and subsequently self-expand into the desired shape and tension when released from the catheter. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein under image guidance (e.g., fluoroscopic, ultrasonic, magnetic resonance, computed tomography, or combinations thereof). For some applications, device  22  comprises a wire. 
     Device  22  is configured for placement along a native tricuspid valve. As such, frame  24  of device  22  is generally circular. For some applications, device  22  comprises an adjustment mechanism  28  which expands and contracts a perimeter of base frame  24 . For some applications, base frame  24  is hollow and is shaped so as to define a lumen, and adjustment mechanism  28  comprises a wire that runs at least partially within the lumen of base frame  24 . In such applications, the wire is pullable and/or twistable to adjust the perimeter of base frame  24 . For some applications, a portion of base frame  24  is collapsible telescopically in response to pulling of the wire of adjustment mechanism  28 . 
     Device  22  is compressible during delivery toward the native heart valve. During delivery of device  22 , device  22  is constrained in a collapsed condition. A flexible push rod can be used to expel the device  22  from a delivery catheter. Free of the catheter, device  22  will self-expand from its compressed state to its preordained configuration, e.g., like that shown in  FIG. 1A . 
       FIG. 1B  shows a system  30  comprising an annulus-marking device  32  comprising a radiopaque material shaped so as to define a base frame  34  having a shape such that it tracks a circumference of a native heart valve annulus and approximates the shape of the annulus. That is, frame  34  has a “D”-shape that tracks the circumference of the native heart valve. Device  32  comprises one or more struts  36  (e.g., three as shown by way of illustration and not limitation). Struts  36  project away from a plane define by base frame  34  and are shaped so as to be placed in the commissures of the native valve. Struts  36  thereby provide an indicator of the location, height, and orientation of the commissures under imaging. Struts  36  are desirably sized and configured to contact tissue near or within the heart valve annulus to brace base frame  34  against migration within the annulus. Struts  36  are spaced apart to rest in engagement with tissue at or near the leaflet commissures (or wherever tissue contact with the struts  36  is intended to occur). For some applications, frame  34  and struts  36  are fabricated from a single piece, or as separate pieces coupled to each other, as mentioned above with respect to device  22 . Also, as mentioned above, struts  36  can provide an indication as to the height of the annulus of the valve, so that when placing device  32 , a height of the annulus can be measured, for example by imaging the struts when in contact with the annulus. 
     Device  32  can be delivered percutaneously, thoracoscopically through the chest, or using open heart surgical techniques. If delivered percutaneously, the device  32  can be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter and subsequently self-expand into the desired shape and tension when released from the catheter. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein under image guidance (e.g., fluoroscopic, ultrasonic, magnetic resonance, computed tomography, or combinations thereof). For some applications, device  32  comprises a wire. 
     Device  32  is configured for placement along a native mitral valve. As such, frame  34  of device  32  is substantially D-shaped and struts  36  are oppositely spaced so as to fit within the commissures of the native mitral valve. For some applications, frame  34  of device  32  is substantially saddle-shaped, such that frame  43  looks like the undulated outer circumference line of a saddle. For some applications, device  32  comprises an adjustment mechanism  38  which expands and contracts a perimeter of base frame  34 . For some applications, base frame  34  is hollow and is shaped so as to define a lumen, and adjustment mechanism  38  comprises a wire that runs at least partially within the lumen of base frame  34 . In such applications, the wire is pullable and/or twistable to adjust the perimeter of base frame  34 . For some applications, a portion of base frame  34  is collapsible telescopically in response to pulling of the wire of adjustment mechanism  38 . 
     Device  32  is compressible during delivery toward the native heart valve. During delivery of device  32 , device  32  is constrained in a collapsed condition. A flexible push rod can be used to expel the device  32  from a delivery catheter. Free of the catheter, device  32  will self-expand from its compressed state to its preordained configuration, e.g., like that shown in  FIG. 1B . 
       FIG. 1C  shows a system  40  comprising an annulus-marking device  42  comprising a radiopaque material shaped so as to define a base frame  44  having a shape such that it tracks a circumference of a native heart valve annulus and approximates the shape of the annulus. That is, frame  44  has a circular shape that tracks the circumference of the native heart valve. Device  42  comprises one or more struts  46  (e.g., three as shown by way of illustration and not limitation). Struts  46  project away from a plane define by base frame  44  and are shaped so as to be placed in the commissures of the native valve. Struts  46  thereby provide an indicator of the location, height, and orientation of the commissures under imaging. Struts  46  are desirably sized and configured to contact tissue near or within the heart valve annulus to brace base frame  44  against migration within the annulus. Struts  46  are spaced apart to rest in engagement with tissue at or near the leaflet commissures (or wherever tissue contact with the struts  46  is intended to occur). For some applications, frame  44  and struts  46  are fabricated from a single piece, or as separate pieces coupled to each other, as mentioned above with respect to device  22 . Also, struts  46  can provide an indication as to the height of the annulus of the valve, so that when placing device  42 , a height of the annulus can be measured, for example by imaging the struts when in contact with the annulus. 
     Device  42  can be delivered percutaneously, thoracoscopically through the chest, or using open heart surgical techniques. The device  42  can be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter and subsequently self-expand into the desired shape and tension when released from the catheter. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein under image guidance (e.g., fluoroscopic, ultrasonic, magnetic resonance, computed tomography, or combinations thereof). For some applications, device  42  comprises a wire. 
     Device  42  is configured for placement along a native mitral valve. As shown, frame  44  of device  42  is generally circular and struts  46  are oppositely spaced so as to fit within the commissures of the native mitral valve. For some applications, frame  44  of device  42  is substantially saddle-shaped. For some applications, device  42  comprises an adjustment mechanism  48  which expands and contracts a perimeter of base frame  44 . For some applications, base frame  44  is hollow and is shaped so as to define a lumen, and adjustment mechanism  48  comprises a wire that runs at least partially within the lumen of base frame  44 . In such applications, the wire is pullable and/or twistable to adjust the perimeter of base frame  44 . For some applications, a portion of base frame  44  is collapsible telescopically in response to pulling of the wire of adjustment mechanism  48 . 
     Device  42  is compressible during delivery toward the native heart valve. During delivery of device  42 , device  42  is constrained in a collapsed condition. A flexible push rod can be used to expel the device  42  from a delivery catheter. Free of the catheter, device  42  will self-expand from its compressed state to its preordained configuration, e.g., like that shown in  FIG. 1C . 
     Reference is now made to  FIGS. 1A-C . Devices  22 ,  32 , and  42  are made, for example by machining, bending, shaping, joining, molding, or extrusion, from a biocompatible metallic or polymer material, or a metallic or polymer material that is suitably coated, impregnated, or otherwise treated with a material to impart biocompatibility, or a combination of such materials. The material is also desirably radiopaque to facilitate fluoroscopic visualization. 
     Reference is now made to  FIGS. 2A-F , which are schematic illustrations of a method and a system  60  for implanting the annulus-marking devices of  FIGS. 1A-C , in accordance with some applications. 
     In  FIG. 2A , device  22  is positioned along an annulus  66  of a native tricuspid valve  62  and device  32  is positioned along an annulus  68  of a native mitral valve  64 . It is to be noted that device  42  can be implanted along mitral valve  64 . As described hereinabove, devices  22  and  32  can be delivered in a constrained configuration into the atrium and then expanded within the atrium. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein. 
     In  FIG. 2B , devices  22  and  32  are adjusted by respective adjusting mechanisms  28  and  38 . In some applications, an adjustment tool  70  engages with a portion of adjusting mechanisms  28  and  38  and pulls and or twists the portion of the adjusting mechanisms  28  and  38  (e.g., a wire of mechanism  28  and  38 ). Frames  24  and  34  are adjusted by tool  70  so as to achieve the desired positioning of devices  22  and  32  respectively along the annulus. 
     In  FIG. 2C , an implant, e.g., an annuloplasty structure  72 , is positioned along annulus  66  of tricuspid valve  62  using a delivery tool  74  which passes structure  72  into the right atrium via the superior vena cava or the inferior vena cava. Structure  72  can comprise a flexible body portion. For some applications, the body portion of structure  72  is shaped so as to define a tubular sleeve through which a plurality of anchors is implanted. The body portion of structure  72  comprises a plurality of radiopaque markers  75 , which are positioned along structure  72  at respective longitudinal sites. The markers provide an indication in a radiographic image (such as a fluoroscopy image) of how much of the body portion has been deployed at any given point during an implantation procedure, in order to enable setting a desired distance between the tissue anchors along the body portion. For some applications, the markers comprise a radiopaque ink. For some applications the markers comprise a radiopaque material or additional radiopaque material, markers, etc. attached to or incorporated in structure  72 . 
     Structure  72  is delivered within a delivery tool  74 . Delivery tool  74  is guided and steered in accordance with imaging guided by annulus-marking device  22 . That is, structure  72  is positioned along annulus  66  and anchored thereto under imaging using annulus-marking device  22  to mark tissue of the annulus and the commissures. 
     In some applications, at least a portion (e.g., at least three, some, all, etc.) of the longitudinal sites of radiopaque markers  75  are longitudinally spaced at a constant interval. In some applications, the longitudinal distance between the distal edges of adjacent markers, and/or the distance between the proximal edges of adjacent markers, is set equal to the desired distance between adjacent anchors. For example, the markers can comprise first, second, and third markers, which first and second markers are adjacent, and which second and third markers are adjacent, and the distance between the proximal and/or distal edges of the first and second markers equal the corresponding distance between the proximal and/or distal edges of the second and third markers. For example, the distance can be between 3 and 15 mm, such as 6 mm, and the longitudinal length of each marker can be between 0.1 and 14 mm, such as 2 mm. (If, for example, the distance were 6 mm and the length were 2 mm, the longitudinal gaps between adjacent markers would have lengths of 4 mm.) 
     Annuloplasty structure  72  is used to repair a dilated valve annulus of tricuspid valve  62 . For some applications, the annuloplasty structure is configured to be placed only partially around the valve annulus (e.g., to assume a C-shape), and, once anchored in place, to be contracted so as to circumferentially tighten the valve annulus. 
     For some applications, structure  72  further comprises an adjusting mechanism, which facilitates contracting and expanding of annuloplasty structure  72  so as to facilitate adjusting of a perimeter of the annulus and leaflets of the cardiac valve. For some applications, the adjusting mechanism comprises a contracting member such as a wire, line, suture, elongate member, etc. extending along the annuloplasty structure  72  and a rotatable structure (e.g., a spool, wheel, spindle, etc.) configured to apply a contracting force to the contracting member so as to longitudinally contract annuloplasty structure  72 . 
     In  FIG. 2D , an implant, e.g., an annuloplasty structure  76 , is positioned along annulus  68  of mitral valve  64  using a delivery tool  74  which passes structure  76  into the left atrium via the superior vena cava or the inferior vena cava and subsequently through the fossa ovalis. In some applications, structure  76  comprises a flexible body portion. For some applications, the body portion of structure  76  is shaped so as to define a tubular sleeve through which a plurality of anchors is implanted. The body portion of structure  76  comprises a plurality of radiopaque markers  75 , which are positioned along structure  76  at respective longitudinal sites. The markers provide an indication in a radiographic image (such as a fluoroscopy image) of how much of the body portion has been deployed at any given point during an implantation procedure, in order to enable setting a desired distance between the tissue anchors along the body portion. For some applications, the markers comprise a radiopaque ink. For some applications the markers comprise a radiopaque material or additional radiopaque material, markers, etc. attached to or incorporated in structure  76 . 
     Structure  76  is delivered within a delivery tool  78 . Delivery tool  78  is guided and steered in accordance with imaging guided by annulus-marking device  32 . That is, structure  76  is positioned along annulus  68  and anchored thereto under imaging using annulus-marking device  32  to mark tissue of the annulus and the commissures. 
     For some applications, at least a portion (e.g., at least three, such as all) of the longitudinal sites of radiopaque markers  75  are longitudinally spaced at a constant interval. For some applications, the longitudinal distance between the distal edges of adjacent markers, and/or the distance between the proximal edges of adjacent markers, is set equal to the desired distance between adjacent anchors. For example, the markers can comprise first, second, and third markers, which first and second markers are adjacent, and which second and third markers are adjacent, and the distance between the proximal and/or distal edges of the first and second markers equal the corresponding distance between the proximal and/or distal edges of the second and third markers. For example, the distance may be between 3 and 15 mm, such as 6 mm, and the longitudinal length of each marker may be between 0.1 and 14 mm, such as 2 mm. (If, for example, the distance were 6 mm and the length were 2 mm, the longitudinal gaps between adjacent markers would have lengths of 4 mm.) 
     Annuloplasty structure  76  is used to repair a dilated valve annulus of mitral valve  64 . For some applications, the annuloplasty structure is configured to be placed only partially around the valve annulus (e.g., to assume a C-shape), and, once anchored in place, to be contracted so as to circumferentially tighten the valve annulus. For some application, annuloplasty structure  76  is implemented using techniques described in U.S. application Ser. No. 12/437,103, filed May 7, 2009 which published as US 2010/0286767, and/or U.S. application Ser. No. 12/689,635, filed Jan. 19, 2010 which published as US 2010/0280604, both of which are assigned to the assignee of the present application and are incorporated herein by reference. 
     For some applications, structure  76  further comprises an adjusting mechanism, which facilitates contracting and expanding of annuloplasty structure  76  so as to facilitate adjusting of a perimeter of the annulus and leaflets of the cardiac valve. For some applications, the adjusting mechanism comprises a contracting member such as a wire, line, suture, elongate member, etc. extending along the annuloplasty structure  76  and a rotatable structure (e.g., a spool, wheel, spindle, etc.) configured to apply a contracting force to the contracting member so as to longitudinally contract annuloplasty structure  76 . 
       FIG. 2E  shows retrieval of annulus-marking device  22  following implantation of annuloplasty structure  72  at annulus  66  of tricuspid valve  62 . Since device  22  is flexible and compressible, device  22  is constrained by pulling device  22  within an extraction tool  80  during the retrieval of device  22  and subsequent removal of device  22  from the body of the subject. That is, device  22  does not function as an implant for such embodiments and is used only to guide implantation of annuloplasty structure  72  (i.e., the implant); rather, device  22  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of annuloplasty structure  72 . 
       FIG. 2F  shows retrieval of annulus-marking device  32  following implantation of annuloplasty structure  76  at annulus  68  of mitral valve  64 . Since device  32  is flexible and compressible, device  32  is constrained by pulling device  32  within an extraction tool  80  during the retrieval of device  32  and subsequent removal of device  32  from the body of the subject. That is, device  32  does not function as an implant for such embodiments and is used only to guide implantation of annuloplasty structure  76  (i.e., the implant); rather, device  32  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of annuloplasty structure  76 . 
     Reference is now made to  FIGS. 1A-C  and  2 A-F. It is to be noted that annulus-marking devices  22 ,  32 , and  42  can be used as a rail for mechanically guiding implantation of the annuloplasty structures described herein. When implanting annuloplasty structure  76  along the annulus, structure  76  can be pushed against the annulus while an anchor is implanted into the annulus, so that the frame of annulus-marking devices  22 ,  32 , and  42  which is less compliant than the annulus tissue provides tactile feedback to an operating physician, and moreover, can also act as a rail at which structure  76  is deflected to the more compliant annulus tissue. That is, the delivery tool which deliver the annuloplasty structure uses the frame of devices  22 ,  32 , and  42  as a tactile and mechanical guide in addition to being a visual guide for moving the delivery tool along the annulus. 
     Reference is now made to  FIGS. 3A-C , which are schematic illustrations of respective annulus-marking devices  92  and  102  which are similar to devices  22  and  32  of  FIGS. 1A-B , respectively, with the exception that devices  92  and  102  each comprise a plurality of radiopaque elements  99  (e.g., radiopaque markers, filaments, wires, extensions, beads, etc.), which are described for example, but are not limited to, radiopaque filaments  99  herein. The plurality of radiopaque filaments  99  function as additional annulus-marking devices. Annulus-marking devices  92  and  102  are configured for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications. The steering procedure is often performed with the aid of imaging, such as fluoroscopy, transesophageal echo, and/or echocardiography. 
       FIG. 3A  shows a system  90  comprising an annulus-marking device  92  comprising a radiopaque material shaped so as to define a base frame  94  having a shape such that it tracks a circumference of a native heart valve annulus and approximates the shape of the annulus. Device  92  comprises one or more struts  96  (e.g., three as shown by way of illustration and not limitation). Struts  96  project away from a plane define by base frame  94  and are shaped so as to be placed in the commissures of the native valve. Struts  96  thereby provide an indicator of the location, height, and orientation of the commissures under imaging. Struts  96  are desirably sized and configured to contact tissue near or within the heart valve annulus to brace base frame  94  against migration within the annulus. Struts  96  are spaced apart to rest in engagement with tissue at or near the leaflet commissures (or wherever tissue contact with the struts  96  is intended to occur). For some applications, frame  94  and struts  96  are fabricated from a single piece, or as separate pieces coupled to each other, as mentioned above with respect to device  22 . Also, struts  96  can provide an indication as to the height of the annulus of the valve, so that when placing device  92 , a height of the annulus can be measured, for example by imaging the struts when in contact with the annulus. 
     Device  92  can be delivered percutaneously, thoracoscopically through the chest, and/or using open heart surgical techniques. Device  92  can be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter and subsequently self-expand into the desired shape and tension when released from the catheter. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein under image guidance (e.g., fluoroscopic, ultrasonic, magnetic resonance, computed tomography, or combinations thereof). For some applications, device  92  comprises a wire. 
     Device  92  is configured for placement along a native tricuspid valve. As such, frame  94  of device  92  is generally circular. For some applications, device  92  comprises an adjustment mechanism  98  which expands and contracts a perimeter of base frame  94 . For some applications, base frame  94  is hollow and is shaped so as to define a lumen, and adjustment mechanism  98  comprises a wire that runs at least partially within the lumen of base frame  94 . In such applications, the wire is pullable and/or twistable to adjust the perimeter of base frame  94 . For some applications, a portion of base frame  94  is collapsible telescopically in response to pulling of the wire of adjustment mechanism  98 . 
     Device  92  is compressible during delivery toward the native heart valve. During delivery of device  92 , device  92  is constrained in a collapsed condition. A flexible push rod can be used to expel the device  92  from a delivery catheter. Free of the catheter, device  92  will self-expand from its compressed state to its preordained configuration, e.g., like that shown in  FIG. 3A . 
       FIG. 3B  shows a system  100  comprising an annulus-marking device  102  comprising a radiopaque material shaped so as to define a base frame  104  having a shape such that it tracks a circumference of a native heart valve annulus and approximates the shape of the annulus. Device  102  comprises one or more struts  106  (e.g., three as shown by way of illustration and not limitation). Struts  106  project away from a plane define by base frame  104  and are shaped so as to be placed in the commissures of the native valve. Struts  106  thereby provide an indicator of the location, height, and orientation of the commissures under imaging. Struts  106  are desirably sized and configured to contact tissue near or within the heart valve annulus to brace base frame  104  against migration within the annulus. Struts  106  are spaced apart to rest in engagement with tissue at or near the leaflet commissures (or wherever tissue contact with the struts  106  is intended to occur). For some applications, frame  104  and struts  106  are fabricated from a single piece, or as separate pieces coupled to each other, as mentioned above with respect to device  22 . Also, struts  106  can provide an indication as to the height of the annulus of the valve, so that when placing device  102 , a height of the annulus can be measured, for example by imaging the struts when in contact with the annulus. 
     Device  102  can be delivered percutaneously, thoracoscopically through the chest, and/or using open heart surgical techniques. Device  102  can be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter and subsequently self-expand into the desired shape and tension when released from the catheter. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein under image guidance (e.g., fluoroscopic, ultrasonic, magnetic resonance, computed tomography, or combinations thereof). For some applications, device  102  comprises a wire. 
     Device  102  is configured for placement along a native mitral valve. As such, frame  104  of device  102  is substantially D-shaped and struts  106  are oppositely spaced so as to fit within the commissures of the native mitral valve. For some applications, frame  104  of device  102  is substantially saddle-shaped. For some applications, device  102  comprises an adjustment mechanism  108  which expands and contracts a perimeter of base frame  104 . For some applications, base frame  104  is hollow and is shaped so as to define a lumen, and adjustment mechanism  108  comprises a wire that runs at least partially within the lumen of base frame  104 . In such applications, the wire is pullable and/or twistable to adjust the perimeter of base frame  104 . For some applications, a portion of base frame  104  is collapsible telescopically in response to pulling of the wire of adjustment mechanism  108 . 
     Device  102  is compressible during delivery toward the native heart valve. During delivery of device  102 , device  102  is constrained in a collapsed condition. A flexible push rod can be used to expel the device  102  from a delivery catheter. Free of the catheter, device  102  will self-expand from its compressed state to its preordained configuration, e.g., like that shown in  FIG. 3B . 
     Frame  104  of device  102  can be circular or another shape. 
     Reference is now made to  FIGS. 3A-C , the plurality of radiopaque filaments  99  comprise radiopaque material (e.g., nitinol or stainless steel) and can be configured to be extremely flexible. Filaments  99  project radially away from base frames  94  and  104 . 
     For some applications, filaments  99  sway with movement of the blood. For some applications, filaments  99  press against tissue of the annulus and tissue coupled thereto (as shown in  FIG. 3C ), such as tissue of an atrial wall  122  as well as tissue of the leaflets of the native valve. Within the entire present disclosure, the term “press against” has the same or a similar meaning as the terms “push against”, “place against” or “align against”, irrespective of a strength of the exerted force. Filaments  99  thus provide enhanced imaging of tissue of valve  64 . That is, when filaments  99  appear bent or pressed, this imaging detects annulus tissue, while when filaments  99  are straight, this could indicate the orifice of the valve.  FIG. 3C  shows a system,  120  in which device  102  is positioned in the native mitral valve  64 . Frame  104  is positioned along the annulus  68  while struts  106  are each placed at commissures  65  and  67 . As mentioned above, struts  106  provide an indication as to the height of the annulus of valve  64 . During positioning of device  102 , the native valve  64  functions normally. 
     Reference is now made to  FIGS. 4A-B , which are schematic illustrations of a system  140  comprising an implant comprising an annuloplasty structure  142  which comprises a body portion  144  and an annulus-marking device which comprises a plurality of radiopaque projections distributed along and attached to body portion  144  and shaped so as to define a plurality of tubular elements  148 , in accordance with some applications. Body portion  144  comprises a flexible material, e.g., a braided fabric mesh. For some applications, body portion  144  is shaped so as to define a sleeve shaped so as to define a lumen therethrough, as shown. For some applications, body portion  144  is flat. Body portion  144  extends along a central longitudinal axis  141  of structure  142 , and the radiopaque projections comprising tubular elements  148  project away from longitudinal axis  141 . 
     Body portion  144  can comprise a braided fabric mesh, e.g., comprising DACRON™. Body portion  144  can be configured to be placed only partially around a cardiac valve annulus (e.g., to assume a C-shape), and, once anchored in place, to be contracted so as to circumferentially tighten the valve annulus. Optionally, structure  142  can be configured to be placed entirely around the valve annulus (e.g., as a closed circle or other closed shape). In order to tighten the annulus, annuloplasty structure  142  comprises a flexible elongated contracting member  145  that extends along body portion  144 . Elongated contracting member  145  comprises a wire, a ribbon, a rope, or a band, which often comprises a flexible and/or superelastic material, e.g., nitinol, polyester, stainless steel, or cobalt chrome. For some applications, contracting member  145  comprises a radiopaque material. For some applications, contracting member  145  comprises a braided polyester suture (e.g., Ticron). For some applications, contracting member  145  is coated with polytetrafluoroethylene (PTFE). For some applications, contracting member  145  comprises a plurality of wires that are intertwined to form a rope structure. 
     The plurality of radiopaque projections comprising tubular elements  148  that comprise a flexible fabric. In some applications, tubular elements  148  and body portion  144  comprise the same material. Tubular elements  148  can be tapered away from axis  141 , as shown. As shown, the distal ends of each element  148  (i.e., the ends of the elements  148  furthest from body portion  144 ) are closed, such that elements  148  are shaped as a pocket. For some applications, elements  148  are each shaped so as to define a windsock. 
     Body portion  144  of structure  142  comprises a plurality of radiopaque markers  146 , which are positioned along structure  142  at respective longitudinal sites. The markers can provide an indication in a radiographic image (such as a fluoroscopy image) of how much of the body portion has been deployed at any given point during an implantation procedure, in order to enable setting a desired distance between tissue anchors  147  along body portion  144 , and thus to indicate placement of anchors  147 . For some applications, the markers comprise a radiopaque ink. For some applications the markers comprise a radiopaque material attached to or incorporated in body portion  144 . 
     As shown, contracting member  145  is coupled to body portion  144  and extends along body portion  144  and through the plurality of tubular elements  148  in a manner in which during application of tension to contracting member  145 , contracting member  145  is configured to change a structural configuration of the plurality of radiopaque projections comprising tubular elements  148 . As shown in  FIG. 4B , contracting member  145  is configured to change the structure configuration of elements  148  by closing an opening  149  (opening  149  is shown in  FIG. 4A ) of each one of tubular elements  148 . In such embodiments, as shown, contracting member  145  extends along the perimeter of each opening  149  of each tubular element  148 . For some applications, contracting member  145  runs along a perimeter of each tubular element  148  such that during application of tension to contracting member  145 , member  145  is configured to compress each tubular element  148  radially toward axis  141  of structure  142 . For some applications, contracting member is configured to change a spatial configuration of tubular elements  148  in sequence. 
     For some applications, contracting member  145  is configured to additionally apply a contracting force to body portion  144  of structure  142  so as to facilitate adjustment of the perimeter of annuloplasty structure  142 . Adjustment of annuloplasty structure  142  can be performed by an adjusting mechanism similar to that described above with respect to annuloplasty structure  72 , for example. For some applications, system  140  comprises an additional contracting member (not shown) configured to adjust a perimeter of body portion  144  while contracting member  145  adjusts the spatial configuration of the plurality of radiopaque projections comprising tubular elements  148 . 
     For some applications, the plurality of radial projections comprising tubular elements  148  are each fully radiopaque. For some applications at least 50% of each projection is radiopaque. The plurality of radial projections comprising tubular elements  148  help facilitate viewing of the tissue of the native heart valve annulus and tissue coupled thereto under imaging. For some applications, the plurality of radial projections comprising tubular elements  148  are placed against and abut the tissue of the annulus and/or tissue coupled thereto (e.g., atrial wall tissue and/or tissue of the leaflets of the native valve). For some applications, at least some of the plurality of radial projections comprising tubular elements  148  are positioned in the path of blood flow. The plurality of radial projections can provide information relating to tissue and/or blood flow responsively to movement of tubular elements  148 . 
     Reference is now made to  FIGS. 5A-B , which are schematic illustrations of a system  160  comprising an implant comprising an annuloplasty structure  162  which comprises a body portion  164  and an annulus-marking device which comprises a plurality of radiopaque projections distributed along and attached to body portion  164  and shaped so as to define a plurality of flat and planar elements  168 , in accordance with some applications. Body portion  164  comprises a flexible material, e.g., a braided fabric mesh. For some applications, body portion  164  is shaped so as to define a sleeve shaped so as to define a lumen therethrough, as shown. For some applications, body portion  164  is flat. Body portion  164  extends along a central longitudinal axis  161  of structure  162 , and the radiopaque projections comprising flat and planar elements  168  project away from longitudinal axis  161 . 
     Body portion  164  can comprise a braided fabric mesh, e.g., comprising DACRON™. Body portion  164  can be configured to be placed only partially around a cardiac valve annulus (e.g., to assume a C-shape), and, once anchored in place, to be contracted so as to circumferentially tighten the valve annulus. Optionally, structure  162  can be configured to be placed entirely around the valve annulus (e.g., as a closed circle or other closed shape). In order to tighten the annulus, annuloplasty structure  162  comprises a flexible elongated contracting member  165  that extends along body portion  164 . Elongated contracting member  165  comprises a wire, a ribbon, a rope, or a band, which can comprise a flexible and/or superelastic material, e.g., nitinol, polyester, stainless steel, or cobalt chrome. For some applications, the contracting member  165  comprises a radiopaque material. For some applications, contracting member  165  comprises a braided polyester suture (e.g., Ticron). For some applications, contracting member  165  is coated with polytetrafluoroethylene (PTFE). For some applications, contracting member  165  comprises a plurality of wires that are intertwined to form a rope structure. 
     The plurality of radiopaque projections comprising flat and planar elements  168  that comprise a flexible fabric. For some applications, flat and planar elements  168  and body portion  164  comprise the same material. Flat and planar elements  168  each have a longest dimension that is measured along an axis that is at a nonzero angle (i.e. not parallel) with respect to longitudinal axis  161  of body portion  164 . 
     Body portion  164  of structure  162  comprises a plurality of radiopaque markers  166 , which are positioned along structure  162  at respective longitudinal sites. The markers can provide an indication in a radiographic image (such as a fluoroscopy image) of how much of the body portion has been deployed at any given point during an implantation procedure, in order to enable setting a desired distance between tissue anchors  167  along body portion  164 , and thus to indicate placement of anchors  167 . For some applications, the markers comprise a radiopaque ink. For some applications the markers comprise a radiopaque material or additional radiopaque material, markers, etc. attached to or incorporated in body portion  164 . 
     As shown, contracting member  165  is coupled to body portion  164  and extends along body portion  164  and through the plurality of flat and planar elements  168  in a manner in which during application of tension to contracting member  165 , contracting member  165  is configured to change a structural configuration of the plurality of radiopaque projections comprising flat and planar elements  168 . As shown in  FIG. 5B , contracting member  165  is configured to change the structure configuration of elements  168  by compressing and/or folding elements  168 . In such embodiments, as shown, contracting member  165  runs along a perimeter of each flat and planar element  168  such that during application of tension to contracting member  165 , member  165  is configured to compress each flat and planar element  168  radially toward axis  161  of structure  162 . For some applications, contracting member is configured to change a spatial configuration of flat and planar elements  168  in sequence. 
     For some applications, contracting member  165  is configured to additionally apply a contracting force to body portion  164  of structure  162  so as to facilitate adjustment of the perimeter of annuloplasty structure  162 . Adjustment of annuloplasty structure  162  can be performed by an adjusting mechanism similar to that described above with respect to annuloplasty structure  72  (contracting member and spool, wheel, spindle, etc.), for example. For some applications, system  160  comprises an additional contracting member (not shown) configured to adjust a perimeter of body portion  164  while contracting member  165  adjusts the spatial configuration of the plurality of radiopaque projections comprising flat and planar elements  168 . 
     For some applications, the plurality of radial projections comprising flat and planar elements  168  are each fully radiopaque. For some applications at least 50% of each projection is radiopaque. The plurality of radial projections comprising flat and planar elements  168  help facilitate viewing of the tissue of the native heart valve annulus and tissue coupled thereto under imaging. For some applications, the plurality of radial projections comprising flat and planar elements  168  are placed against and abut the tissue of the annulus and/or tissue coupled thereto (e.g., atrial wall tissue and/or tissue of the leaflets of the native valve). For some applications, at least some of the plurality of radial projections comprising flat and planar elements  168  are positioned in the path of blood flow. The plurality of radial projections can provide information relating to tissue and/or blood flow responsively to movement of flat and planar elements  168 . 
     In some applications, anchors  167  comprise a biocompatible material such as stainless steel 316 LVM. For some applications, anchors  167  comprise nitinol. For some applications, anchors  167  are coated fully or partially with a non-conductive material. 
     Reference is now made to  FIG. 6A , which is a schematic illustration of a system  170  comprising a tissue anchor  176  comprising a distal tissue-coupling element  173  having a longitudinal axis  175  measured from a distal end to a proximal end of distal tissue-coupling element  173  and an annulus-marking device having a plurality of radiopaque elements, e.g., filaments  99 , coupled to the tissue anchor  176 . The filaments  99  or other radiopaque elements comprise a radiopaque material and project away from axis  175 , in accordance with some applications. Distal tissue-coupling element  173  is configured for anchoring into tissue of a native heart valve annulus  68 . 
     In some embodiments, the plurality of radiopaque filaments  99  comprise radiopaque material (e.g., nitinol or stainless steel) and are configured to be extremely flexible. Filaments  99  project away from anchor  176 . Filaments  99  are configured for aiding implantation of cardiac devices, e.g., an annuloplasty structure  172 , under the guidance of imaging, in accordance with some applications. Implantation of anchors  176  and annuloplasty structure  172  is often performed with the aid of imaging, such as fluoroscopy, transesophageal echo, and/or echocardiography. 
     For some applications, filaments  99  sway with movement of the blood. For some applications, filaments  99  press against tissue of the annulus and tissue coupled thereto (e.g., such as tissue of an atrial wall  122  as well as tissue of leaflet  123  of the native valve) prior to placement of a portion of structure  172  along annulus  68  and prior to puncturing of tissue of annulus  68  by the distal tip of anchor  176 , as shown in view A of  FIG. 6A . That is, the distal tip of anchor  176  punctures through a portion of a body portion  174  of structure  172  and brings filaments  99  through fabric of body portion  174  such that filaments  99  can be pressed against tissue of the annulus and tissue coupled thereto. As such, filaments  99  thus provide enhanced imaging of tissue of the cardiac valve  64 . 
     It is to be noted that although system  170  is shown on mitral valve  64 , system  170  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     In some applications, anchors  176  comprise a biocompatible material such as stainless steel 316 LVM. For some applications, anchors  176  comprise nitinol. For some applications, anchors  176  are coated fully or partially with a non-conductive material. 
     For some applications, each distal tissue-coupling element  173  of anchors  176  is hollow and filaments  99  can be compressed in a lumen of the hollow element  173  during delivery of anchor  176  to the atrium of the heart and expand from within the lumen once inside the atrium. 
     Annuloplasty structure  172  can comprise a braided fabric mesh, e.g., comprising DACRON™. Annuloplasty structure  172  can be configured to be placed only partially around a cardiac valve annulus (e.g., to assume a C-shape), and, once anchored in place, to be contracted so as to circumferentially tighten the valve annulus. Optionally, structure  172  can be configured to be placed entirely around the valve annulus (e.g., as a closed circle or other closed shape). 
     As shown, for some applications, the annulus-marking device comprising filaments  99  is coupled to the distal end of distal tissue-coupling element  173 . Structure  172  comprises a fabric, and the annulus-marking device comprising filaments  99  is configured to pass through the fabric of structure  172 . It is to be noted that additional filaments  99  can be coupled to distal tissue-coupling element  173  or to any other portion of anchor  176 . 
     It is to be noted that for some applications, filaments  99  can be coupled to the anchor driver used to drive the anchor into tissue. 
     Reference is now made to  FIG. 6B , which is a schematic illustration of a system  180  comprising a tissue anchor  176  comprising a distal tissue-coupling element  173  having a longitudinal axis  175  measured from a distal end to a proximal end of distal tissue-coupling element  173  and an annulus-marking device having a plurality of radiopaque filaments  99  coupled to the tissue anchor  176 , filaments  99  comprise a radiopaque material and project away from axis  175 , in accordance with some applications. Distal tissue-coupling element  173  is configured for anchoring into tissue of a native heart valve annulus  68 . 
     The plurality of radiopaque filaments  99  comprise radiopaque material (e.g., nitinol or stainless steel) and can be configured to be extremely flexible. Filaments  99  are coupled to a proximal head  177  of anchor  176  and project away from anchor  176 . Filaments  99  are configured for aiding implantation of cardiac devices, e.g., an annuloplasty structure  182 , under the guidance of imaging, in accordance with some applications. Implantation of anchors  176  and annuloplasty structure  172  is often performed with the aid of imaging, such as fluoroscopy, transesophageal echo, and/or echocardiography. 
     For some applications, filaments  99  sway with movement of the blood. Filaments  99  thus provide enhanced imaging of tissue of the cardiac valve  64 . 
     Annuloplasty structure  182  comprises a flat band by way of illustration and not limitation. Structure  182  comprises a braided fabric or braided metal and is not tubular in shape. 
     It is to be noted that although system  180  is shown on mitral valve  64 , system  180  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     For some applications, anchors  176  comprise a biocompatible material such as stainless steel 316 LVM. For some applications, anchors  176  comprise nitinol. For some applications, anchors  176  are coated fully or partially with a non-conductive material. 
     Annuloplasty structure  182  can comprise a braided fabric mesh, e.g., comprising DACRON™. Annuloplasty structure  182  can be configured to be placed only partially around a cardiac valve annulus (e.g., to assume a C-shape), and, once anchored in place, to be contracted so as to circumferentially tighten the valve annulus. Optionally, structure  182  can be configured to be placed entirely around the valve annulus (e.g., as a closed circle or other closed shape). 
     As shown, structure  182  is flat, and head  177  is disposed above the flat band following implantation of anchor  176 . Once implanted in the tissue, filaments  99  provide an indication of their placement above the band of structure  182 . 
     Reference is now made to  FIGS. 7A-C , which are schematic illustrations of a system  190  comprising an implant comprising an annuloplasty structure  192  which comprises a body portion  194  and an annulus-marking device which comprises one or more (e.g., two, as shown) planar radiopaque fins  196 , in accordance with some applications. Body portion  194  comprises a flexible material, e.g., a braided fabric mesh. For some applications, body portion  194  is shaped so as to define a sleeve shaped so as to define a lumen therethrough, as shown. For some applications, body portion  194  is flat. Body portion  194  extends along a central longitudinal axis  191  of structure  192 , and radiopaque fins  196  project away from longitudinal axis  191 . Radiopaque fins  196  each have a longest dimension that is measured along longitudinal axis  191 . 
     Body portion  194  can comprise a braided fabric mesh, e.g., comprising DACRON™. Body portion  194  can be configured to be placed only partially around a cardiac valve annulus (e.g., to assume a C-shape), and, once anchored in place, to be contracted so as to circumferentially tighten the valve annulus. Optionally, structure  192  can be configured to be placed entirely around the valve annulus (e.g., as a closed circle or other closed shape). In order to tighten the annulus, annuloplasty structure  192  comprises a flexible elongated contracting member (not shown) that extends along body portion  194 . The contracting member comprises a wire, a ribbon, a rope, or a band, which often comprises a flexible and/or superelastic material, e.g., nitinol, polyester, stainless steel, or cobalt chrome. For some applications, the contracting member comprises a radiopaque material. For some applications, contracting the member comprises a braided polyester suture (e.g., Ticron). For some applications, the contracting member is coated with polytetrafluoroethylene (PTFE). For some applications, the contracting member comprises a plurality of wires that are intertwined to form a rope structure. 
     Planar radiopaque fins  196  comprise a flexible fabric. In some applications, fins  196  and body portion  194  comprise the same material. 
     Body portion  194  of structure  192  comprises a plurality of radiopaque markers  195 , which are positioned along structure  192  at respective longitudinal sites. The markers may provide an indication in a radiographic image (such as a fluoroscopy image) of how much of the body portion has been deployed at any given point during an implantation procedure, in order to enable setting a desired distance between tissue anchors  198  along body portion  194 . For some applications, the markers comprise a radiopaque ink. For some applications the markers comprise a radiopaque material or additional radiopaque material, markers, etc. attached to or incorporated in body portion  164 . 
       FIG. 7B  shows a transverse cross-section of structure  172  showing body portion  194  and fins  196 . 
     In some applications, fins  196  are each fully radiopaque. For some applications at least 50% of each fin  196  is radiopaque. Fins  196  help facilitate viewing of the tissue of the native heart valve annulus and tissue coupled thereto under imaging. For some applications, as shown in  FIG. 7C , fins  196  are placed against and abut the tissue of the annulus and/or tissue coupled thereto (e.g., tissue of atrial wall  122  and/or tissue of the leaflets of the native valve). For some applications, fins  196  are positioned in the path of blood flow and provide information relating to tissue and/or blood flow responsively to movement of fins  196 . 
     As shown in  FIG. 7C , anchors  198  that anchor structure  192  to tissue of annulus  68  are designated for implantation in-between fins  196 . 
     For some applications, fins  196  comprise shape-memory wires which help them expand to assume their shape. For some applications, the fabric of fins  196  is thinner than the fabric of body portion  194 . 
     It is to be noted that although system  190  is shown on mitral valve  64 , system  190  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 8A-B , which are schematic illustrations of a system  200  comprising an annulus-marking device  202  for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications. Device  202  comprises a frustoconical scaffolding  203  having a plurality of struts  204  collectively arranged in a frustoconical shape. Scaffolding  203  is collapsible and expandable. Annulus-marking device  202  is configured to run alongside at least one side of body portion  210  of an implant  209  (e.g., an annuloplasty structure, as shown) configured for placement along a native heart valve annulus  68  of a mitral valve  64  of the subject. As shown, device  202  surrounds a given portion of body portion  210 , as it is frustoconical in shape. Body portion  210  of implant  209  comprises a flexible material and has a longitudinal axis that runs along the length of body portion  210  (e.g., when the body portion is straightened). Body portion  210  comprises radiopaque markings  211  to aid in imaging for accurate delivery of anchors to annulus  68  in order to anchor implant  209  to tissue of annulus  68 . 
     Scaffolding  203  comprises radiopaque material (e.g., nitinol or stainless steel) and is flexible. Scaffolding  203  is coupled to a proximal ring  206  at a proximal end of scaffolding  203 . For some applications, scaffolding  203  comprises ring  206 . Ring  206  surrounds at least a portion of body portion  210  of implant  209  and is moveable proximally and distally with respect to body portion  210  in a manner in which scaffolding  203  is moveable to multiple locations along body portion  210  of implant  209 . 
     Annulus-marking device  202  is coupled to a delivery tool  208  which is configured to deliver implant  209  to annulus  68 . Annulus-marking device  202  is retrievable upon removal of delivery tool  208  from the subject. For some applications, scaffolding  203  and ring  206  slide with respect to tool  208 . For some applications, ring  206  is fixedly coupled to tool  208 , and scaffolding  203  moves proximally and distally with respect to body portion  210  responsively to movement of tool  208 . Delivery tool  208  is configured to surround a portion of body portion  210  of implant  209 , and annulus-marking device  202  is configured to surround body portion  210  of implant  209  at least in part, e.g., entirely surround a portion of body portion  210 . 
     For some applications, a plurality of radiopaque elements, such as radiopaque filaments  99  (or other radiopaque markers, wires, extensions, beads, etc.), are coupled to scaffolding  203  at a distal portion thereof. The plurality of radiopaque elements or filaments  99  function as additional annulus-marking devices. Annulus-marking device  202  is configured for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications. The steering procedure is performed with the aid of imaging, such as fluoroscopy, transesophageal echo, and/or echocardiography. 
     Device  202  can be delivered percutaneously, thoracoscopically through the chest, or using open heart surgical techniques. If delivered percutaneously, device  202  may be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter and subsequently self-expand into the desired shape and tension when released from the catheter. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein under image guidance (e.g., fluoroscopic, ultrasonic, magnetic resonance, computed tomography, or combinations thereof). For some applications, device  202  comprises a wire. 
     The plurality of radiopaque filaments  99  comprise radiopaque material (e.g., nitinol or stainless steel) and can be configured to be extremely flexible. For some applications, filaments  99  sway with movement of the blood. For some applications, filaments  99  press against tissue of the annulus and tissue coupled thereto (as shown in  FIG. 8 ), such as tissue of an atrial wall  122  as well as tissue of leaflets  123  of the native valve. Filaments  99  thus provide enhanced imaging of tissue of valve  64 . 
     For some applications, delivery tool  208  comprises a fin (not shown, but shown as fin  227  in  FIG. 10B ) that is coupled to a distal portion of delivery tool  208  and to a portion of scaffolding  203  in a manner in which movement of the fin responsively to blood flow rotationally orients scaffolding  203  with respect to body portion  210  of implant  209 . In such a manner, the operating physician is able to discern between tissue of the atrial wall and leaflet tissue under the aid of imaging. 
     Reference is now made to  FIG. 8B . For some applications, the plurality of radiopaque elements or radiopaque filaments  99  comprise a first subset of radiopaque filaments having a first length and a second subset of filaments having a second length that is greater than the first length. The first and second subsets are configured to rotationally orient scaffolding  203  with respect to implant  209 . That is, the second subset of filaments  99  having a longer length will orient scaffolding  203  in a manner in which the second subset of longer filaments  99  will align against tissue of leaflet  123  and the first subset of shorter filaments  99  will align against tissue of atrial wall  122 . For some applications, the plurality of radiopaque filaments  99  comprises a first subset of radiopaque filaments having a first rigidity and a second subset of filaments having a second rigidity that is greater than the first length. The first and second subsets are configured to rotationally orient scaffolding  203  with respect to implant  209 . That is, the second subset of filaments  99  having a greater rigidity will orient scaffolding  203  in a manner in which the second subset of more rigid filaments  99  will align against tissue of leaflet  123  and the first subset of less rigid filaments  99  will align against tissue of atrial wall  122 . 
     Subsequently to implanting of implant  209 , annulus-marking device  202  is retrieved. Since device  202  is flexible and compressible, device  202  is constrained within the tool during the retrieval of device  202  and subsequent removal of device  202  from the body of the subject. That is, device  202  does not function as an implant for such embodiments and is used only to guide implantation of implant  209 ; rather, device  202  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of implant  209 . 
     It is to be noted that although system  200  is shown on mitral valve  64 , system  200  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is made to  FIGS. 9A-B , which are schematic illustrations of a navigational-based guidance system  230 , which employs one or more longitudinal guides  232  configured to facilitate guidance of an implant  231  to specific portions of annulus  68  by the guides contacting a surface of the valve (e.g., the annulus, commissure, and/or leaflets of the valve), in accordance with some applications. Guide  232  comprises a flexible material (e.g., a flexible metal such as nitinol or stainless steel), and each guide  232  is radiopaque. A plurality of eyelets  234  are disposed along a lateral outer surface of a body portion  233  of implant  231 , and each guide  232  (e.g., a distal portion thereof) is disposed within at least some of the eyelets (e.g., the guide is threaded through the eyelets). Eyelets  234  can comprise suture or fabric. 
     In some applications, eyelets  234  are arranged in longitudinal rows along the length of body portion  233 , and each guide  232  is disposed within the eyelets of a respective row. In some applications, the eyelets of each row are disposed at the same longitudinal site as a corresponding eyelet of each other row. For some applications body portion  233  comprises a plurality of radiopaque markers  235 , which are positioned along the body portion at respective longitudinal sites. For some applications the eyelets of each row are disposed at the same longitudinal site as a corresponding radiopaque marker. Though, optionally, the eyelets can be disposed between radiopaque markers. Guides  232  are disposed at respective circumferential positions around body portion  233  (e.g., the longitudinal axis thereof). In  FIGS. 9A-B , each of the three guides is shown as being disposed at about 120 degrees around body portion  233  from the adjacent guides, but the scope includes other arrangements, such as two guides disposed opposite each other. 
     For some applications, each guide  232  comprises a wire with a looped portion  238  such that the guide has (1) two parallel linear portions of the wire, and (2) the looped portion at a distal end portion  236  of the guide. 
     For some applications, distal end portion  236  of each guide  232  is biased (e.g., shape-set) to protrude radially outward from body portion  233 . Such biasing may confer a desired behavior on the guide, e.g., during distal movement of the guide. For example, when the guide is moved distally against tissue, the biasing may facilitate splaying of the guide over the tissue (e.g., as described hereinbelow). Alternatively or additionally, after the guide has been withdrawn proximally from a given eyelet, when the guide is subsequently moved distally again, the biasing may inhibit (e.g., prevent) re-threading of the guide into the given eyelet. 
     Body portion  233  is configured to be advanced distally out of delivery tool  208  and anchored to annulus  68  using anchors. 
     Guides  232  are placed (e.g., pushed) against tissue of the valve, e.g., by virtue of being already disposed distally to a distal end of body portion  233 , or by being advanced distally after the distal end of the body portion has itself been placed against tissue of the valve. Each guide  232  (e.g., looped portion  238  thereof) thereby comprises a tissue-engaging portion that is configured to be placed in contact with tissue of the subject. 
     In one or more ways, the behavior of guides  232  in response to being placed against the tissue of the valve facilitates guidance by viewing of body portion  233  (e.g., positioning of the body portion on the annulus). For example: 
     Resistance of a guide to being pushed further distally may indicate that the guide is in contact with tissue that resists forces applied by the guide. For example, the distal end of the guide may be abutting annulus  68  and/or a wall  122  of the atrium (as shown in  FIG. 9B ). Conversely, lack of resistance of a guide to being pushed further distally may indicate that the distal end of the guide is not in contact with tissue that resists forces applied by the guide. For example, the distal end of the guide may be moving between leaflets  123  of the valve (e.g., at a commissure), and/or may be pushing a leaflet  123  downward (e.g., into the ventricle). Such resistance (or lack thereof) can be detected mechanically (e.g., as tactile feedback to the operating physician and/or by an extracorporeal control unit). Since guides  232  comprise radiopaque material, such resistance (or lack thereof) can be detected via imaging (e.g., fluoroscopically). 
     Similarly, the position, orientation and/or shape of a guide (e.g., with respect to one or more other guides, body portion  233  of implant  231 , tissue of the valve, etc.) may indicate against what, if anything, the guide is disposed. Imaging techniques such as fluoroscopy can be used to identify this position, orientation and/or shape of the guide. For example, if the distal end of a guide is positioned at the same height (i.e., at the same place on a superior-inferior axis of the subject) as the distal end of body portion  233 , this may indicate that body portion  233  and guide  232  abut the same surface (e.g., annulus  68 ). Conversely, if the distal end of guide  232  is positioned lower than body portion  233 , this may indicate that the body portion  233  is disposed against annulus  68 , while guide  232  has passed toward or into the ventricle. Movement (e.g., beating) of the guide may indicate that the guide is disposed against a leaflet of the valve, and that the leaflet is moving the guide as the heart beats. Such imaging may be facilitated by one or more components comprising radiopaque markings. For some applications, each guide  232  has different radiopaque markings, so as to facilitate identification during imaging. 
     One or more of the guides  232  may inhibit movement of body portion  233  of implant  231 . For example, if a guide extends between leaflets at a commissure, the guide may inhibit movement of body portion  233  away from the commissure. 
     Guides  232  may be configured and/or selected, either collectively or individually, such that the guides behave in a particular manner upon interaction with tissue. For example, the guides may be configured and/or selected to be (1) sufficiently rigid so as to provide tactile feedback upon abutting tissue, and/or (2) sufficiently flexible so as to splay over tissue, not to damage tissue, and/or to be movable by beating leaflets. 
       FIG. 9B  shows body portion  233  having been placed against annulus  68  of the subject in a vicinity of left fibrous trigone. Guides  232  have been pushed distally, and have splayed across annulus  68 , e.g., due to resistance of the annulus (see view A of  FIG. 9B ). As described hereinabove, this can be detected mechanically and/or by imaging. Guide  232   a , which has also been pushed distally, extends between leaflets  123  at the commissure (see view B of  FIG. 9B ). As described hereinabove, this can be detected mechanically and/or using imaging. The position, orientation and/or shape of each guide, alone and/or in combination with the other guides and/or elements indicates that the portion of body portion  233  is positioned against firm tissue that is close to the commissure, which for some applications is the preferred position for anchoring of the portion of body portion  233 . Identification (e.g., mechanically and/or by imaging) of which guide is in which position may further indicate the rotational orientation of body portion  233 . 
     Once the desired position has been identified, an anchor (e.g., a first anchor) is used to anchor body portion  233 . For some applications, one or more of guides  232  can be withdrawn slightly proximally before anchoring, e.g., so as to reduce a likelihood of inadvertently anchoring the guide to the tissue. Subsequently, additional portions of body portion  233  are anchored to annulus  68 . In some applications, guides  232  are moved proximally with respect to body portion  233 . This process can be repeated for each anchor until implant  231  is fully implanted. 
     It is to be noted that although system  230  is shown on mitral valve  64 , system  230  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 10A-B , which are schematic illustrations of a system  220  comprising an annulus-marking device  222  for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications. Device  222  comprises a generally-triangular scaffolding  223  having a plurality of struts  204  collectively arranged in a generally triangular shape. Scaffolding  223  is collapsible and expandable. Annulus-marking device  222  is configured to run alongside at least one side of body portion  210  of an implant  209  (e.g., an annuloplasty structure, as shown) configured for placement along a native heart valve annulus  68  of a mitral valve  64  of the subject. As shown, device  222  is positioned ahead and in advance of body portion  210 . Device  222  leads implant  209 . Body portion  210  of implant  209  comprises a flexible material and has a longitudinal axis that runs along the length of body portion  210  (e.g., when the body portion is straightened). Body portion  210  comprises radiopaque markings to aid in imaging for accurate delivery of anchors to annulus  68  in order to anchor implant  209  to tissue of annulus  68 . 
     Scaffolding  223  comprises radiopaque material (e.g., nitinol or stainless steel) and is flexible. Scaffolding  223  is coupled to a proximal ring  226  at a proximal end of scaffolding  223 . For some applications, scaffolding  223  comprises ring  226 . Ring  226  surrounds at least a portion of body portion  210  of implant  209  and is moveable proximally and distally with respect to body portion  210  in a manner in which scaffolding  223  is moveable to multiple locations along body portion  210  of implant  209 . For some applications, scaffolding  223  is semitubular. 
     Annulus-marking device  222  is coupled to a delivery tool  208  which is configured to deliver implant  209  to annulus  68 . Annulus-marking device  222  is retrievable upon removal of delivery tool  208  from the subject. For some applications, scaffolding  223  and ring  226  are configured to slide with respect to tool  208 . For some applications, scaffolding  223  and ring  226  are configured to rotate with respect to delivery tool  208 . For some applications, ring  226  is fixedly coupled to tool  208 , and scaffolding  223  moves proximally and distally with respect to body portion  210  responsively to movement of tool  208 . Delivery tool  208  is configured to surround a portion of body portion  210  of implant  209 , and annulus-marking device  222  is configured to surround body portion  210  of implant  209  at least in part, e.g., a single side of body portion  210 , as shown. 
     For some applications, scaffolding  223  is planar and triangular, as shown. For some applications, scaffolding  223  is semi-tubular. 
     For some applications, a plurality of radiopaque elements, such as radiopaque filaments  99  (or other radiopaque markers, wires, extensions, beads, etc.) are coupled to scaffolding  223  at a distal portion thereof. The plurality of radiopaque elements or filaments  99  function as additional annulus-marking devices. Annulus-marking device  222  is configured for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications. The steering procedure is performed with the aid of imaging, such as fluoroscopy, transesophageal echo, and/or echocardiography. In some embodiments, a first subset of filaments  99  touch atrial wall  122 , a second subset of filaments  99  touch annulus  68 , a third subset of filaments  99  touch leaflet  123 , while a fourth subset of filaments  99  extend over the orifice of the valve and between leaflets  123 . 
     Device  222  may be delivered percutaneously, thoracoscopically through the chest, or using open heart surgical techniques. If delivered percutaneously, device  222  may be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter and subsequently self-expand into the desired shape and tension when released from the catheter. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein under image guidance (e.g., fluoroscopic, ultrasonic, magnetic resonance, computed tomography, or combinations thereof). For some applications, device  222  comprises a wire. 
     The plurality of radiopaque filaments  99  comprise radiopaque material (e.g., nitinol or stainless steel) and can be configured to be extremely flexible. For some applications, filaments  99  sway with movement of the blood. For some applications, filaments  99  press against tissue of the annulus and tissue coupled thereto (as shown in  FIG. 10 ), such as tissue of an atrial wall  122 . Filaments  99  thus provide enhanced imaging of tissue of valve  64 . 
     Reference is now made to  FIG. 10B . For some applications, delivery tool  208  comprises a fin  227  that is coupled to a distal portion of delivery tool  208  and to a portion of scaffolding  223  in a manner in which movement of fin  227  responsively to blood flow rotationally orients scaffolding  223  with respect to body portion  210  of implant  209 . In such a manner, the operating physician is able to discern between tissue of the atrial wall and leaflet tissue under the aid of imaging. For some applications, fin  227  is radiopaque. For some applications, a distal portion of fin  227  extends into the ventricle. 
     Reference is now made to  FIGS. 8B and 10A -B. For some applications, the plurality of radiopaque filaments  99  comprises a first subset of radiopaque filaments having a first length and a second subset of filaments having a second length that is greater than the first length. The first and second subsets are configured to rotationally orient scaffolding  223  with respect to implant  209 . That is, the second subset of filaments  99  having a longer length will orient scaffolding  223  in a manner in which the second subset of longer filaments  99  will align against tissue of leaflet  123  and the first subset of shorter filaments  99  will align against tissue of atrial wall  122 . For some applications, the plurality of radiopaque filaments  99  comprises a first subset of radiopaque filaments having a first rigidity and a second subset of filaments having a second rigidity that is greater than the first length. The first and second subsets are configured to rotationally orient scaffolding  223  with respect to implant  209 . That is, the second subset of filaments  99  having a greater rigidity will orient scaffolding  223  in a manner in which the second subset of more rigid filaments  99  will align against tissue of leaflet  123  and the first subset of less rigid filaments  99  will align against tissue of atrial wall  122 . 
     For some applications, a first subset of filaments  99  touch atrial wall  122 , a second subset of filaments  99  touch annulus  68 , a third subset of filaments  99  touch leaflet  123 , while a fourth subset of filaments  99  extend over the orifice of the valve and between leaflets  123 . 
     Reference is again made to  FIGS. 10A-B . In some applications, device  222  is configured for placement between implant  209  and tissue of atrial wall  122 , as shown. 
     Subsequently to implanting of implant  209 , annulus-marking device  222  is retrieved. Since device  222  is flexible and compressible, device  222  is constrained within the tool during the retrieval of device  222  and subsequent removal of device  222  from the body of the subject. That is, device  222  does not function as an implant for such embodiments and is used only to guide implantation of implant  209 ; rather, device  222  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of implant  209 . 
     It is to be noted that although system  220  is shown on mitral valve  64 , system  220  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 11A-C , which are schematic illustrations of a system  250  comprising an annulus-marking device  252  comprising a tubular stent body  254  having a central longitudinal axis  251  and a plurality of extensions  256  coupled to a proximal end of tubular stent body  254  and projecting away from longitudinal axis  251  of stent body  254 , in accordance with some applications. Annulus-marking device  252  is configured for placement within a native heart valve of the subject, e.g., mitral valve  64 , as shown, the tricuspid valve, or any other cardiac valve. 
     The plurality of extensions  256  are configured for placement along a circumference of annulus  68  of valve  64 . In some applications, prior to implantation of an implant along annulus  68 , annulus-marking device is positioned within valve  64  and thus configured to provide a guide for implantation of the implant along the annulus during implantation. For some applications, tubular stent body  254  comprises two or more leaflets in order to regulate blood flow while device  252  is positioned in valve  64 . Device  252  is compressible during delivery toward valve  64  and expandable from a compressed state for positioning in the native heart valve  64 . Once device  252  is positioned within valve  64 , the valve is imaged using imaging, e.g., fluoroscopy. Extensions  256  provide an indication as to the circumference of annulus  68 . 
     Device  252  may be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter. Additionally, device  252  is made from radiopaque material to facilitate fluoroscopic visualization. In some applications, tissue of valve annulus  68  and tissue coupled thereto is viewed using the plurality of extensions  256 . Additionally, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  252  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the plurality of extensions  256  against the tissue. For some applications, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  252  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the plurality of extensions  256  responsively to movement of the tissue. 
     Subsequently to the positioning of device  252  within valve  64 , and under imaging, an implant comprising an annuloplasty structure  259  is positioned along annulus  68 , as shown in  FIG. 11B . Structure  259  comprises a body portion  260 , e.g., a tubular body portion, through which a plurality of anchors  264  are deployed. Structure  259  comprises a plurality of a plurality of radiopaque markers  262 , which are positioned along structure  259  at respective longitudinal sites. The markers may provide an indication in a radiographic image (such as a fluoroscopy image) of how much of the body portion has been deployed at any given point during an implantation procedure, in order to enable setting a desired distance between the tissue anchors along the body portion. For some applications, the markers comprise radiopaque ink. For some applications the markers comprise a radiopaque material attached to or incorporated in body portion  164 . 
     Anchors  264  are delivered to valve  64  in order to anchor structure  259  to annulus  68  by deploying each anchor  264  of the plurality of anchors  264  between adjacent extensions  256  of device  252 . In addition to the guidance under imaging provided by radiopaque extensions  256 , markers  262  of structure  259  aid in deployment of anchors  264 . 
     Following anchoring of structure  259  to annulus  68 , annulus-marking device  252  is constrained within a catheter such that tubular stent body  254  collapses and extensions  256  trail behind body  254  in a manner in which extensions  256  slide from under annuloplasty structure  259  implanted along annulus  68 . Annulus-marking device  252  is retrieved and removed from the body of the subject. 
     Subsequently to implanting of structure  259 , annulus-marking device  252  is retrieved. Since device  252  is flexible and compressible, device  252  is constrained within a tool during the retrieval of device  252  and subsequent removal of device  252  from the body of the subject. That is, device  252  does not function as an implant for such embodiments and is used only to guide implantation of annuloplasty structure  259  (i.e., the implant); rather, device  252  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of annuloplasty structure  259 . 
     It is to be noted that although system  250  is shown on mitral valve  64 , system  250  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 12A-B , which are schematic illustrations of a system  270  for facilitating imaging of cardiac tissue during implantation of a cardiac implant, the system  270  comprising an annulus-marking device comprising a mapping catheter  272 , in accordance with some applications. Mapping catheter  272  is introduced percutaneously (e.g., transvascularly) toward mitral valve  64 , as shown. It is to be noted that catheter  272  may be advanced to the tricuspid valve and/or to any other cardiac valve. Mapping catheter  272  comprises a plurality of mapping subunits  274 . For some applications, subunits  274  comprise electrodes and mapping of valve  64  is performed electrophysiologically using electroanatomic mapping systems using the electrodes. For some applications, subunits  274  comprise magnets and mapping of valve  64  is performed by generating a magnetic field and under magnetic imaging. For some applications, subunits  274  comprise radiopaque material and imaging is performed, such as under fluoroscopy. 
     As shown in  FIG. 12B , once a map of valve  64  is generated using mapping catheter  272 , an implant, e.g., an annuloplasty structure  271 , is placed at valve  64  using the map as a guide. In some applications, structure  271  comprises a flexible body portion  275 . For some applications, the body portion of structure  271  is shaped so as to define a tubular sleeve through which a plurality of anchors  277  is implanted. The body portion of structure  271  comprises a plurality of radiopaque markers  273 , which are positioned along structure  271  at respective longitudinal sites. The markers may provide an indication in a radiographic image (such as a fluoroscopy image) of how much of the body portion has been deployed at any given point during an implantation procedure, in order to enable setting a desired distance between the tissue anchors along the body portion. For some applications, the markers comprise radiopaque ink. For some applications the markers comprise a radiopaque material or additional radiopaque material, markers, etc. attached to or incorporated in body portion  164 . 
     For some applications, mapping catheter  272  is removed prior to implanting of structure  271  and structure  271  is implanted under the guidance of a map  276  generated by mapping catheter  272 , wherein map  276  can be stored and displayed by an imaging device. For some applications, mapping catheter  272  remains at annulus  68  during implantation of structure  271  and is viewed under fluoroscopy. For some applications, mapping catheter  272  facilitates viewing and mapping of tissue of the native heart valve annulus and tissue coupled thereto using mapping catheter  272 . For some applications, mapping catheter  272  facilitates viewing and mapping of tissue of the native heart valve annulus and tissue coupled thereto by viewing mapping catheter  272  against the tissue. For some applications, mapping catheter  272  facilitates viewing and mapping of tissue of the native heart valve annulus and tissue coupled thereto by viewing movement of mapping catheter  272  responsively to movement of the tissue. 
     It is to be noted that although system  270  is shown on mitral valve  64 , system  270  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 13 , which is a schematic illustration of a system  280  comprising an annulus-marking device  282  comprising a plurality of expandable elements  286  which form device  282  into a generally spherical shape for facilitating imaging of cardiac tissue during implantation of a cardiac implant, in accordance with some applications. Device  282  comprises a flexible, radiopaque material, e.g., nitinol or stainless steel, which facilitates collapsing and expanding of device  282 . 
     As shown, device  282  aids in imaging implantation of a cardiac implant, e.g., an annuloplasty structure  288 , as shown. Structure  288  comprises a body portion  290  which comprises a flexible material and has a longitudinal axis that runs along the length of body portion  290  (e.g., when the body portion is straightened). Body portion  290  comprises radiopaque markings  292  to aid in imaging for accurate delivery of anchors  296  to annulus  68  in order to anchor implant  288  to tissue of annulus  68 . 
     Annulus-marking device  282  is delivered using a delivery tool  284  which is configured to deliver device  282  to the left atrium in a compressed state. Device  282  is configured to be expanded from its compressed state once deployed from within a lumen of tool  284 . Annulus-marking device  282  is retrievable upon removal of delivery tool  284  from the subject. That is, device  282  is constrained within the lumen of tool  284  once the cardiac implant has been implanted at annulus  68 . Device  282  may be delivered percutaneously, thoracoscopically through the chest, or using open heart surgical techniques. If delivered percutaneously, device  282  may be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter and subsequently self-expand into the desired shape and tension when released from the catheter. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein under image guidance (e.g., fluoroscopic, ultrasonic, magnetic resonance, computed tomography, or combinations thereof). For some applications, device  282  comprises a wire. 
     Once inside the atrium, the plurality of expandable elements  286  expand radially within the atrium such that the plurality of expandable elements  286  provides an indication as to a location of the native heart valve annulus  68  of native heart valve  64 . It is to be noted that although device  282  is being used in the left atrium, device  282  may be used in the right atrium, the left ventricle, and the right ventricle. 
     The plurality of expandable elements  286  collectively form annulus-marking device  282  into a generally spherical shape. As shown, the plurality of expandable elements  286  comprise a plurality of curved wires. For some applications, plurality of expandable elements  286  surround a central shaft  285 . A proximal end and a distal end of each expandable element  286  is coupled to shaft  285 . 
     A collective proximal diameter Di 1  of the proximal ends of the plurality of expandable elements  286  is equal to a collective distal diameter Di 3  of the distal ends of the plurality of expandable elements  286 . A collective middle diameter Di 2  of the plurality of expandable elements  286  is greater than collective proximal diameter Di 1  and greater than collective distal diameter Di 3 . 
     Annulus  68  is then imaged using fluoroscopy. In some applications, annulus-marking device  282  is imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing the plurality of expandable elements  286  against the tissue. For some applications, annulus-marking device  282  is imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing movement of the plurality of expandable elements  286  responsively to movement of the tissue. For either application, annulus-marking device  282  is imaged with respect to the tissue of the native heart valve annulus  68 , tissue of at least one leaflet, and tissue of an atrial wall  122 . 
     Reference is now made to  FIGS. 3A-B  and  13 . For some applications, annulus-marking device  282  is coupled to a plurality of radiopaque elements or filaments  99 . In some applications, annulus-marking device  282  and elements or filaments  99  are imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing the plurality of expandable elements  286  and radiopaque elements or filaments  99  against the tissue. For some applications, annulus-marking device  282  is imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing movement of the plurality of expandable elements  286  and of filaments  99  responsively to movement of the tissue. For either application, annulus-marking device  282  and radiopaque element or filaments  99  are imaged with respect to the tissue of the native heart valve annulus  68 , tissue of at least one leaflet, and tissue of an atrial wall  122 . 
     Reference is again made to  FIG. 13 . Annuloplasty structure  288  is implanted under the guidance of fluoroscopy using annulus-marking device  282  as a guide. Annuloplasty structure  288  is positioned between annulus-marking device  282  and atrial wall  122 . A respective anchor  296  is deployed to anchor structure  288  at a site along annulus  68  that is marked between successive curved wires of elements  286 . 
     Once annuloplasty structure  288  is implanted, device  282  is constrained within tool  284  and extracted from the subject. 
     Subsequently to implanting of structure  288 , annulus-marking device  282  is retrieved. Since device  282  is flexible and compressible, device  282  is constrained within a tool during the retrieval of device  282  and subsequent removal of device  282  from the body of the subject. That is, device  282  does not function as an implant for such embodiments and is used only to guide implantation of annuloplasty structure  288  (i.e., the implant); rather, device  282  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of annuloplasty structure  288 . 
     It is to be noted that although system  280  is shown on mitral valve  64 , system  280  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 14 , which is a schematic illustration of a system  300  comprising an annulus-marking device  302  comprising a plurality of expandable elements  301  which form device  302  into a generally spherical, or generally bulbous shape for facilitating imaging of cardiac tissue during implantation of a cardiac implant, in accordance with some applications. The plurality of expandable elements  301  of device  302  comprise woven radiopaque fibers comprising a flexible, radiopaque material, e.g., nitinol or stainless steel, which facilitates collapsing and expanding of device  302 . The plurality of expandable elements  301  of device  302  collectively assumes a mesh. 
     As shown, device  302  aids in imaging implantation of a cardiac implant, e.g., an annuloplasty structure  308 , as shown. Structure  308  comprises a body portion  307  which comprises a flexible material and has a longitudinal axis that runs along the length of body portion  307  (e.g., when the body portion is straightened). Body portion  307  comprises radiopaque markings  309  to aid in imaging for accurate delivery of anchors  310  to annulus  68  in order to anchor structure  308  to tissue of annulus  68 . Structure  308  is delivered using a delivery tool  305 . 
     Annulus-marking device  302  is delivered using a delivery tool  304  which is configured to deliver device  302  to the left atrium in a compressed state. Device  302  is configured to be expanded from its compressed state once deployed from within a lumen of tool  304 . Annulus-marking device  302  is retrievable upon removal of delivery tool  304  from the subject. That is, device  302  is constrained within the lumen of tool  304  once the cardiac implant has been implanted at annulus  68 . Device  302  may be delivered percutaneously, thoracoscopically through the chest, or using open heart surgical techniques. If delivered percutaneously, device  302  may be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter and subsequently self-expand into the desired shape and tension when released from the catheter. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein under image guidance (e.g., fluoroscopic, ultrasonic, magnetic resonance, computed tomography, or combinations thereof). For some applications, device  302  comprises a wire. 
     It is to be noted that although device  302  is shown as being delivered via the aorta, any suitable delivery path may be used in order to deliver device  302  into the atrium. 
     Once inside the atrium, the plurality of expandable elements  301  expand radially within the atrium such that the plurality of expandable elements  301  provides an indication as to a location of the native heart valve annulus  68  of native heart valve  64 . It is to be noted that although device  302  is being used in the left atrium, device  302  may be used in the right atrium, the left ventricle, and the right ventricle. 
     Annulus  68  is then imaged using fluoroscopy. In some applications, annulus-marking device  302  is imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing the plurality of expandable elements  301  against the tissue. For some applications, annulus-marking device  302  is imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing movement of the plurality of expandable elements  301  responsively to movement of the tissue. For either application, annulus-marking device  302  is imaged with respect to the tissue of the native heart valve annulus  68 , tissue of at least one leaflet, and tissue of an atrial wall  122 . 
     Reference is now made to  FIGS. 3A-B  and  14 . For some applications, annulus-marking device  302  is coupled to a plurality of radiopaque elements, such as radiopaque filaments  99 , or other radiopaque markers, wires, extensions, beads, etc. In some applications, annulus-marking device  302  and radiopaque elements or filaments  99  are imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing the plurality of expandable elements  301  and radiopaque elements or filaments  99  against the tissue. For some applications, annulus-marking device  302  is imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing movement of the plurality of expandable elements  301  and of radiopaque elements or filaments  99  responsively to movement of the tissue. For either application, annulus-marking device  302  and radiopaque elements or filaments  99  are imaged with respect to the tissue of the native heart valve annulus  68 , tissue of at least one leaflet, and tissue of an atrial wall  122 . 
     Annuloplasty structure  308  is implanted under the guidance of fluoroscopy using annulus-marking device  302  as a guide. Annuloplasty structure  308  is positioned between annulus-marking device  302  and atrial wall  122 . A respective anchor  310  is deployed to anchor structure  308  at a site along annulus  68  that is marked between successive curved wires of elements  301 . 
     Once annuloplasty structure  308  is implanted, device  302  is constrained within tool  304  and extracted from the subject. 
     Subsequently to implanting of structure  308 , annulus-marking device  302  is retrieved. Since device  302  is flexible and compressible, device  302  is constrained within a tool during the retrieval of device  302  and subsequent removal of device  302  from the body of the subject. That is, device  302  does not function as an implant for such embodiments and is used only to guide implantation of annuloplasty structure  308  (i.e., the implant); rather, device  302  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of annuloplasty structure  308 . 
     It is to be noted that although system  300  is shown on mitral valve  64 , system  300  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 15 , which is a schematic illustration of a system  320  comprising an annulus-marking device  321  comprising a guidewire  324  that runs alongside an implant, e.g., an annuloplasty structure  322 , in accordance with some applications. Guidewire  324  extends from within a delivery tool  323  and is disposed between leaflets  123  (e.g., posterior leaflet and anterior leaflet), often at a commissure of the valve. Guidewire  324  is at least partly stiff, and provides resistance, which facilitates positioning of structure  322 . Guidewire  324  may also provide tactile feedback to the operating physician. 
     Structure  322  comprises a body portion which comprises a flexible material and has a longitudinal axis that runs along the length of the body portion (e.g., when the body portion is straightened). The body portion comprises radiopaque markings  325  to aid in imaging for accurate delivery of anchors to annulus  68  in order to anchor structure  322  to tissue of annulus  68 . 
     In addition to providing tactile feedback, guidewire  324  may also facilitate positioning of the annuloplasty structure  322  by facilitating imaging (e.g., fluoroscopy). For example, the presence of guidewire  324  and/or the shape thereof (e.g., bending due to being pressed into the commissure) is visible in fluoroscopic imaging, and can be used to facilitate identification of the position and angle of annuloplasty structure  322  with respect to tissues. 
     Guidewire  324  extends proximally through tool  323  and can extend to outside of the body of the subject. Guidewire  324  can be removed by pulling subsequent to the deployment of one or more tissue anchors in order to anchor structure  322 . 
     Reference is now made to  FIGS. 3A-B  and  15 . It is to be noted that guidewire  324  can be coupled to a plurality of filaments  99  and can be shaped in any suitable shape. For example, a distal end of guidewire  324  may be helical. 
     It is to be noted that although system  320  is shown on mitral valve  64 , system  320  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 16A-C , which are schematic illustrations of a system  350  comprising an annulus-marking device  352  comprising a tubular stent body  354  having a central longitudinal axis  251  and a frame  356  coupled to a proximal end of tubular stent body  354  and projecting away from longitudinal axis  351  of stent body  354 , in accordance with some applications. Annulus-marking device  352  is configured for placement within a native heart valve of the subject, e.g., mitral valve  64 , as shown, the tricuspid valve, or any other cardiac valve. 
     Frame  356  is configured for placement along at least a part of a circumference of annulus  68  of valve  64 . In some applications, prior to implantation of an implant along annulus  68 , annulus-marking device is positioned within valve  64 . For some applications, tubular stent body  354  comprises two or more leaflets in order to regulate blood flow while device  352  is positioned in valve  64 . Device  352  is compressible during delivery toward valve  64  and expandable from a compressed state for positioning in the native heart valve  64 . Once device  352  is positioned within valve  64 , the valve is imaged using imaging, e.g., fluoroscopy. Frame  356  provides an indication as to the circumference of annulus  68 . 
     Device  352  may be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter. Additionally, device  352  is made from radiopaque material to facilitate fluoroscopic visualization. In some applications, tissue of valve annulus  68  and tissue coupled thereto is viewed using the frame  356 . Additionally, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  352  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing frame  356  against the tissue. In other words, viewing the tissue of the native heart valve annulus  68  and tissue coupled thereto comprises imaging annulus-marking device  352  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto when frame  356  is placed against the tissue. For some applications, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  352  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the frame  356  responsively to movement of the tissue. In other words, viewing the tissue of the native heart valve annulus  68  and tissue coupled thereto comprises imaging annulus-marking device  352  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto when frame  356  is moved responsively to movement of the tissue. 
     Subsequently to the positioning of device  352  within valve  64 , and under imaging, an implant comprising an annuloplasty structure  359  is positioned along annulus  68 , as shown in  FIG. 16B . Structure  359  comprises a body portion  260 , e.g., a tubular body portion, through which a plurality of anchors  355  are deployed. Structure  359  comprises a plurality of radiopaque markers  357 , which are positioned along structure  359  at respective longitudinal sites. The markers may provide an indication in a radiographic image (such as a fluoroscopy image) of how much of the body portion has been deployed at any given point during an implantation procedure, in order to enable setting a desired distance between the tissue anchors along the body portion. For some applications, the markers comprise radiopaque ink. For some applications the markers comprise a radiopaque material attached to or incorporated in body portion  164 . 
     Anchors  355  are delivered to valve  64  in order to anchor structure  359  to annulus  68  by deploying each anchor  355  of the plurality of anchors  355  within frame  356  of device  352 . In addition to the guidance under imaging provided by frame  356 , markers  357  of structure  359  aid in deployment of anchors  355 . 
     Following anchoring of structure  359  to annulus  68 , annulus-marking device  352  is constrained within a catheter such that tubular stent body  354  collapses and frame  356  slides proximally around structure  359  and proximally away from annulus  68 . Annulus-marking device  352  is retrieved and removed from the body of the subject. 
     Subsequently to the implanting of structure  359 , annulus-marking device  352  is retrieved. Since device  352  is flexible and compressible, device  352  is constrained within a tool during the retrieval of device  352  and subsequent removal of device  352  from the body of the subject. That is, device  352  does not function as an implant for such embodiments and is used only to guide implantation of annuloplasty structure  359  (i.e., the implant); rather, device  352  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of annuloplasty structure  359 . 
     It is to be noted that although system  350  is shown on mitral valve  64 , system  350  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 17A-C , which are schematic illustrations of a system  370  comprising an annulus-marking device  372  comprising a plurality of expandable elements  376  which form device  372  into a generally umbrella shape for facilitating imaging of cardiac tissue during implantation of a cardiac implant, in accordance with some applications. Device  372  comprises a flexible, radiopaque material, e.g., nitinol or stainless steel, which facilitates collapsing and expanding of device  372 . For some applications, the plurality of expandable elements  376  form device  372  into a generally pear shape. For some applications, the plurality of expandable elements  376  form device  372  into a partially-spherical shape. For some applications, the plurality of expandable elements  376  form device  372  into a partially-bulbous shape. 
     As shown, device  372  aids in imaging implantation of a cardiac implant, e.g., an annuloplasty structure  373 , as shown. Structure  373  comprises a body portion  375  which comprises a flexible material and has a longitudinal axis that runs along the length of body portion  375  (e.g., when the body portion is straightened). Body portion  375  comprises radiopaque markings  377  to aid in imaging for accurate delivery of anchors  379  to annulus  68  in order to anchor structure  373  to tissue of annulus  68 . 
     Annulus-marking device  372  is delivered using a delivery tool  374  which is configured to deliver device  372  to the left atrium in a compressed state. Device  372  is configured to be expanded from its compressed state once deployed from within a lumen of tool  374 . Annulus-marking device  372  is retrievable upon removal of delivery tool  374  from the subject. That is, device  372  is constrained within the lumen of tool  374  once the cardiac implant has been implanted at annulus  68 . Device  372  may be delivered percutaneously, thoracoscopically through the chest, or using open heart surgical techniques. If delivered percutaneously, device  372  may be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter and subsequently self-expand into the desired shape and tension when released from the catheter. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein under image guidance (e.g., fluoroscopic, ultrasonic, magnetic resonance, computed tomography, or combinations thereof). For some applications, device  372  comprises a wire. 
     Once inside the atrium, the plurality of expandable elements  376  expand radially within the atrium such that the plurality of expandable elements  376  provides an indication as to a location of the native heart valve annulus  68  of native heart valve  64 . It is to be noted that although device  372  is being used in the left atrium, device  372  may be used in the right atrium, the left ventricle, and the right ventricle. 
     The plurality of expandable elements  376  collectively form annulus-marking device  372  into a generally umbrella shape. 
     Annulus  68  is then imaged using fluoroscopy. In some applications, annulus-marking device  372  is imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing the plurality of expandable elements  376  against the tissue. For some applications, annulus-marking device  372  is imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing movement of the plurality of expandable elements  376  responsively to movement of the tissue. For either application, annulus-marking device  372  is imaged with respect to the tissue of the native heart valve annulus  68 , tissue of at least one leaflet, and tissue of an atrial wall  122 . 
     For some applications, in order to facilitate positioning of the plurality of expandable elements  376 , a guidewire  371  extends from within tool  374  and is disposed between leaflets  123  (e.g., posterior leaflet and anterior leaflet), often at a commissure of the valve. For some applications, a proximal section of each one of the plurality of expandable elements  376  is coupled to guidewire  371 . Guidewire  371  is at least partly stiff, and provides resistance, which facilitates positioning of the plurality of expandable elements  376 . Guidewire  371  may also provide tactile feedback to the operating physician. 
     In addition to mechanical effects such as biasing of the plurality of expandable elements  376  and providing tactile feedback, guidewire  371  may also facilitate positioning of the plurality of expandable elements  376  and/or of annuloplasty structure  373  by facilitating imaging. For example, the presence of guidewire  371  and/or the shape thereof (e.g., bending due to being pressed into the commissure) is visible in fluoroscopic imaging, and can be used to facilitate identification of the position and angle of the plurality of expandable elements  376  and/or of annuloplasty structure  373  with respect to tissues. 
     Guidewire  371  extends proximally through tool  374  and can extend to outside of the body of the subject. Guidewire  371  can be removed by pulling subsequent to the deployment of one or more tissue anchors  379 . 
     As shown, the plurality of expandable elements  376  comprise a plurality of curved wires each having a curved section  378  at a distal end portion thereof, as shown in  FIG. 17A . Structure  373  is placed within a concave section of each curved section  378  of the plurality of expandable elements  376 . 
     As shown in  FIGS. 17A-B , structure  373  is delivered subsequently to placement of the plurality of expandable elements  376 . It is to be noted that for some applications, structure  373  is delivered together with annulus-marking device  372 . For such applications, structure  373  is coupled to the plurality of expandable elements  376  in a manner in which curved sections  378  wrap around body portion  375  of structure  373 , and structure  373  is delivered toward the annulus within tool  374 . 
     A collective proximal diameter Di 1  of the proximal ends of the plurality of expandable elements  376  is smaller than a collective distal diameter Di 3  of the distal ends of the plurality of expandable elements  376 . A collective middle diameter Di 2  of the plurality of expandable elements  376  is greater than collective proximal diameter Di 1  and greater than collective distal diameter Di 3 . 
     Reference is now made to  FIGS. 3A-B  and  17 A-C. For some applications, annulus-marking device  372  is coupled to a plurality of radiopaque elements such as radiopaque filaments  99 , radiopaque markers, radiopaque wires, radiopaque extensions, radiopaque beads, etc. In some applications, annulus-marking device  372  and radiopaque elements or filaments  99  are imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing the plurality of expandable elements  376  and radiopaque elements or filaments  99  against the tissue. For some applications, annulus-marking device  372  is imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing movement of the plurality of expandable elements  376  and of radiopaque elements or filaments  99  responsively to movement of the tissue. For either application, annulus-marking device  372  and radiopaque elements or filaments  99  are imaged with respect to the tissue of the native heart valve annulus  68 , tissue of at least one leaflet, and tissue of an atrial wall  122 . 
     Reference is again made to  FIG. 17B . Annuloplasty structure  373  is implanted under the guidance of fluoroscopy using annulus-marking device  372  as a guide. Curved sections  378  are positioned between annuloplasty structure  373  and atrial wall  122 . A respective anchor  379  is deployed to anchor structure  373  at a site along annulus  68  that is marked between successive curved wires of elements  376 . 
     Following anchoring of structure  373  to annulus  68 , annulus-marking device  372  is constrained within tool  374 . As device  372  is constrained, elements  376  slide from under annuloplasty structure  373  implanted along annulus  68 . Annulus-marking device  372  is retrieved and removed from the body of the subject. During the retrieving of annulus-marking device  372 , curved sections  378  of elements  376  are sliding from under annuloplasty structure  373  implanted along annulus  68 . Annulus-marking device  372  is retrieved and removed from the body of the subject. 
     Subsequently to implanting of structure  373 , annulus-marking device  372  is retrieved. Since device  372  is flexible and compressible, device  372  is constrained within a tool during the retrieval of device  372  and subsequent removal of device  372  from the body of the subject. That is, device  372  does not function as an implant for such embodiments and is used only to guide implantation of annuloplasty structure  373  (i.e., the implant); rather, device  372  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of annuloplasty structure  373 . 
     It is to be noted that although system  370  is shown on mitral valve  64 , system  370  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 18 , which is a schematic illustration of a system  360  comprising an annulus-marking device  362  comprising a guidewire  364  that has a distal end portion that is curved upwards and bends toward a ventricular surface  361  of valve  64 , in accordance with some applications. The distal end portion of guidewire  364  is configured to contact the ventricular surface of annulus  68  and/or the ventricular wall. Guidewire  364  is configured to facilitate imaging of annulus  68  by imaging movement of the distal end portion thereof along a perimeter of the ventricular surface of annulus  68 . 
     Guidewire  364  extends from within a delivery tool  365  and is disposed between leaflets  123  (e.g., posterior leaflet and anterior leaflet). Guidewire  364  has a distal curved section that curves upward toward ventricular surface  361 . Guidewire  364  is at least partly stiff, which facilitates imaging-guided (e.g., under fluoroscopy) positioning of a cardiac device such as an annuloplasty structure (not shown). Guidewire  364  may also provide tactile feedback to the operating physician. For example, the presence of guidewire  364  and/or the shape thereof (e.g., bending due to being pressed into the commissure) is visible in fluoroscopic imaging, and can be used to facilitate identification of the position and angle of an annuloplasty structure with respect to tissues. 
     Guidewire  364  extends proximally through tool  365  and can extend to outside of the body of the subject. Guidewire  364  can be removed by pulling subsequent to the deployment of one or more tissue anchors in order to anchor the annuloplasty structure to annulus  68 . 
     In some applications, guidewire  364  moves circumferentially around a ventricular surface  361  of annulus  68  in order to generate a map  366 . For some applications, map  366  is generated prior to implantation of the annuloplasty structure. For some applications, guidewire  364  moves in conjunction with implantation of the annuloplasty structure at the atrial surface of annulus  68 . 
     Reference is now made to  FIGS. 3A-B  and  18 . It is to be noted that guidewire  364  can be coupled to a plurality of elements or filaments  99  and can be shaped in any suitable shape. For example, a distal end of guidewire  364  may be helical. 
     It is to be noted that although system  360  is shown on mitral valve  64 , system  360  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 19A-C , which are schematic illustrations of a system  380  comprising an annulus-marking device  382  comprising a plurality of expandable elements  384  which form device  382  into a generally umbrella shape for facilitating imaging of cardiac tissue during implantation of a cardiac implant, in accordance with some applications. Device  382  comprises a flexible, radiopaque material, e.g., nitinol or stainless steel, which facilitates collapsing and expanding of device  382 . For some applications, the plurality of expandable elements  384  form device  382  into a generally pear shape. For some applications, the plurality of expandable elements  384  form device  382  into a partially-spherical shape. For some applications, the plurality of expandable elements  384  form device  382  into a partially-bulbous shape. 
     As shown, device  382  aids in imaging implantation of a cardiac implant, e.g., an annuloplasty structure  386 , as shown. Structure  386  comprises a body portion which comprises a flexible material and has a longitudinal axis that runs along the length of the body portion (e.g., when the body portion is straightened). The body portion comprises radiopaque markings  387  to aid in imaging for accurate delivery of anchors to annulus  68  in order to anchor structure  386  to tissue of annulus  68 . 
     Annulus-marking device  382  is delivered using a delivery tool  381  which is configured to deliver device  382  to the left atrium in a compressed state. Device  382  is configured to be expanded from its compressed state once deployed from within a lumen of tool  381 . Annulus-marking device  382  is retrievable upon removal of delivery tool  381  from the subject. That is, device  382  is constrained within the lumen of tool  381  once the cardiac implant has been implanted at annulus  68 . Device  382  may be delivered percutaneously, thoracoscopically through the chest, or using open heart surgical techniques. If delivered percutaneously, device  382  may be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter and subsequently self-expand into the desired shape and tension when released from the catheter. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein under image guidance (e.g., fluoroscopic, ultrasonic, magnetic resonance, computed tomography, or combinations thereof). For some applications, device  382  comprises a wire. 
     For some applications, the plurality of expandable elements  384  guide tool  381  to the appropriate place along annulus  68 . 
     Once inside the atrium, the plurality of expandable elements  384  expand radially within the atrium such that the plurality of expandable elements  384  provides an indication as to a location of the native heart valve annulus  68  of native heart valve  64 . In some applications, the plurality of expandable elements  384  comprise a shape-memory material that enables elements  384  to expand to a given shape within the heart. It is to be noted that although device  382  is being used in the left atrium, device  382  may be used in the right atrium, the left ventricle, and the right ventricle. 
     The plurality of expandable elements  384  collectively form annulus-marking device  382  into a generally umbrella shape. 
     Annulus  68  is then imaged using fluoroscopy. In some applications, annulus-marking device  382  is imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing the plurality of expandable elements  384  against the tissue. For some applications, annulus-marking device  382  is imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing movement of the plurality of expandable elements  384  responsively to movement of the tissue. For either application, annulus-marking device  382  is imaged with respect to the tissue of the native heart valve annulus  68 , tissue of at least one leaflet, and tissue of an atrial wall  122 . 
     As shown in  FIG. 19A , structure  386  is delivered subsequently to placement of the plurality of expandable elements  384 . It is to be noted that for some applications, structure  386  is delivered together with annulus-marking device  382 . Annuloplasty structure  386  is implanted under the guidance of fluoroscopy using annulus-marking device  382  as a guide. Annuloplasty structure  386  is positioned between annulus-marking device  382  and atrial wall  122 . A respective anchor is deployed to anchor structure  386  at a site along annulus  68  that is marked between successive curved wires of elements  384  and in between radiopaque markings  387  of structure  386 . 
     Reference is now made to  FIG. 19A . A collective proximal diameter Di 1  of the proximal ends of the plurality of expandable elements  384  is smaller than a collective distal diameter Di 3  of the distal ends of the plurality of expandable elements  384 . A collective middle diameter Di 2  of the plurality of expandable elements  384  is greater than collective proximal diameter Di 1  and greater than collective distal diameter Di 3 . 
     Reference is now made to  FIGS. 19A-C . The plurality of expandable elements  384  comprise a very flexible material and design that allows elements  384  to assume the shape of the cavity that they are opened in, e.g., the left atrium, as shown. 
     In  FIG. 19A , a respective distal end  385  of each expandable element  384  of device  382  is positioned within the atrium. That is, distal ends  385  remain at the atrial surface of annulus  68 . 
     For some applications, distal ends  385  are pushed into the ventricle, as shown in  FIG. 19B . In such applications, the plurality of expandable elements  384  are each made to bend at a middle section thereof collectively forming a bent section  389 , and it is at this bent section  389  that the operating physician determines using imaging that this is the location of the annulus. 
     Reference is now made to  FIG. 19C . For some applications, a radiopaque helical stent  388  is delivered to the ventricle between the plurality of expandable elements  384 , and between leaflets  123  of valve  64 . As shown, a distal end portion comprising a distal end of stent  388  is positioned within the ventricle and is sued for imaging the ventricle. 
     Reference is now made to  FIGS. 3A-B  and  19 A-C. For some applications, annulus-marking device  382  is coupled to a plurality of radiopaque elements, such as radiopaque filaments  99 . In some applications, annulus-marking device  382  and radiopaque elements or filaments  99  are imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing the plurality of expandable elements  384  and radiopaque elements or filaments  99  against the tissue. For some applications, annulus-marking device  382  is imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing movement of the plurality of expandable elements  384  and of radiopaque elements or filaments  99  responsively to movement of the tissue. For either application, annulus-marking device  382  and radiopaque elements or filaments  99  are imaged with respect to the tissue of the native heart valve annulus  68 , tissue of at least one leaflet, and tissue of an atrial wall  122 . 
     Following anchoring of structure  386  to annulus  68 , annulus-marking device  382  is constrained within tool  381 . Annulus-marking device  382  is retrieved and removed from the body of the subject. 
     Subsequently to implanting of structure  386 , annulus-marking device  382  is retrieved. Since device  382  is flexible and compressible, device  382  is constrained within a tool during the retrieval of device  382  and subsequent removal of device  382  from the body of the subject. That is, device  382  does not function as an implant for such embodiments and is used only to guide implantation of annuloplasty structure  386  (i.e., the implant); rather, device  382  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of annuloplasty structure  386 . 
     It is to be noted that although system  380  is shown on mitral valve  64 , system  380  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 19A-C . It is to be noted that annulus-marking device  382  can comprises a braided mesh of radiopaque material, e.g., fabric or metal. For some applications, annulus-marking device  382  comprises an expandable radiopaque balloon. 
     Reference is now made to  FIGS. 20A-B , which are schematic illustrations of a system  390  comprising an annulus-marking device  392  comprising a toroidal stent  394 , in accordance with some applications. Toroidal stent  394  is configured to be compressed and constrained within a delivery system. Stent  394  expands within the atrium and is placed against annulus tissue at a surface of valve  64 . For some applications, stent  394  may be placed along an atrial surface. For some applications, stent  394  may be placed along a ventricular surface. Annulus-marking device  392  comprises radiopaque material 
     For some applications, stent  394  is shaped so as to define a complete toroid. For some applications, stent  394  is shaped so as to define a discontinuous generally-toroidal shape. For such applications, opposing ends of stent  394  may overlap to form stent  394  into a toroid. For some applications, stent  394  can be shaped as a coil having a perimeter that corresponds to a perimeter of annulus  68  of valve  64 . 
     Device  392  may be delivered percutaneously, thoracoscopically through the chest, or using open heart surgical techniques. If delivered percutaneously, device  392  may be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter and subsequently self-expand into the desired shape and tension when released from the catheter. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein under image guidance (e.g., fluoroscopic, ultrasonic, magnetic resonance, computed tomography, or combinations thereof). For some applications, device  392  comprises a plurality of struts, e.g., a plurality of wires. 
     For some applications, stent  394  comprises a plurality of radiopaque elements, such as radiopaque filaments  99 , at least at an inner surface of stent  394 . In some implementations, the plurality of radiopaque elements or filaments projects inwardly toward the orifice of valve  64 . The plurality of radiopaque elements or filaments  99  comprise radiopaque material (e.g., nitinol or stainless steel) and can be configured to be extremely flexible. Radiopaque elements or filaments  99  are configured for aiding implantation of cardiac devices, e.g., an annuloplasty structure  396 , under the guidance of imaging, in accordance with some applications. Implantation of annuloplasty structure  396  is performed with the aid of imaging, such as fluoroscopy, transesophageal echo, and/or echocardiography. 
     For some applications, radiopaque elements or filaments  99  sway with movement of the blood. For some applications, radiopaque elements or filaments  99  press against tissue of the annulus and tissue coupled thereto, such as tissue of an atrial wall  122  as well as tissue of leaflets  123  of the native valve (as shown). Radiopaque elements or filaments  99  thus provide enhanced imaging of tissue of valve  64 . For example, in some embodiments, when radiopaque filaments  99  appear bent or pressed, this imaging detects annulus tissue, while when filaments  99  are straight, this could indicate the orifice of the valve. 
     Device  392  can be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter. Additionally, device  392  is made from radiopaque material to facilitate fluoroscopic visualization. In some applications, tissue of valve annulus  68  and tissue coupled thereto is viewed by imaging stent  394 . Additionally, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  392  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing the stent  394  against the tissue. For some applications, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  392  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of the stent  394  responsively to movement of the tissue. 
     Reference is now made to  FIG. 20B , which shows implantation of annuloplasty structure  396  which comprises a body portion  399 . Body portion  399  comprises a flexible material, e.g., a braided fabric mesh. For some applications, body portion  399  is shaped so as to define a sleeve shaped so as to define a lumen therethrough, as shown. For some applications, body portion  399  is flat. 
     Body portion  399  can comprise a braided fabric mesh, e.g., comprising DACRON™. Body portion  399  can be configured to be placed only partially around a cardiac valve annulus (e.g., to assume a C-shape), and, once anchored in place, to be contracted so as to circumferentially tighten the valve annulus. Optionally, structure  396  can be configured to be placed entirely around the valve annulus (e.g., as a closed circle or other closed shape). In order to tighten the annulus, annuloplasty structure  396  comprises a flexible elongated contracting member that extends along body portion  399 . 
     Structure  396  comprises a plurality of radiopaque markers  398 , which are positioned along structure  396  at respective longitudinal sites. The markers may provide an indication in a radiographic image (such as a fluoroscopy image) of how much of the body portion has been deployed at any given point during an implantation procedure, in order to enable setting a desired distance between the tissue anchors along the body portion. For some applications, the markers comprise radiopaque ink. For some applications the markers comprise a radiopaque material attached to or incorporated in body portion  164 . 
     As shown, structure  396  is implanted between an external surface of stent  394  and tissue of atrial wall  122 . 
     Subsequently to implanting of structure  396 , annulus-marking device  392  is retrieved. Since device  392  is flexible and compressible, device  392  is constrained within a tool during the retrieval of device  392  and subsequent removal of device  392  from the body of the subject. That is, device  392  does not function as an implant for such embodiments and is used only to guide implantation of annuloplasty structure  396  (i.e., the implant); rather, device  392  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of annuloplasty structure  396 . 
     It is to be noted that although system  390  is shown on mitral valve  64 , system  390  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 21-27 , which are schematic illustrations of respective annulus marking devices comprising implant-leading devices for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications. 
     Reference is now made to  FIG. 21 , which is a schematic illustration of a system  330  comprising an annulus-marking device  332  comprising an implant-leading device  336  that runs in advance of an implant, e.g., an annuloplasty structure, in its implantation path, upstream thereof, in accordance with some applications. Implant-leading device  336  extends from within a delivery tool  331  and travels along a perimeter of annulus  68 . As successive portions of the annuloplasty structure are extended from within delivery tool  331  and are positioned along successive portions of annulus  68 , implant-leading device  336  guides the successive portions of the annuloplasty structure under imaging as implant-leading device  336  comprises a radiopaque material (e.g., nitinol or stainless steel). Implant-leading device  336  comprises a wire  337  and at least one generally flat, geometric radiopaque unit  335  at a distal end thereof, e.g., a square of radiopaque material. For some applications, as shown, implant-leading device  336  comprises wire  337  and three generally flat, geometric radiopaque units  335  at a distal end thereof. 
     Implant-leading device  336  is relatively small and provides an indication of a specific section of annulus  68  immediately preceding the placement of the successive portion of the annuloplasty structure along annulus  68 . Implant-leading device  336  is at least partly stiff, and provides resistance, which facilitates positioning of the annuloplasty structure. Implant-leading device  336  may also provide tactile feedback to the operating physician. 
     The annuloplasty structure comprises a body portion  333  which comprises a flexible material and has a longitudinal axis that runs along the length of the body portion (e.g., when implant is straightened). Body portion  333  comprises radiopaque markings  334  to aid in imaging for accurate delivery of anchors to annulus  68  in order to anchor the annuloplasty structure to tissue of annulus  68 . 
     In addition to providing tactile feedback, implant-leading device  336  may also facilitate positioning of the annuloplasty structure by facilitating imaging (e.g., fluoroscopy) and mechanical guidance. For example, the presence of implant-leading device  336  and/or the shape thereof (e.g., bending due to being pressed against an atrial wall) is visible in fluoroscopic imaging, and can be used to facilitate identification of the position and angle of the annuloplasty structure with respect to tissues. 
     Implant-leading device  336  extends proximally through tool  331  and can extend to outside of the body of the subject. Implant-leading device  336  can be removed by pulling subsequent to the deployment of one or more tissue anchors in order to anchor the annuloplasty structure. In some applications, implant-leading device  336  is constrained within tool  331  in order to be retrieved and removed from the body of the subject. 
     Reference is now made to  FIGS. 3A-B  and  21 . It is to be noted that implant-leading device  336  can be coupled to a plurality of elements or filaments  99  and can be shaped in any suitable shape. 
     It is to be noted that although system  330  is shown on mitral valve  64 , system  330  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 22 , which is a schematic illustration of a system  340  comprising an annulus-marking device  344  comprising an implant-leading device  345  that runs in advance of an implant, e.g., an annuloplasty structure  341 , in its implantation path, upstream thereof, in accordance with some applications. Annulus-marking device  344  extends from within a delivery tool  343  and travels along a perimeter of annulus  68 . As successive portions of annuloplasty structure  341  are extended from within delivery tool  343  and are positioned along successive portions of annulus  68 , annulus-marking device  344  guides the successive portions of structure  341  under imaging as annulus-marking device  344  comprises a radiopaque material (e.g., nitinol or stainless steel). Annulus-marking device  344  comprises (1) a wire that is shaped in a loop and, for some applications, (2) a structure comprising a mesh  346 . The wire is generally deflectable to be pushed against tissue of annulus  68  and abut tissue of annulus  68  such that annulus-marking device  344  facilitates providing an image of a large percentage, e.g., at least 50% or at least 60%, of a perimeter of annulus  68 . Additionally, the wire of annulus-marking device  344  pushes against a first portion of annulus  68  in order to apply a pushing force to the opposite portion of annulus  68  (i.e., the portion of annulus  68  at which delivery tool  343  is positioned) such that system  340  ensures that annuloplasty structure  341  is properly positioned at a juncture between tissue of annulus  68  and tissue of atrial wall  122 . In such a manner, device  344  ensures that annuloplasty structure  341  is positioned outside the external perimeter of the wire that is shaped as a loop, responsively to the pushing of device  344  against tissue of annulus  68  such that annuloplasty structure  341  is implanted along annulus  68  and not on leaflet  123 . For some applications, device  344  does not lead the implant, but rather, device  344  is positioned in conjunction with implantation of the implant. 
     Mesh  346  also spans a portion of the orifice of the valve at leaflets  123  and comprises a braided, radiopaque mesh that does not interfere with blood flow during implantation of annuloplasty structure  341  on a beating heart. Annulus-marking device  344  is at least partly stiff, and provides resistance, which facilitates positioning of structure  341 . Annulus-marking device  344  may also provide tactile feedback to the operating physician. 
     It is to be noted that device  344  may be provided without mesh  346  such that device  344  comprises only the wire that is shaped in a loop. 
     Structure  341  comprises a body portion which comprises a flexible material and has a longitudinal axis that runs along the length of the body portion (e.g., when the body portion is straightened). The body portion comprises radiopaque markings  342  to aid in imaging for accurate delivery of anchors to annulus  68  in order to anchor structure  341  to tissue of annulus  68 . 
     In addition to providing tactile feedback, annulus-marking device  344  may also facilitate positioning of the annuloplasty structure  341  by facilitating imaging (e.g., fluoroscopy) and mechanical guidance. For example, the presence of annulus-marking device  344  and/or the shape thereof (e.g., bending due to being pressed against an atrial wall) is visible in fluoroscopic imaging, and can be used to facilitate identification of the position and angle of annuloplasty structure  341  with respect to tissues. 
     Annulus-marking device  344  extends proximally through tool  343  and can extend to outside of the body of the subject. Annulus-marking device  344  can be removed by pulling subsequent to the deployment of one or more tissue anchors in order to anchor structure  341 . In some applications, annulus-marking device  344  is constrained within tool  343  in order to be retrieved and removed from the body of the subject. For some applications, device  344  is delivered together with the annuloplasty structure  341  in tool  343 . For some applications, device  344  is delivered in a separate tool to tool  343  used to deliver annuloplasty structure  341 . 
     Reference is now made to  FIGS. 3A-B  and  22 . It is to be noted that annulus-marking device  344  can be coupled to a plurality of elements or filaments  99  and can be shaped in any suitable shape. 
     It is to be noted that although system  340  is shown on mitral valve  64 , system  340  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 23 , which is a schematic illustration of a system  400  comprising an annulus-marking device  402  comprising an implant-leading device  404  that runs in advance of an implant, e.g., an annuloplasty structure  406 , in its implantation path, upstream thereof, in accordance with some applications. Implant-leading device  404  extends from within a delivery tool  405  and travels along a perimeter of annulus  68 . As successive portions of annuloplasty structure  406  are extended from within delivery tool  405  and are positioned along successive portions of annulus  68 , implant-leading device  404  guides the successive portions of structure  406  under imaging as implant-leading device  404  comprises a radiopaque material (e.g., nitinol or stainless steel). Implant-leading device  404  comprises a wire and a generally flat, bulbous radiopaque unit at a distal end thereof, e.g., a teardrop-shaped structure, comprising radiopaque material. For such applications, implant-leading device  404  is relatively small and provides an indication of a specific section of annulus  68  immediately preceding the placement of the successive portion of structure  406  along annulus  68 . Implant-leading device  404  is at least partly stiff, and provides resistance, which facilitates positioning of structure  406 . Implant-leading device  404  may also provide tactile feedback to the operating physician. 
     Structure  406  comprises a body portion which comprises a flexible material and has a longitudinal axis that runs along the length of the body portion (e.g., when the body portion is straightened). The body portion comprises radiopaque markings  408  to aid in imaging for accurate delivery of anchors to annulus  68  in order to anchor structure  406  to tissue of annulus  68 . 
     In addition to providing tactile feedback, implant-leading device  404  may also facilitate positioning of the annuloplasty structure  406  by facilitating imaging (e.g., fluoroscopy) and mechanical guidance. For example, the presence of implant-leading device  404  and/or the shape thereof (e.g., bending due to being pressed against an atrial wall  122 ) is visible in fluoroscopic imaging, and can be used to facilitate identification of the position and angle of annuloplasty structure  406  with respect to tissues. 
     Implant-leading device  404  extends proximally through tool  405  and can extend to outside of the body of the subject. Implant-leading device  404  can be removed by pulling subsequent to the deployment of one or more tissue anchors in order to anchor structure  406 . In some applications, implant-leading device  404  is constrained within tool  405  in order to be retrieved and removed from the body of the subject. 
     Reference is now made to  FIGS. 3A-B  and  23 . It is to be noted that implant-leading device  404  can be coupled to a plurality of elements or filaments  99  and can be shaped in any suitable shape. 
     It is to be noted that although system  400  is shown on mitral valve  64 , system  400  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 24 , which is a schematic illustration of a system  410  comprising an annulus-marking device  412  comprising an implant-leading device  414  that runs in advance of an implant, e.g., an annuloplasty structure  413 , in its implantation path, upstream thereof, in accordance with some applications. Implant-leading device  414  extends from within a delivery tool  411  and travels along a perimeter of annulus  68 . As successive portions of annuloplasty structure  413  are extended from within delivery tool  411  and are positioned along successive portions of annulus  68 , implant-leading device  414  guides the successive portions of structure  413  under imaging as implant-leading device  414  comprises a radiopaque material (e.g., nitinol or stainless steel). Implant-leading device  414  comprises a deflectable wire shaped in a petal-shaped loop, e.g., a puddle-shaped structure, comprising radiopaque material. For such applications, implant-leading device  414  facilitates providing an image of a large percentage, e.g., at least 30% or at least 40%, of valve  64 . Device  414  spans a portion of the orifice of the valve at leaflets  123 , and the wireframe of device  414  does not interfere with blood flow during implantation of annuloplasty structure  413  on a beating heart. Implant-leading device  414  is at least partly stiff, and provides resistance, which facilitates positioning of structure  413 . Implant-leading device  414  may also provide tactile feedback to the operating physician. 
     Structure  413  comprises a body portion which comprises a flexible material and has a longitudinal axis that runs along the length of the body portion (e.g., when the body portion is straightened). The body portion comprises radiopaque markings  415  to aid in imaging for accurate delivery of anchors to annulus  68  in order to anchor structure  413  to tissue of annulus  68 . 
     In addition to providing tactile feedback, implant-leading device  414  may also facilitate positioning of the annuloplasty structure  413  by facilitating imaging (e.g., fluoroscopy) and mechanical guidance. For example, the presence of implant-leading device  414  and/or the shape thereof (e.g., bending due to being pressed against an atrial wall  122 ) is visible in fluoroscopic imaging, and can be used to facilitate identification of the position and angle of annuloplasty structure  413  with respect to tissues. 
     Implant-leading device  414  extends proximally through tool  411  and can extend to outside of the body of the subject. Implant-leading device  414  can be removed by pulling subsequent to the deployment of one or more tissue anchors in order to anchor structure  413 . In some applications, implant-leading device  414  is constrained within tool  411  in order to be retrieved and removed from the body of the subject. 
     Reference is now made to  FIGS. 3A-B  and  24 . It is to be noted that implant-leading device  414  can be coupled to a plurality of elements or filaments  99  and can be shaped in any suitable shape. 
     It is to be noted that although system  410  is shown on mitral valve  64 , system  410  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 25 , which is a schematic illustration of a system  420  comprising an annulus-marking device  422  comprising an implant-leading device  424  that runs in advance of an implant, e.g., an annuloplasty structure  425 , in its implantation path, upstream thereof, in accordance with some applications. Implant-leading device  424  extends from within a delivery tool  421  and travels along a perimeter of annulus  68 . As successive portions of annuloplasty structure  425  are extended from within delivery tool  421  and are positioned along successive portions of annulus  68 , implant-leading device  424  guides the successive portions of structure  425  under imaging as implant-leading device  424  comprises a radiopaque material (e.g., nitinol or stainless steel). Implant-leading device  424  comprises a deflectable wire shaped in a petal-shaped loop, e.g., a pear-shaped structure, comprising radiopaque material. For such applications, implant-leading device  424  is relatively small and provides an indication of a specific section of annulus  68  immediately preceding the placement of the successive portion of structure  425  along annulus  68 . Implant-leading device  424  is at least partly stiff, and provides resistance, which facilitates positioning of structure  425 . Implant-leading device  424  may also provide tactile feedback to the operating physician. 
     Structure  425  comprises a body portion which comprises a flexible material and has a longitudinal axis that runs along the length of the body portion (e.g., when the body portion is straightened). The body portion comprises radiopaque markings  423  to aid in imaging for accurate delivery of anchors to annulus  68  in order to anchor structure  425  to tissue of annulus  68 . 
     In addition to providing tactile feedback, implant-leading device  424  may also facilitate positioning of the annuloplasty structure  425  by facilitating imaging (e.g., fluoroscopy) and mechanical guidance. For example, the presence of implant-leading device  424  and/or the shape thereof (e.g., bending due to being pressed against an atrial wall  122 ) is visible in fluoroscopic imaging, and can be used to facilitate identification of the position and angle of annuloplasty structure  425  with respect to tissues. 
     Implant-leading device  424  extends proximally through tool  421  and preferably to outside of the body of the subject. Implant-leading device  424  can be removed by pulling subsequent to the deployment of one or more tissue anchors in order to anchor structure  425 . In some applications, implant-leading device  424  is constrained within tool  421  in order to be retrieved and removed from the body of the subject. 
     Reference is now made to  FIGS. 3A-B  and  25 . It is to be noted that implant-leading device  424  can be coupled to a plurality of elements or filaments  99  and can be shaped in any suitable shape. 
     It is to be noted that although system  420  is shown on mitral valve  64 , system  420  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 26 , which is a schematic illustration of a system  430  comprising an annulus-marking device  432  comprising an implant-leading device  434  that runs alongside an implant, e.g., an annuloplasty structure  435 , in accordance with some applications. For some applications, device  432  is integral with the body portion of structure  435 . Implant-leading device  434  extends from within a delivery tool  431  and travels along a perimeter of annulus  68 . As successive portions of annuloplasty structure  435  are extended from within delivery tool  431  and are positioned along successive portions of annulus  68 , implant-leading device  434  guides the successive portions of structure  435  under imaging as implant-leading device  434  comprises a radiopaque material (e.g., nitinol or stainless steel). Implant-leading device  434  comprises a deflectable radiopaque wire or a radiopaque fabric. Implant-leading device  434  may also provide tactile feedback to the operating physician. 
     Structure  435  comprises a body portion which comprises a flexible material and has a longitudinal axis that runs along the length of the body portion (e.g., when body portion is straightened). The body portion comprises radiopaque markings  436  to aid in imaging for accurate delivery of anchors to annulus  68  in order to anchor structure  435  to tissue of annulus  68 . 
     In addition to providing tactile feedback, implant-leading device  434  may also facilitate positioning of the annuloplasty structure  435  by facilitating imaging (e.g., fluoroscopy) and mechanical guidance. For example, the presence of implant-leading device  434  and/or the shape thereof (e.g., bending due to being pressed against an atrial wall  122 ) is visible in fluoroscopic imaging, and can be used to facilitate identification of the position and angle of annuloplasty structure  435  with respect to tissues. 
     Reference is now made to  FIGS. 3A-B  and  26 . It is to be noted that implant-leading device  434  can be coupled to a plurality of elements or filaments  99  and can be shaped in any suitable shape. 
     It is to be noted that although system  430  is shown on mitral valve  64 , system  430  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 27 , which is a schematic illustration of a system  440  comprising an annulus-marking device  442  that is positioned in the orifice of valve  64  in advance of an implant, e.g., an annuloplasty structure  446 , in accordance with some applications. For some applications, annulus-marking device  442  leads and is placed in advance of the implant such that device  442  functions as an implant-leading device  444 . Annulus-marking device  442  extends from within a delivery tool  447 . For some applications, annulus-marking device  442  comprises a stent-like mesh, e.g., a fabric or metal mesh, that is positioned partially within the orifice of valve  64  and does not significantly interfere with function of valve  64 . For some applications, annulus-marking device  442  comprises a tubular stent. As successive portions of annuloplasty structure  446  are extended from within its delivery tool  445  and are positioned along successive portions of annulus  68 , annulus-marking device  442  guides the successive portions of structure  446  under imaging as annulus-marking device  442  comprises a radiopaque material (e.g., nitinol or stainless steel). For some applications, annulus-marking device  442  comprises a balloon made of nylon. 
     Annulus-marking device  442  provides an indication of a specific section of annulus  68  immediately preceding the placement of the successive portion of structure  446  along annulus  68 . Annulus-marking device  442  is at least partly stiff, and provides resistance, which facilitates positioning of structure  446 . Annulus-marking device  442  may also provide tactile feedback to the operating physician. 
     Annuloplasty structure  446  comprises a body portion which comprises a flexible material and has a longitudinal axis that runs along the length of the body portion (e.g., when the body portion is straightened). The body portion comprises radiopaque markings to aid in imaging for accurate delivery of anchors to annulus  68  in order to anchor structure  446  to tissue of annulus  68 . 
     In addition to providing tactile feedback, annulus-marking device  442  may also facilitate positioning of the annuloplasty structure  446  by facilitating imaging (e.g., fluoroscopy) and mechanical guidance. For example, the presence of annulus-marking device  442  and/or the shape thereof (e.g., bending due to being pressed against an atrial wall) is visible in fluoroscopic imaging, and can be used to facilitate identification of the position and angle of annuloplasty structure  446  with respect to tissues. Additionally, annulus-marking device  442  ensures that tool  445  is positioned at an external perimeter of annulus-marking device  442  such that it is positioned between device  442  and atrial wall  122 . 
     Annulus-marking device  442  can be removed by being pulled and constrained within tool  447  in order to be retrieved and removed from the body of the subject. 
     In some applications, as shown, annulus-marking device  442  is delivered toward valve  64  in a delivery tool  447  that is separate from delivery tool  445  used to deliver the implant. For some applications, annulus-marking device  442  and the implant may be delivered from the same delivery tool. 
     Reference is now made to  FIGS. 3A-B  and  27 . It is to be noted that annulus-marking device  442  can be coupled to a plurality of elements or filaments  99  and can be shaped in any suitable shape. 
     It is to be noted that although system  440  is shown on mitral valve  64 , system  440  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. It is to be further noted that although device  442  is shown as being delivered via the ventricle, device  442  may be delivered to the valve using any suitable delivery method into the atrium, e.g., transvascularly or using a minimally-invasive approach. 
     Reference is again made to  FIGS. 21-27 . It is to be noted that all implant-leading devices can be coupled to an electronic beeping gauge, e.g., a multimeter. For some applications, the implant-leading devices may be delivered to and removed from the valve using the same delivery tool as that which delivers the annuloplasty structure. For some applications, the implant-leading devices may be delivered to and removed from the valve using a separate delivery tool to the delivery tool that delivers the annuloplasty structure. For some applications, the implant-leading devices move responsively to movement of cardiac tissue such as tissue of annulus  68 , tissue of atrial wall  122 , and/or tissue of leaflets  123 . Under imaging, such movement of the respective implant-leading devices responsively to movement of a given tissue, provides an indication as to the tissue types and an indication as to the layout of valve  64  in order to more efficiently facilitate implantation of the cardiac device. 
     Reference is now made to  FIGS. 28A-B , which are schematic illustrations of a system  450  comprising an annulus-marking device  452  comprising a plurality of radiopaque pins  454  which facilitate implantation of an implant, e.g., an annuloplasty structure  458 , in accordance with some applications. Pins  454  comprise flexible, radiopaque material, e.g., nitinol or stainless steel. Each pin  454  comprises a respective barb  453  in order to ensure pin  454  remains within tissue of annulus  68 . Each one of the plurality of pins has a longest width of 0.5-3.0 mm. It is to be noted that pins  454  function as indicator pins and the function of barb  453  is to anchor only themselves to tissue. Pins  454  do not function to anchor any device to tissue except for themselves; therefore, pins  454  function only as radiopaque indicator pins. 
     Pins  454  are implanted using a delivery system  456  which operates under imaging, e.g., echocardiography or fluoroscopy, and using tactile feedback in order to deploy a plurality of pins  454  along annulus  68 . For some applications, as shown, a proximal portion of each pin  454  is exposed from tissue of annulus. For some applications, pins  454  are deployed fully within tissue of annulus  68 . 
     Once the plurality of pins  454  have been deployed, a map is generated under imaging. For some applications, a map is not generated, but rather, pins  454  function as markers real-time during placement of the implant along annulus  68 . In either embodiment, pins  454  facilitate imaging of valve  64 . For some applications, pins  454  facilitate imaging of the heart valve annulus by facilitating imaging of movement of the plurality of pins responsively to movement of the annulus. 
     It is to be noted that although system  450  is shown on mitral valve  64 , system  450  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
       FIG. 28B  shows implantation of annuloplasty structure  458  at annulus  68 . Structure  458  comprises a body portion  457  which comprises a flexible material and has a longitudinal axis that runs along the length of body portion  457  (e.g., when the body portion is straightened). Body portion  457  comprises radiopaque markings  459  to aid in imaging for accurate delivery of anchors to annulus  68  in order to anchor structure  458  to tissue of annulus  68 . A plurality of tissue anchors  455  are deployed in order to fasten and anchor structure  458  to tissue of annulus  68 . Pins  454  do not interfere with placement of structure  458  and/or with deploying of anchors  455 . For some applications, each anchor  455  is deployed in a vicinity of pin  454  and between markings  459  of structure  458 . For some applications, a larger number of tissue anchors  455  are deployed than the number of pins  454  deployed in annulus  68 . 
     Once structure  458  is implanted at annulus  68 , pins  454  remain within tissue of annulus  68 . 
     Reference is now made to  FIG. 29 , which is a schematic illustration of a system  460  for facilitating imaging of annulus  68  of heart valve  64  using an annulus-marking device  462  comprising a plurality of radiopaque pins  464 , in accordance with some applications. Pins  464  are disposed within a distal end of a catheter  461  and move proximally in response to a force applied thereto. Pins  464  move distally once that force is removed. If no force is applied to pins  464 , they remain static. Pins  464  comprise a radiopaque material that is visible under imaging, e.g., fluoroscopy. Pins  464  move proximally and distally in response to variations in the topography of the tissue of annulus  68 . 
     The movement of pins  464  indicated valleys  466  and peaks  468  in tissue of annulus  68 . That is, each pin  464  has a distal end  465  that pushes against tissue of annulus  68 . In response, tissue of annulus  68  applied a force to pins  464 . For some applications, an image of annulus  68  is generated, and based on that image, an implant, e.g., an annuloplasty structure, is implanted using the image pins  464  generated as a guide. For some applications, the implant is implanted during movement of pins  464  along annulus  68 . For some applications, the implant is implanted only once pins  464  have moved fully around annulus  68  and have generated a map of the topography of valve  64 . In either embodiment, the implant is implanted under the guidance of imaging. 
     For some applications, the implant is delivered using the same catheter  461  that delivers pins  464  to annulus  68 . That is, as shown, there is a central lumen  463  within catheter  461  between the plurality of radiopaque pins  464 . 
     For some applications, pins  464  move proximally in response to movement of pins  464  against hard tissue of annulus  68  and remain static when pins  464  move around softer tissue such as tissue of atrial wall  122  and tissue of leaflets  123 . 
     Pins  464  and catheter  461  are retrieved and removed from the body of the subject following the imaging. For some applications, pins  464  are retrieved subsequently to implantation of the implant. For example, for embodiments in which the implant is implanted while pins  464  move around annulus  68 , pins  464  are removed only subsequently to implantation of the implant. For some applications, pins  464  are retrieved prior to implantation of the implant. For example, for some embodiments, an image and/or map of the topography of valve  64  is generated in advance of implantation, and the implant is implanted using the map as a guide. 
     For some applications, the position of pins  464  and/or the contact of each pin  464  with tissue of annulus  68  is detected electronically, e.g., from an extracorporeal location. For example, each pin  464  comprises an electronic position detector, e.g., a piezoelectric sensor. 
     It is to be noted that although system  460  is shown on mitral valve  64 , system  460  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 30A-B , which are schematic illustrations of a system  470  for facilitating imaging of cardiac tissue using at least one annulus-marking device  472  comprising a distal frame wire  475  and a plurality of radiopaque elements, such as radiopaque filaments  99  or other radiopaque markers, radiopaque wires, radiopaque extension, etc. System  470  comprises a multilumen tube  474  from which annulus-marking device  472  expands.  FIG. 30A  shows annulus-marking device  472  in an expanded state and exposed from within tube  474 . In the expanded state of device  472 , distal frame wire  475  assumes an expanded shape in which it assumes a generally linear configuration. The plurality of radiopaque elements or filaments  99  coupled to the distal frame wire  475  comprise radiopaque material and project away from distal frame wire  475  in the expanded state of annulus-marking device  472 . 
     For some applications, elements or filaments  99  sway with movement of the blood. For some applications, elements or filaments  99  press against tissue of the annulus and tissue coupled thereto, such as tissue of an atrial wall as well as tissue of the leaflets of the native valve. Elements or filaments  99  thus provide enhanced imaging of tissue of the valve. For example, in some implementations, when filaments  99  appear bent or pressed, this imaging detects annulus tissue, while when filaments  99  are straight, this could indicate the orifice of the valve. 
     For each annulus-marking device  472 , at least one central rod  473  is coupled to a middle portion of distal frame wire  475  and disposed primarily and slidable within a primary sublumen  478 , or a collecting lumen, of multilumen tube  474 . Central rod  473  is configured to constrain distal frame wire  475  and the plurality of radiopaque filaments  99  from the expanded state of the annulus-marking device  472  (shown in  FIG. 30A ) and pull distal frame wire  475  and the plurality of radiopaque elements or filaments  99  within primary sublumen  478  of multilumen tube  474  during constraining of annulus-marking device  472  (shown in  FIG. 30B ). 
     For each annulus-marking device  472 , at least two peripheral wires  471  are coupled to distal frame wire  475  at opposite end portions thereof. Peripheral wires  471  are disposed primarily and slidable within respective secondary sublumens  479  of multilumen tube  474 . Peripheral wires  471  are configured to stabilize distal frame wire  475  in the expanded state of annulus-marking device  472  by applying a downward pushing force to the ends of distal frame wire  475  in order to ensure that wire  475  assumes the generally linear state and is in a tense state. 
     Annulus-marking device  472  is compressible during delivery toward the native heart valve, and expandable from a compressed state for positioning along the native heart valve annulus. Annulus-marking device  472  guides implantation of an implant, e.g., an annuloplasty structure, by providing a guide for implantation of the implant along the annulus during implantation since the plurality of elements or filaments  99  are positioned along the annulus and are radiopaque and visible under imaging, e.g., fluoroscopy. The implant can be delivered through a central lumen  476  of multilumen tube  474 . Once the implant has been implanted along the annulus, annulus-marking device  472  is retrieved as is described hereinbelow with reference to  FIG. 30B . 
     As shown in  FIG. 30A , for some applications, system  470  comprises first and second annulus-marking devices  472   a  and  472   b . Each device  472  has a respective central rod  473  as well as respective peripheral wires  471 . For embodiments in which system  470  comprises first and second annulus-marking device  472   a  and  472   b , tube  474  has (a) first and second primary sublumens  478  for sliding therethrough of respective first and second central rods  473 , and (b) four secondary sublumens  479  for sliding therethrough of a respective wire of four peripheral wires  471 . 
     For some applications, system  470  independently controls each one of first and second annulus-marking devices  472   a  and  472   b . That is each one of first and second annulus-marking devices  472   a  and  472   b  may be expanded from within tube  474  or constrained within tube  474 , independently. 
     Each primary sublumen  478  is typically larger than each secondary sublumen  479  since distal frame wire  475  and the plurality of elements or filaments  99  are pulled through primary sublumen  478  as is described hereinbelow. 
     It is to be noted that elements or filaments  99 , distal frame wire  475 , central rod  473 , and peripheral wires  471  are radiopaque and comprise flexible material, e.g., nitinol or stainless steel. For some applications, distal frame wire  475  is instead a textile strip. 
       FIG. 30B  shows retrieval of annulus-marking device  472  into primary sublumen  478 . Central rod  473  is pulled proximally such that the center of distal frame wire  475  is pulled toward primary sublumen  478 . Peripheral wires  471  are released and slide distally within their respective secondary sublumens  479 . Peripheral wires  471  trail behind distal frame wire  475  as wire  475  and radiopaque elements or filaments  99  are pulled proximally into sublumen  478 . Ultimately, distal frame wire  475  and the plurality of elements or filaments are collected into primary sublumen  478 , or the collecting lumen. 
     It is to be noted that system  470  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 31 , which is a schematic illustration of a system  480 , for facilitating imaging of cardiac tissue using first, second, third, and fourth annulus-marking devices  472   a ,  472   b ,  472   c , and  472   d , in accordance with some applications. It is to be noted that system  480  is similar to system  470  described hereinabove with reference to  FIGS. 30A-B  with the exception that system  480  comprises a larger number of annulus-marking devices  472  than system  470 , and like reference numbers refer to like parts. Each device  472  has a respective central rod  473  as well as respective peripheral wires  471 . For embodiments in which system  480  comprises first, second, third, and fourth annulus-marking devices  472   a ,  472   b ,  472   c , and  472   d , tube  474  has (a) first, second, third, and fourth primary sublumens  478  for sliding therethrough of respective first, second, third, and fourth central rods  473 , and (b) eight secondary sublumens  479  for sliding therethrough of a respective wire of eight peripheral wires  471 . 
     Preferably, system  480  independently controls each one of first, second, third, and fourth annulus-marking devices  472   a ,  472   b ,  472   c , and  472   d . That is each one of first, second, third, and fourth annulus-marking devices  472   a ,  472   b ,  472   c , and  472   d  may be expanded from within tube  474  or constrained within tube  474 , independently. 
     It is to be noted that system  480  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 30A-B  and  31 . In some applications, once each one of annulus-marking device  472  has been positioned along the annulus, the implant is implanted over the filaments  99 . When filaments  99  are configured to be very thin, they avoid interfering with and entangling with the implant and slide from underneath the implanted implant during retrieval of annulus-marking device  472  from the body of the subject. It is to be further noted that any one and any number of annulus-marking devices  472  may be expanded from within tube  474  at a given time and they can be retrieved and expanded any number of times during a single procedure. 
     Reference is now made to  FIG. 32 , which is schematic illustration of a system  490  comprising an annulus-marking device  492  comprising a plurality of expandable elements  494 , e.g., stent struts, which form device  492  into a generally semi-spherical, or partially spherical shape for facilitating imaging of cardiac tissue during implantation of a cardiac implant, in accordance with some applications. Device  492  comprises a flexible, radiopaque material, e.g., nitinol or stainless steel, which facilitates collapsing and expanding of device  492 . For some applications, the plurality of expandable elements  494  form device  492  into a partially-bulbous shape. 
     As shown, device  492  aids in imaging implantation of a cardiac implant, e.g., an annuloplasty structure  496 , as shown. Structure  496  comprises a body portion which comprises a flexible material and has a longitudinal axis that runs along the length of the body portion (e.g., when the body portion is straightened). The body portion comprises radiopaque markings to aid in imaging for accurate delivery of anchors  498  to annulus  68  in order to anchor structure  496  to tissue of annulus  68 . 
     Annulus-marking device  492  is delivered using a delivery tool  493  which is configured to deliver device  492  to the left atrium in a compressed state. Device  492  is configured to be expanded from its compressed state once deployed from within a lumen of tool  493 . Annulus-marking device  492  is retrievable upon removal of delivery tool  493  from the subject. That is, device  492  is constrained within the lumen of tool  493  once the cardiac implant has been implanted at annulus  68 . Device  492  may be delivered percutaneously, thoracoscopically through the chest, or using open heart surgical techniques. If delivered percutaneously, device  492  may be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter and subsequently self-expand into the desired shape and tension when released from the catheter. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein under image guidance (e.g., fluoroscopic, ultrasonic, magnetic resonance, computed tomography, or combinations thereof). For some applications, device  492  comprises a wire. 
     Once inside the atrium, the plurality of expandable elements  494  expand radially within the atrium such that the plurality of expandable elements  494  provides an indication as to a location of the native heart valve annulus  68  of native heart valve  64 . For some applications, the plurality of expandable elements  494  comprise a shape-memory material that enables expandable elements  494  to expand to a given shape within the heart. It is to be noted that although device  492  is being used in the left atrium, device  492  may be used in the right atrium, the left ventricle, and the right ventricle. That is, for some applications, a distal end of device  492  enters and is positioned within the ventricle. 
     The plurality of expandable elements  494  collectively form annulus-marking device  492  into a generally umbrella shape. 
     The plurality of expandable elements  494  comprise a very flexible material and design that allows elements  494  to assume the shape of the cavity that they are opened in, e.g., the left atrium, as shown. 
     Annulus-marking device  492  is coupled to a plurality of radiopaque elements, such as radiopaque filaments  99  or other radiopaque markers, wires, extensions, etc. For some applications, annulus-marking device  492  and radiopaque elements or filaments  99  are imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing the plurality of expandable elements  494  and elements or filaments  99  against the tissue. For some applications, annulus-marking device  492  is imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing movement of the plurality of expandable elements  494  and of elements or filaments  99  responsively to movement of the tissue. For either application, annulus-marking device  492  and elements or filaments  99  are imaged with respect to the tissue of the native heart valve annulus  68 , tissue of at least one leaflet, and tissue of an atrial wall  122 . 
     As shown, the distal end of device  492  is positioned within the atrium. That is, the distal end remains at the atrial surface of annulus  68 . 
     For some applications, the distal ends of device  492  is pushed into the ventricle. In such applications, the plurality of expandable elements  494  are each made to bend at a middle section thereof collectively forming a bent section, and it is at this bent section that the operating physician determines using imaging that this is the location of the annulus. 
     Annulus  68  is then imaged using fluoroscopy. For some applications, annulus-marking device  492  is imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing the plurality of expandable elements  494  against the tissue. For some applications, annulus-marking device  492  is imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing movement of the plurality of expandable elements  494  responsively to movement of the tissue. For either application, annulus-marking device  492  is imaged with respect to the tissue of the native heart valve annulus  68 , tissue of at least one leaflet, and tissue of an atrial wall  122 . 
     Structure  496  is delivered subsequently to placement of the plurality of expandable elements  494 . It is to be noted that for some applications, structure  496  is delivered together with annulus-marking device  492 . Annuloplasty structure  496  is implanted under the guidance of fluoroscopy using annulus-marking device  492  as a guide. Annuloplasty structure  496  is positioned between annulus-marking device  492  and atrial wall  122 . A respective anchor  498  is deployed to anchor structure  496  at a site along annulus  68  that is marked between the radiopaque markings of structure  496 . 
     Anchors  498  are delivered while some elements or filaments  99  may be underneath structure  496 . 
     Subsequently to implanting of structure  496 , annulus-marking device  492  is retrieved. Since device  492  is flexible and compressible, device  492  is constrained within a tool during the retrieval of device  492  and subsequent removal of device  492  from the body of the subject. That is, device  492  does not function as an implant for such embodiments and is used only to guide implantation of annuloplasty structure  496  (i.e., the implant); rather, device  492  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of annuloplasty structure  496 . Annulus-marking device  492  is constrained within tool  493 . Annulus-marking device  492  is retrieved and removed from the body of the subject. In some embodiments, filaments  99  are soft and flexible, such that they trail behind elements  494  in a manner in which filaments  99  slide from under annuloplasty structure  496  implanted along annulus  68 . 
     It is to be noted that although system  490  is shown on mitral valve  64 , system  490  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 33A-B , which are schematic illustrations of a system  500  comprising an annulus-marking device  502  comprising a plurality of inflatable fingers  504 , which form device  502  into a glove shape, for facilitating imaging of cardiac tissue during implantation of a cardiac implant, in accordance with some applications. Device  502  comprises a flexible, radiopaque material, e.g., nitinol or stainless steel, which facilitates folding, inflating, and deflating of device  502 . For some applications, device  502  is octopus-shaped. For some applications, device  502  comprises 4-10 fingers  504 , e.g., 8 fingers  504 . For some applications of the present invention device  502  comprises nylon. For some applications of the present invention device  502  comprises a noncompliant balloon. 
     For some applications, device  502  itself is radiopaque. For some applications, device  502  is inflated with radiopaque fluid. 
     Device  502  is delivered using a delivery tool  506 . For some applications, device  502  surrounds a distal portion of tool  506 . Tool  506  is shaped to define a lumen through which a cardiac implant, e.g., an annuloplasty structure  507 , is delivered, as shown in  FIG. 33B . That is, the implant is implanted while annulus-marking device  502  guides implantation thereof and ensures that the implant is implanted at the correct location of annulus  68  of valve  64 . Since device  502  is radiopaque, implantation of the implant can be guided under fluoroscopy only, e.g., using two angles. 
     For some applications, during delivery of device  502 , device  502  is in a deflated state and folded into a compressed state within an external catheter  508 . Once inside the atrium, catheter  508  is retracted and device  502  is exposed and inflated, e.g., with saline and or with a radiopaque fluid, to assume an inflated and expanded state. For some applications, tool  506  comprises an inflation lumen for delivering fluid to device  502 . Tool  506  is then advanced toward annulus  68  and using tactile feedback, device  502  is pressed against annulus  68 , as shown in  FIG. 33B . Under fluoroscopy, device  502  is imaged in order to determine whether tool  506  is appropriately positioned along annulus  68 . That is, if the physician sees that a number of fingers  504  of device  502  are bent, the physician determines that tool  506  is in the right place along annulus  68 , e.g., on annulus  68 , against atrial wall  122 , and/or at the hinge. If the physician detects movement of any number of fingers  504 , the physician determines that tool  506  is positioned at least partially along leaflet  123  of valve  64 . 
     If the physician detects some fingers  504  bent and some straight and moving, the physician can determine that tool is positioned partially on the annulus and partially on the leaflet. 
     For some applications, the physician compares an overall configuration of device  502  and its fingers  504  to an ideal configuration of device  502  which is indicative of correct positioning of tool  506  at annulus  68  in order to facilitate proper positioning of the implant along annulus  68 . The physician can determine proximity of tool  506  to atrial wall  122 . For some applications, device  502  is used to measure the height of annulus  68  by viewing the shape and/or movement of the fingers  504  when device  502  is placed against tissue of the annulus. 
     It is to be noted that inflation and deflation can occur multiple times during a single procedure. 
     It is to be noted that although system  500  is shown on mitral valve  64 , system  500  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 34A-C , which are schematic illustrations of a system  510  comprising an annulus-marking device  512  comprising a radiopaque material shaped to define a plurality of concentric wire loops  514  connected by a scaffolding  519  and a wire loop frame  517  coupled to scaffolding  519  and concentric with respect to the plurality of concentric wire loops  514 , in accordance with some applications. Wire loop frame  517  is configured for placement along at least a part of a circumference of annulus  68  of native heart valve  64 , while the plurality of concentric wire loops  514  span the orifice of valve  64 , e.g., above leaflets  123  at the atrial surface of valve  64 . Annulus-marking device  512  is compressible to a compressed state during delivery toward valve  64 , and expandable from a compressed state for positioning in valve  64 , to an expanded state. 
     Frame  517  comprises the outer-most wire loops  514   a  and  514   b . Frame  517  defines a space  516  for implanting an implant, e.g., an annuloplasty structure, along annulus  68  using annulus-marking device  512  as a guide for implantation of the implant along annulus  68  and within a space defined by frame  517 , under imaging, e.g., fluoroscopy. 
     Annulus-marking device  512  comprises a flexible radiopaque material, e.g., nitinol or stainless steel. 
     Device  512  is deployed from within a delivery tool and is expanded either due to the shape-memory property of device  512  and/or using a pushing tool  511  which pushes distally on a locking ring  518  that is disposed in a center of the plurality of concentric wire loops  514  and acts via struts on wire loops  514  and/or scaffolding  519  and/or loop frame  517 . Locking ring  518  helps transition device  512  from the compressed state to the expanded state by pushing distally ring  518 . Pushing on locking ring  518  also locks in place the configuration of device  512 . Locking ring  518  also helps transition device  512  from the expanded state to the compressed state by pulling proximally ring  518 . 
     As shown in  FIG. 34B , device  512  is expanded radially until it pushes against atrial wall  122 . 
     As shown in  FIG. 34C , device  512  is placed such that the plurality of concentric wire loops  514  are at the orifice of valve  64 , wire loop frame  517  is disposed along at least a part of a circumference of annulus  68 . 
     Once device  512  is positioned, annulus  68  is viewed under imaging annulus-marking device  512  with respect to the tissue of annulus  68  and the tissue coupled thereto by viewing annulus-marking device  512  against the tissue. For some applications, movement of annulus-marking device  512  responsively to movement of the tissue is viewed. 
     Annulus-marking device comprises a plurality of radiopaque elements or radiopaque filaments  99  coupled to loops  514 . Under fluoroscopy, the physician is able to determine where leaflets are due to undulations of filaments  99  responsively to movement of leaflets and/or blood flowing through valve  64 . For some applications, if the physician sees that some filaments  99  are not moving, the physician can determine that that portion of device  512  is positioned at annulus  68 . 
     Once the implant is implanted along the annulus, device  512  is retrieved. During retrieval, frame  517  slides around the implant while the radiopaque filaments  99  coupled to loops  514   a  and  514   b  slide from under the implant. Device  512  is constrained within the tool and extracted from the subject. Device  512  is transitioned from the expanded state to the compressed state by pulling proximally on locking ring  518 . 
     Since device  512  has a low profile, it does not interfere significantly with blood flow. 
     Subsequently to the implanting of the implant, annulus-marking device  512  is retrieved. Since device  512  is flexible and compressible, device  512  is constrained within a tool during the retrieval of device  512  and subsequent removal of device  512  from the body of the subject. That is, device  512  does not function as an implant for such embodiments and is used only to guide implantation of the implant; rather, device  512  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation the implant. 
     It is to be noted that although system  510  is shown on mitral valve  64 , system  510  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 35 , which is a schematic illustration of a system  520  comprising an annulus-marking device  522  comprising a plurality of inflatable petals  526 , which form device  522  into a flower shape, for facilitating imaging of cardiac tissue during implantation of a cardiac implant, in accordance with some applications. Device  522  comprises a flexible, radiopaque material, e.g., nitinol or stainless steel, which facilitates expanding and compressing of device  522 . For some applications, device  522  comprises 4-10 petals  526 , e.g., 5 petals  526 . For some applications, device  522  itself is radiopaque. For some applications, petals  526  are not inflatable. 
     A largest petal  524  of the plurality of petals  526  is configured for placement in between leaflets  123  of valve  64 , e.g., at the commissure, as shown by way of illustration and not limitation. 
     Device  522  is delivered using a delivery tool. For some applications, device  522  surrounds a distal portion of the tool. For some applications, the tool is shaped to define a lumen through which a cardiac implant, e.g., an annuloplasty structure, is delivered. That is, the implant is implanted while annulus-marking device  522  guides implantation thereof and ensures that the implant is implanted at the correct location of annulus  68  of valve  64 . Since device  522  is radiopaque, implantation of the implant can be guided under fluoroscopy only, e.g., using two angles. 
     For some applications, during delivery of device  522 , device  522  is in a compressed state within an external catheter. Once inside the atrium, the external catheter is retracted and device  522  is allowed to expand to assume a shape because of its shape-memory material. The tool is then advanced toward annulus  68  and using tactile feedback, device  522  is pressed against annulus  68 . Under fluoroscopy, device  522  is imaged in order to determine whether the tool is appropriately positioned along annulus  68 . That is, if the physician sees that a number of petals  526  of device  522  are bent, the physician determines that the tool is in the right place along annulus  68 , e.g., on annulus  68 , against atrial wall  122 , and/or at the hinge. If the physician detects movement of any number of petals  526 , the physician determines that the tool is positioned at least partially along leaflet  123  of valve  64 . 
     If the physician detects some petals  526  bent and some straight and moving, the physician can determine that tool is positioned partially on the annulus and partially on the leaflet. 
     For some applications, the physician compares an overall configuration of device  522  and its petals  526  to an ideal configuration of device  522  which is indicative of correct positioning of the tool at annulus  68  in order to facilitate proper positioning of the implant along annulus  68 . The physician can determine proximity of the tool to atrial wall  122 . For some applications, device  522  is used to measure the height of annulus  68  in a manner described above. 
     It is to be noted that although system  520  is shown on mitral valve  64 , system  520  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 36 , which is a schematic illustration of a system,  530  comprising a guidewire  534  having a distal end portion that is coupled to an annulus-marking device  532  comprising a plurality of radiopaque elements, such as radiopaque filaments  99 , in accordance with some applications. In some embodiments, guidewire  534  and filaments  99  comprise a flexible, radiopaque material, e.g., nitinol or stainless steel. The distal end portion of guidewire  534  has shape-memory and is configured to assume a curved or helical shape, as shown. The distal end portion of guidewire  534  may surround a delivery tube  536 . Delivery tube  536  is configured to facilitate delivery and implantation of a cardiac implant, e.g., an annuloplasty structure. That is, under imaging, the plurality of filaments  99  are imaged as a concentrated fluoroscopic unit around tube  536  such that the appropriate position of tube  536  can be determined. Additionally, filaments  99  of device  532  guide implantation of the implant by facilitating guiding of tube  536  along annulus  68  under imaging. It is to be noted that the scope of the present invention includes the use of guidewire  534  and device  532  independently of tube  536 . 
     For some applications, device  532  comprises a large number of filaments  99  such that a relatively concentrated fluoroscopic image is achieved in a given region under imaging. 
     For some applications, annulus-marking device  532  is imaged with respect to the tissue of the native heart valve annulus  68  and the tissue coupled thereto by viewing movement of the plurality of filaments  99  responsively to movement of the tissue. For either application, annulus-marking device  532  is imaged with respect to the tissue of the native heart valve annulus  68 , tissue of at least one leaflet, and tissue of an atrial wall  122 . For some applications, filaments  99  of device  532  help facilitate measuring of a height of the native heart valve annulus  68 . 
     For some applications, if the physician sees movement of filaments  99 , the physician can determine that device  532  is positioned at least partially along leaflet  123 . 
     The distal portion of guidewire  534  and device  532  are typically advanced into a heart chamber of the subject, (e.g., a left atrium, as shown). For some applications, the distal portion of guidewire  534  and device  532  are advanced into a ventricle of the subject using a transvascular approach or a transapical approach. For some applications, the distal portion of guidewire  534  and device  532  are advanced into an atrium of the subject using a transvascular approach or a minimally-invasive approach. For applications in which the distal portion of guidewire  534  and device  532  are advanced into the ventricle, the device is positioned in a subannular space, e.g., the subannular groove of valve  64 . 
     Device  532  and guidewire  534  and tube  536  are retrieved and extracted from the body of the subject following implantation of the cardiac implant. 
     It is to be noted that although system  530  is shown on mitral valve  64 , system  530  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 37A-G , which are schematic illustrations of a system  540  comprising an annulus-marking device  542  comprising a first radiopaque loop  544  and a second radiopaque loop  546  configured to gyrate with respect to first radiopaque loop  544 , in accordance with some applications. As shown in  FIGS. 37B-C , second radiopaque loop  546  is configured to pivot and tilt with respect to first radiopaque loop  544 . Device  542  comprises a flexible, radiopaque material (nitinol or stainless steel) such that it is compressible to a compressed state during delivery toward the native heart valve, and expandable from a compressed state for positioning in the native heart valve to an expanded state. 
     Each loop  544  and  546  comprises a wire frame surrounded at least in part by a respective spring  543  and  547  which expand and compress in order to facilitate proper positioning and alignment of loops  544  and  546  within valve  64 . For some applications, springs  543  and  547  help loops  544  and  546  apply pressure to tissue of valve  64  and/or to tissue surrounding valve  64 . 
     Device  542  is delivered within a chamber of the heart (e.g., a left atrium, as shown in  FIG. 37D ) and is allowed to expand due to its shape memory material. 
       FIG. 37E  shows tilting of second radiopaque loop  546  with respect to first radiopaque loop  544  and allowing of second radiopaque loop  546  to pivot along a plane that is at a non-zero angle with respect to a plane of first radiopaque loop  544 . For some applications, movement of second loop  546  is aided responsively to downward pushing of first loop  544 . As shown in  FIG. 37E , annulus-marking device  542  is positioned in its fully expanded state at least in part within native heart valve  64  in a manner in which (1) first radiopaque loop  544  is disposed between leaflets  123  of valve  64 , an upper portion of first radiopaque loop  544  is disposed within the atrium and a lower portion of first radiopaque loop  544  is disposed within the left ventricle of the heart, and (2) second radiopaque loop  546  is disposed along an atrial surface of annulus  68  of valve  64 . 
     For some applications, first radiopaque loop  544  is positioned between leaflets  123  such that it applies a force to commissures of valve  64 . 
     As shown in  FIG. 37E , in the fully expanded state of device  542 , second radiopaque loop is moveable vertically along a portion  545  (shown in  FIGS. 37A and 37E ) of first radiopaque loop  544 . Such moving enables proper positioning of loop  546  along annulus  68 . For some applications, such moving of second radiopaque loop  546  vertically along portion  545  of the first radiopaque loop comprises measuring a height of annulus  68 . 
     Once device  542  is in place, tissue of native heart valve annulus  68  and tissue coupled thereto is viewed under imaging annulus-marking device  542 . Device  542  is imaged while placed against the tissue. For some applications, tissue of native heart valve annulus  68  and tissue coupled thereto is imaged by viewing movement of annulus-marking device  542  responsively to movement of the tissue. 
       FIG. 37F  shows an implant, e.g., an annuloplasty structure  549 , being implanted along annulus  68  using annulus-marking device  542  as a guide for implantation of the implant under imaging, e.g., fluoroscopy. The implant is positioned between loop  546  and tissue of atrial wall  122 . The implant is delivered using a delivery tool  550 , and a plurality of anchors  551  are used to anchor the implant to tissue of annulus  68 . 
     Following the implantation of the implant, device  542  is retrieved by pivoting and tilting second radiopaque loop  546  with respect to first radiopaque loop  544 . Device  542  is constrained within a tool and extracted from the body of the subject. That is, device  542  does not function as an implant for such embodiments and is used only to guide implantation of the implant; rather, device  542  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of the implant. 
       FIG. 37G  shows annuloplasty structure  549  at annulus  68  in presence of annulus-marking device  542 . 
     It is to be noted that although system  540  is shown on mitral valve  64 , system  540  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 38 , which is a schematic illustration of a system  560  comprising an annulus-marking device  562  comprising two or more expandable wires  564 , a central pole  565 , and at least one ultrasound transducer  568  slidable along and rotational with respect to central pole  565 , in accordance with some applications. Device  562  comprises a flexible, radiopaque material (nitinol or stainless steel) such that it is compressible to a compressed state during delivery toward the native heart valve, and expandable from a compressed state for positioning in the native heart valve to an expanded state. Wires  564  are connected at their respective proximal and distal ends to central pole  565 . It is to be noted that although two wires  564  are shown, the scope of the present invention includes devices  562  comprising any suitable number of wires  564 . Collectively, wires  564  form a frame of device  562 . 
     Each wire  564  comprises a flexible metal which expands and compresses in order to facilitate proper positioning and alignment of wires  564  within valve  64 . For some applications, wires  564  apply pressure to tissue of valve  64  and/or to tissue surrounding valve  64 . Each wire  564  is shaped so as to define a respective indented section  566  which fits annulus  68 . Wires  564  are positioned between leaflets  123  such that they apply a force to commissures of valve  64 . 
     Device  562  is delivered within valve  64  and is allowed to expand due to its shape memory material. 
     Annulus-marking device  562  is positioned in its fully expanded state at least in part within native heart valve  64  in a manner in which the two or more expandable wires  564  are disposed between leaflets  123  of valve  64 , an upper portion of each expandable wire  564  is disposed within the atrium, and a lower portion of each expandable wire  564  being disposed within the ventricle. 
     For some applications, wires  564  are positioned between leaflets  123  such that they apply a force to commissures of valve  64 . 
     In the fully expanded state of device  562 , at least one radiopaque marker  563  (e.g., two markers  563 , as shown) is moveable vertically along a respective wire  564  toward annulus  68  until marker  563  abuts annulus  68 . For some applications, each marker  563  comprises a wire frame and a plurality of radiopaque elements, such as radiopaque filaments  99 , etc. For some applications, such moving of markers  563  vertically along wires  564  comprises measuring a height of annulus  68 . Wire  564  acts as a guide for markers  563  to reach the commissures of valve  64 . 
     Once device  562  is in place, tissue of native heart valve annulus  68  and tissue coupled thereto is viewed under imaging annulus-marking device  562 . Device  562  is imaged while placed against the tissue. For some applications, tissue of native heart valve annulus  68  and tissue coupled thereto is imaged by viewing movement of annulus-marking device  562  responsively to movement of the tissue. For some applications, tissue is viewed by ultrasound transducer  568 . A height of annulus  68  is also measured using transducer  568 . For some applications, transducer  568  measures an inter-commissural distance. For some applications, transducer  568  faces outward, in particular radially outward. 
     For some applications, central pole  565  is hollow, and ultrasound transducer  568  is slidable within pole  565 . It is to be noted that the scope of the present application includes any number of ultrasound transducers in device  562 . For some applications, device  562  can comprise a single ultrasound transducer. For either application in which transducer  568  slides within or around pole  565 , ultrasound transducer  568  is advanced along pole  565  in a vicinity of annulus  68 . For some applications, ultrasound transducer  568  is advanced along pole  565  until it is in line with indented section  566  of wires  564 . 
     An implant, e.g., an annuloplasty structure, (not shown) can be implanted along annulus  68  using annulus-marking device  562  as a guide for implantation of the implant under imaging, e.g., ultrasound and fluoroscopy. The implant is positioned between wires  564  and tissue of atrial wall  122 . The implant is delivered using a delivery tool, and a plurality of anchors are used to anchor the implant to tissue of annulus  68 . 
     Following the implantation of the implant, device  562  is retrieved by constraining device  562  within a tool and extracted from the body of the subject. That is, device  562  does not function as an implant for such embodiments and is used only to guide implantation of the implant; rather, device  562  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of the implant. 
     It is to be noted that although system  560  is shown on mitral valve  64 , system  560  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 39 , which is a schematic illustration of a system  570  comprising an annulus-marking device  572  comprising a clip comprising radiopaque material and first and second jaws  571  and  573  coupled together at a hinge point  574  and first and second filaments  576  and  577  extending from respective ends or from respective roots of first and second jaws  571  and  573 , in accordance with some applications. For some applications, device  572  is introduced within the ventricle of the heart, either by a transapical approach or by a transaortic approach. The clip clips together both leaflets  123  of valve  64 . During the clipping, first filament  576  extends along and abuts an atrial surface of one leaflet  123  in a manner in which an end of first filament  576  is positioned in a vicinity of a hinge of annulus  68  of the valve in a vicinity of atrial wall  122 . Second filament  577  extends along and abuts a ventricular surface of leaflet  123  in a manner in which an end of second filament  577  is positioned in a subannular groove of the valve in a vicinity of a ventricular wall. 
     It is to be noted that the clip can comprise any number of filaments coupled thereto. These filaments can extend circumferentially along annulus  68 . 
     Device  572  comprises radiopaque material, e.g., nitinol or stainless steel. Filaments  576  and  577  can be super-elastic and, for some applications, have shape-memory material. 
     For some applications, clipping leaflets  123  together creates a double orifice in valve  64 . As shown, filament  576  extends from a ventricular space toward the atrial surface, e.g., through one of the orifices created by the clip. 
     Once device  572  is in place, tissue of native heart valve annulus  68  and tissue coupled thereto is viewed under imaging annulus-marking device  572 . Device  572  is imaged while placed against the tissue. For some applications, tissue of native heart valve annulus  68  and tissue coupled thereto is imaged by viewing movement of annulus-marking device  572  responsively to movement of the tissue, e.g., to movement of filaments  576  and  577 . 
     Under the guidance of imaging, e.g., fluoroscopy, using device  572 , an implant, e.g., an annuloplasty structure  578  is implanted along annulus  68  of the subject. A plurality of anchors  579  are used to implant structure  578  to annulus  68 . 
     For some applications, device  572  remains implanted within the body of the subject once structure  578  is implanted. For some applications, implanting structure  578  affixes at least one of filaments  576  and  577  to valve  64 . For some applications, device  572  is retrieved following the implanting. 
     It is to be noted that although system  570  is shown on mitral valve  64 , system  570  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 40 , which is a schematic illustration of a system  580  comprising an annulus-marking device  582  comprising a clamp comprising radiopaque material and first and second arms  583  and  585  coupled together at a hinge point, and first and second tongs, or curved elements  584  and  586  extending from respective ends of first and second arms  583  and  585 , in accordance with some applications. For some applications, device  582  is introduced within the ventricle of the heart, either by a transapical approach or by a transaortic approach. The clamp clamps around a single leaflet  123  of valve  64 . During the clamping, first curved element  584  extends along and abuts an atrial surface of one leaflet  123  in a manner in which curved element is positioned in a vicinity of a hinge of annulus  68  of the valve in a vicinity of atrial wall  122 . Second curved element  586  extends along and abuts a ventricular surface of leaflet  123  in a manner in which second curved element  586  is positioned in a subannular groove of valve  64  in a vicinity of a ventricular wall. 
     It is to be noted that the clamp can comprise any number of arms  583  and  585  coupled thereto. Device  582  comprises radiopaque material, e.g., nitinol or stainless steel. 
     Once device  582  is in place, tissue of native heart valve annulus  68  and tissue coupled thereto is viewed under imaging annulus-marking device  582 . Device  582  is imaged while placed against the tissue. For some applications, tissue of native heart valve annulus  68  and tissue coupled thereto is imaged by viewing movement of annulus-marking device  582  responsively to movement of the tissue. 
     Under the guidance of imaging, e.g., fluoroscopy, using device  582 , an implant, e.g., an annuloplasty structure  588  is implanted along annulus  68  of the subject. A plurality of anchors  589  are used to implant structure  588  to annulus  68 . 
     Once structure  588  is implanted, device  582  is retrieved from the body of the subject. 
     It is to be noted that although system  580  is shown on mitral valve  64 , system  580  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. It is to be further noted that although device  582  is shown as being delivered transapically, device  582  may be delivered to the valve using any suitable delivery method into the atrium or into the ventricle, e.g., transvascularly or using a minimally-invasive approach. 
     Reference is now made to  FIG. 41 , which is a schematic illustration of a system  590  comprising an annulus-marking device  592  comprising a balloon having an upper inflatable section  594 , a lower inflatable section  596 , and a central waist  595  between the upper and lower inflatable sections  594  and  596 , in accordance with some applications. Upper inflatable section  594  is inflatable to assume a generally paddle shape, or any other shape having a relatively small width and one or more flat surfaces  598 , while lower inflatable section  596  is inflatable to assume a spherical shape. The balloon of device  592  is delivered to valve  64  using a catheter, such as during transvascular approach or during a minimally-invasive procedure. The balloon is positioned within the heart such that upper inflatable section  594  is disposed within an atrium of the heart, lower inflatable section  596  is disposed within a ventricle of the heart, and central waist  595  is disposed between leaflets  123  of valve  64 . As shown, in one view of the balloon of device  592 , the balloon assumes an hourglass shape, e.g., at at least one cross-section thereof. In a second view, as shown, upper inflatable section  594  is narrower and has a lower profile than lower inflatable section  596 . 
     The balloon is inflated such that upper inflatable section  594  expands to assume the generally paddle shape, and lower inflatable section  596  expands to assume the spherical shape. For some applications, the balloon comprises a radiopaque material. For some applications, the balloon is inflated using radiopaque fluid. For some applications, upper inflatable section  594  is less compliant than lower inflatable section  596 . For some applications, upper inflatable section  594  is noncompliant. 
     Once the balloon is inflated, under imaging guidance, e.g., under fluoroscopy, an implant, e.g., an annuloplasty structure  599 , is implanted at annulus  68  of valve  64  using annulus-marking device  592  as a guide. For some applications, structure  599  is implanted between an external surface of upper inflatable section  594  and a surface of atrial wall  122 . 
     For some applications, tissue of native heart valve annulus  68  and tissue coupled thereto is viewed using the balloon of device  592 . The tissue is viewed by imaging annulus-marking device  592  with respect to the tissue of native heart valve annulus  68  and the tissue coupled thereto by viewing upper and lower inflatable sections  594  and  596  placed against the tissue. Annulus-marking device  592  is imaged with respect to the tissue of native heart valve annulus  68 , tissue of at least one leaflet  123 , and tissue of atrial wall  122 . 
     For some applications, at least one surface, e.g., an upper surface, of upper inflatable section  594  is slanted (not shown) in order to reduce the chance that the delivery system used to deliver annuloplasty structure  599  abuts a surface of lower inflatable section  596  and reduces the chance that the delivery system drives an anchor into lower inflatable section  596 . 
     Once structure  599  is implanted along annulus  68 , annulus-marking device  592  is retrieved. For some applications, device  592  is deflated and then constrained within a tool and extracted from the body of the subject. 
     It is to be noted that although system  590  is shown on mitral valve  64 , system  590  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 42 , which is a schematic illustration of a system  600  comprising an annulus-marking device  602  comprising a balloon configured to be positioned in the ventricle of the heart of the subject, in accordance with some applications. The balloon of device  602  is inflatable to assume a spherical shape. The balloon of device  602  is delivered to valve  64  using a catheter  608 , e.g., during transvascular approach or during a minimally-invasive procedure. 
     The balloon is inflated such that it expands to assume the spherical shape. For some applications, the balloon comprises a radiopaque material. For some applications, the balloon includes a toroidal marking  604  which is at an upper surface of the balloon. For such applications, toroidal marking  604  is radiopaque and serves as a guide for implantation of the implant along annulus  68 . For some applications, the balloon is inflated using radiopaque fluid. For some applications, the balloon of device  602  is compliant. For some applications, the balloon of device  602  is noncompliant. 
     For either embodiment, the balloon is inflated, and the inflation is controlled in order to prevent puncture of the balloon. The balloon is expanded toward the leaflet hinge point. 
     For some applications of the present invention the balloon comprises a magnetic substance, e.g., filaments, within a space defined by the balloon. For such applications, marking  604  is therefore defined by a collection of the magnetic substance at the upper surface of the balloon of device  602 , thereby marking annulus  68  of valve  64  from a ventricular surface of valve  64 . As such, catheter  608  comprises a magnet  606  at a distal end thereof. Magnet  606  is configured to draw the magnetic substance toward the upper surface of the balloon in order to form the magnetic substance into marking  604 . 
     Once the balloon is inflated, under imaging guidance, e.g., under fluoroscopy, an implant, e.g., an annuloplasty structure  607 , is implanted at annulus  68  of valve  64  using annulus-marking device  602  as a guide. Structure  607  is implanted using a plurality of tissue anchors  609 . 
     For some applications, tissue of native heart valve annulus  68  and tissue coupled thereto is viewed using the balloon of device  602 . The tissue is viewed by imaging annulus-marking device  602  with respect to the tissue of native heart valve annulus  68  and the tissue coupled thereto by viewing the balloon placed against the tissue. Annulus-marking device  602  is imaged with respect to the tissue of native heart valve annulus  68 , tissue of at least one leaflet  123 , and tissue of atrial wall  122 . 
     For some applications, at least one surface, e.g., an upper surface, of device  602  is slanted (not shown) in order to reduce the chance that the delivery system used to deliver annuloplasty structure  607  abuts a surface of the balloon of device  602  and reduces the chance that the delivery system drives an anchor into the balloon. 
     Once structure  607  is implanted along annulus  68 , annulus-marking device  602  is retrieved. For some applications, device  602  is deflated and then constrained within a tool and extracted from the body of the subject. 
     It is to be noted that although system  600  is shown on mitral valve  64 , system  600  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 43 , which is a schematic illustration of a system  610  comprising an annulus-marking device  612  comprising one or more balloons having an upper inflatable element  614  and a lower inflatable element  616 , in accordance with some applications. Upper inflatable element  614  is inflatable to assume a generally toroidal shape, and lower inflatable element  616  is inflatable to assume a toroidal shape. Device  612  is delivered to valve  64  using a catheter  613 , e.g., during transvascular approach or during a minimally-invasive procedure. Device  612  is positioned within the heart such that upper inflatable element  614  is disposed within an atrium of the heart and lower inflatable element  616  is disposed within a ventricle of the heart. For some applications, upper and lower inflatable elements are discrete and are delivered and inflated separately (configuration not shown). For some applications, as shown, device  612  comprises a central waist  615  between the upper and lower inflatable elements  614  and  616 , and central waist  615  is disposed between leaflets  123  of valve  64 . That is, for such an embodiment, device  612  comprises a single balloon including elements  614  and  616  and waist  615  and assumes an hourglass shape at at least one cross-element thereof. 
     Once positioned at valve  64 , upper inflatable element  614  is inflated and expands to assume the toroidal shape, and lower inflatable element  616  is inflated and expands to assume the toroidal shape. For some applications, device  612  comprises a radiopaque material. For some applications, device  612  is inflated using radiopaque fluid. For some applications, upper inflatable element  614  and lower inflatable element  616  comprise compliant material. For some applications, upper inflatable element  614  and lower inflatable element  616  comprise noncompliant material. Elements  614  and  616  are inflated to any suitable pressure depending on the compliance of the material of elements  614  and  616 . 
     Once elements  614  and  616  are inflated, catheter  613  applies a pushing force downward to upper inflatable element  614  and/or a pulling force upward to lower inflatable element  616  in order to position device  612  properly within valve  64 . Once inflated elements  614  and  616  are positioned properly, under imaging guidance, e.g., under fluoroscopy, an implant, e.g., an annuloplasty structure  618 , is implanted at annulus  68  of valve  64  using annulus-marking device  612  as a guide. For some applications, structure  618  is implanted between an external surface of upper inflatable element  614  and a surface of atrial wall  122 . 
     For some applications, tissue of native heart valve annulus  68  and tissue coupled thereto is viewed using device  612 . The tissue is viewed by imaging annulus-marking device  612  with respect to the tissue of native heart valve annulus  68  and the tissue coupled thereto by viewing upper and lower inflatable elements  614  and  616  placed against the tissue. Annulus-marking device  612  is imaged with respect to the tissue of native heart valve annulus  68 , tissue of at least one leaflet  123 , and tissue of atrial wall  122 . 
     For some applications, at least one surface, e.g., an upper surface, of upper inflatable element  614  is slanted (not shown) in order to reduce the chance that the delivery system used to deliver annuloplasty structure  618  abuts a surface of lower inflatable element  616  and reduces the chance that the delivery system drives an anchor into lower inflatable element  616 . 
     Once structure  618  is implanted along annulus  68  by a plurality of anchors  619 , annulus-marking device  612  is retrieved. For some applications, device  612  is deflated and then constrained within a tool and extracted from the body of the subject. 
     It is to be noted that although system  610  is shown on mitral valve  64 , system  610  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 44 , which is a schematic illustration of a system  620  comprising an annulus-marking device  622  comprising a balloon  624  configured to be positioned in the ventricle of the heart of the subject, in accordance with some applications. Balloon  624  of device  622  is inflatable to assume a toroidal shape. Balloon  624  of device  622  is delivered to valve  64  using a catheter  628 , e.g., during transvascular approach or during a minimally-invasive procedure. 
     Balloon  624  is inflated such that it expands to assume the toroidal shape. For some applications, balloon  624  comprises a radiopaque material. For some applications, balloon  624  is inflated using radiopaque fluid. For some applications, balloon  624  of device  622  is compliant. For some applications, balloon  624  of device  622  is noncompliant. 
     For either embodiment, balloon  624  is inflated and the inflation is controlled in order to prevent puncture of the balloon. Balloon  624  is expanded toward the leaflet hinge point. 
     For some applications of the present invention balloon  624  comprises a magnetic substance  626 , e.g., filaments, within a space defined by balloon  624 . For such applications, a marker of device  622  is therefore defined by a collection of magnetic substance  626  at the upper surface of balloon  624  of device  622 , thereby marking annulus  68  of valve  64  from a ventricular surface of valve  64 . For such applications, magnetic substance forms a toroidal marking that serves as a guide for implantation of the implant along annulus  68 . As such, catheter  628  comprises a magnet  625  at a distal end portion thereof. Magnet  625  is configured to draw magnetic substance  626  toward the upper surface of balloon  624  in order to form magnetic substance  626  into the marker. For some applications, magnet  625  is toroidal. For some applications, magnet  625  is circular and flat. 
     Once the balloon is inflated, under imaging guidance, e.g., under fluoroscopy, an implant, e.g., an annuloplasty structure  627 , is implanted at annulus  68  of valve  64  using annulus-marking device  622  as a guide. Structure  627  is implanted using a plurality of tissue anchors  629 . Structure  627  can be positioned between an external surface of magnet  625  and atrial wall  122 . 
     For some applications, tissue of native heart valve annulus  68  and tissue coupled thereto is viewed using balloon  624  of device  622 . The tissue is viewed by imaging annulus-marking device  622  with respect to the tissue of native heart valve annulus  68  and the tissue coupled thereto by viewing balloon  624  against the tissue. Annulus-marking device  622  is imaged with respect to the tissue of native heart valve annulus  68 , tissue of at least one leaflet  123 , and tissue of atrial wall  122 . 
     For some applications, at least one surface, e.g., an upper surface, of device  622  is slanted (not shown) in order to reduce the chance that the delivery system used to deliver annuloplasty structure  627  abuts a surface of the balloon of device  622  and reduces the chance that the delivery system drives an anchor into balloon  624 . 
     Once structure  627  is implanted along annulus  68 , annulus-marking device  622  is retrieved. For some applications, device  622  is deflated and then constrained within a tool and extracted from the body of the subject. 
     It is to be noted that although system  620  is shown on mitral valve  64 , system  620  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 45 , which is a schematic illustration of a system  630  comprising an annulus-marking device  632  comprising at least one magnetic element, e.g., first and second magnetic elements  634  and  636 , configured to be positioned at respective atrial and ventricular surface of valve  64  of the subject, in accordance with some applications. For some applications, first and second magnetic elements  634  and  636  each comprise a wire. For some applications, first and second magnetic elements  634  and  636  are each circular. For some applications, first and second magnetic elements  634  and  636  are each toroidal, e.g., ring-shaped. For some applications, first and second magnetic elements  634  and  636  are partially ring-shaped, e.g., C-shaped. For some applications, first and second magnetic elements  634  and  636  each comprise flat, circular discs. For applications in which first and second magnetic elements  634  and  636  each comprise flat, circular discs, elements  634  and  636  apply pressure to leaflets  123 . 
     Device  632  is delivered to valve  64  using a catheter, e.g., during transvascular approach or during a minimally-invasive procedure. 
     Positioning of second magnetic element  636  at the ventricular surface generates a magnetic field. The magnetic field helps ensure proper positioning of device  632  with respect to tissue of valve  64 . The magnetic field helps prevent movement of device  632  with respect to tissue of valve  64 . For some applications, the magnetic field helps adjust a size of elements  634  and  636 . 
     Once device  632  is positioned at valve  64 , under imaging guidance, e.g., under fluoroscopy, an implant, e.g., an annuloplasty structure  637 , is implanted at annulus  68  of valve  64  using annulus-marking device  632  as a guide. Structure  637  is implanted using a plurality of tissue anchors  639 . Structure  637  can be positioned between an external surface of magnetic element  634  and atrial wall  122 . 
     For some applications, tissue of native heart valve annulus  68  and tissue coupled thereto is viewed using device  632 , i.e., using magnetic elements  634  and  636 . The tissue is viewed by imaging annulus-marking device  632  with respect to the tissue of native heart valve annulus  68  and the tissue coupled thereto by viewing elements  634  and  636  placed against the tissue. Annulus-marking device  632  is imaged with respect to the tissue of native heart valve annulus  68 , tissue of at least one leaflet  123 , and tissue of atrial wall  122 . 
     Once structure  637  is implanted along annulus  68 , annulus-marking device  632  is retrieved. For some applications, device  632  is constrained within a tool and extracted from the body of the subject. 
     It is to be noted that although system  630  is shown on mitral valve  64 , system  630  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 46 , which is a schematic illustration of a system  640  comprising an annulus-marking device  642  comprising at least one magnetic element  644  configured to be positioned at respective atrial and ventricular surface of valve  64  of the subject, in accordance with some applications. For some applications, element  644  comprises a wire. For some applications, magnetic element  644  is circular. For some applications, magnetic element  644  is toroidal. For some applications, magnetic element  644  comprises a flat, circular disc. Device  642  is delivered to valve  64  using a catheter  646 , e.g., during transvascular approach or during a minimally-invasive procedure. 
     Once positioned at the atrial and/or ventricular surface of valve  64 , a magnetic field is generated, e.g., by an external magnetic field generator. The magnetic field helps ensure proper positioning of device  642  with respect to tissue of valve  64 . The magnetic field helps prevent movement of device  642  with respect to tissue of valve  64 . For some applications, the magnetic field helps adjust a size of magnetic element  644 . 
     For applications in which magnetic element  644  is positioned at the atrial surface (e.g., approaching valve  64  via the fossa ovalis), the magnetic field is generated from the ventricle of the heart, e.g., transapically, from within the ventricle using a catheter, or from a magnet positioned outside the body of the subject. 
     For applications in which magnetic element  644  is positioned at the ventricular surface (e.g., approaching valve  64  via the aorta), the magnetic field is generated from the ventricle of the heart, e.g., from within the atrium using a catheter, or from a magnet positioned outside the body of the subject. 
     Once device  642  is positioned at valve  64 , under imaging guidance, e.g., under fluoroscopy, an implant, e.g., an annuloplasty structure, is implanted at annulus  68  of valve  64  using annulus-marking device  642  as a guide. For some applications, the delivery tool used to deliver and implant the implant comprises a metallic element or a magnet which is attracted by annulus-marking device  642 . For applications in which element  644  is positioned at the atrial surface, the annuloplasty structure can be positioned between an external surface of magnetic element  644  and atrial wall  122 . 
     For some applications, tissue of native heart valve annulus  68  and tissue coupled thereto is viewed using device  642 , i.e., using magnetic element  644 . The tissue is viewed by imaging annulus-marking device  642  with respect to the tissue of native heart valve annulus  68  and the tissue coupled thereto by viewing element  644  placed against the tissue. Annulus-marking device  642  is imaged with respect to the tissue of native heart valve annulus  68 , tissue of at least one leaflet  123 , and tissue of atrial wall  122 . 
     Once the annuloplasty structure is implanted along annulus  68 , annulus-marking device  642  is retrieved. For some applications, device  642  is constrained within a tool and extracted from the body of the subject. 
     It is to be noted that although system  640  is shown on mitral valve  64 , system  640  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 47 , which is a schematic illustration of a system  650  comprising an annulus-marking device  652  comprising a coil-shaped wire  654  that is placed along annulus  68  in advance of implantation of an implant, e.g., an annuloplasty structure  662 , in accordance with some applications. Annulus-marking device  652  extends from within a delivery tool  651  and travels along at least a portion of a perimeter of annulus  68  (e.g., a posterior portion of annulus  68  of mitral valve  64 , as shown). Annulus-marking device  652  acts as a guide for implantation of structure  662  under imaging guidance, e.g., fluoroscopy. As successive portions of annuloplasty structure  662  are extended from within its delivery tool  660  and are positioned along successive portions of annulus  68 , annulus-marking device  652  guides the successive portions of structure  662  under imaging as annulus-marking device  652  comprises a radiopaque material (e.g., nitinol or stainless steel). Annulus-marking device  652  comprises wire  654  that is shaped in a coil or spring. Wire  654  is generally deflectable to be pushed against tissue of annulus  68  and abut tissue of annulus  68  such that annulus-marking device  652  facilitates providing an image of a large percentage, e.g., at least 50% or at least 60%, of a perimeter of annulus  68 . Additionally, wire  654  applies a pushing force against a portion of annulus  68  in such that system  650  ensures that annuloplasty structure  662  is properly positioned at a juncture between tissue of annulus  68  and tissue of the atrial wall at an external perimeter of annulus-marking device  652 . In such a manner, device  652  ensures that annuloplasty structure  662  is positioned outside the external perimeter of wire  654  that is shaped as a coil or spring, responsively to the pushing of device  652  against tissue of annulus  68  such that annuloplasty structure  662  is implanted along annulus  68  and not on any part of the leaflets of valve  64 . 
     Device  652  does not interfere with blood flow during implantation of annuloplasty structure  662  on a beating heart. Annulus-marking device  652  is at least partly stiff, and provides resistance, which facilitates positioning of structure  662 . Annulus-marking device  652  can also provide tactile feedback to the operating physician. 
     For some applications, device  652  is positioned along an atrial surface of annulus  68  of valve  64 . A first end portion of wire  654  of device  652  is positioned at a first commissure  61  of valve  64 , e.g., an anteriolateral commissure. The first end portion of wire  654  is reversibly anchored in place at commissure  61  using a first anchor  656  which is deployed within the ventricle of the heart. For some applications, anchor  656  comprises a “T”-shaped anchor which reversibly catches tissue of the valve at the ventricular surface of the valve. Once the first end portion of wire  654  of device  652  is anchored to first commissure  61 , successive portions of wire  654  of device  652  are deployed from within tool  651  and positioned along a portion of the perimeter of annulus  68  of valve  64 . During the positioning of the successive portions of wire  654  of device  652 , due to the shape-memory property of wire  654 , device  652  assumed a curved shape and conforms to the shape of annulus  68 . For some applications, device  652  applies a pushing force against tissue of annulus  68  in order to properly position device  652 , and thereby annuloplasty structure  662 , properly along annulus  68 . For some applications, device  652  applies the pushing force due to the spring shape of coil-shaped wire  654  which has a tendency to expand radially and apply a radial pushing force against tissue of annulus  68 . 
     Once coil-shaped wire  654  has been positioned along the posterior perimeter of valve  64 , a second end portion of wire  654  of device  652  is positioned at a second commissure  63  of valve  64 , e.g., a posterolateral commissure. The second end portion of wire  654  is reversibly anchored in place at commissure  63  using a second anchor  658  which is deployed within the ventricle of the heart. For some applications, anchor  656  comprises a “T”-shaped anchor which reversibly catches tissue of the valve at the ventricular surface of the valve. 
     Structure  662  comprises a body portion which comprises a flexible material and has a longitudinal axis that runs along the length of the body portion (e.g., when the body portion is straightened). The body portion comprises radiopaque markings to aid in imaging for accurate delivery of anchors to annulus  68  in order to anchor structure  662  to tissue of annulus  68 . 
     In addition to providing tactile feedback, annulus-marking device  652  can also facilitate positioning of annuloplasty structure  662  by facilitating imaging (e.g., fluoroscopy) and mechanical guidance. For example, the presence of annulus-marking device  652  and/or the shape thereof (e.g., bending due to being pressed against an atrial wall) is visible in fluoroscopic imaging, and can be used to facilitate identification of the position and angle of annuloplasty structure  662  with respect to tissues. 
     Annulus-marking device  652  can be removed by pulling subsequent to the deployment of one or more tissue anchors in order to anchor structure  662 . For some applications, annulus-marking device  652  is decoupled from commissures  61  and  63  by disengaging anchors  656  and  658  and device  652  is constrained within a tool in order to be retrieved and removed from the body of the subject. For some applications, device  652  is delivered together with the annuloplasty structure  662  in tool  660 . 
     Following the implantation of the implant, device  652  is retrieved by constraining device  652  within a tool and extracted from the body of the subject. That is, device  652  does not function as an implant for such embodiments and is used only to guide implantation of the implant; rather, device  652  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of the implant. 
     Device  652  can be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter. Additionally, device  652  is made from radiopaque material to facilitate fluoroscopic visualization. For some applications, tissue of valve annulus  68  and tissue coupled thereto is viewed using device  652 . Additionally, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  652  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing device  652  placed against the tissue. For some applications, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  652  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of device  652  responsively to movement of the tissue. 
     Reference is now made to  FIGS. 3A-B  and  47 . Annulus-marking device  652  can be coupled to a plurality of elements or filaments  99  and can be shaped in any suitable shape. Although system  650  is shown on mitral valve  64 , system  650  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 48 , which is a schematic illustration of a system  670  comprising an annulus-marking device  672  that is positioned in the orifice of valve  64  in advance of an implant, e.g., an annuloplasty structure  674 , in accordance with some applications. For some applications, annulus-marking device  672  leads and is placed in advance of the implant. Annulus-marking device  672  extends from within a delivery tool  671 . For some applications, annulus-marking device  672  comprises an expandable element that assumes a spherical, generally spherical, bulbous, generally bulbous, ovoid, generally ovoid, teardrop, generally teardrop shape, etc. For some applications, annulus-marking device  672  comprises a stent-like mesh  673  comprising a plurality of expandable radiopaque elements (e.g., struts) coupled together to form a mesh (e.g., a fabric mesh or metal mesh), that is positioned partially within the orifice of valve  64  and does not significantly interfere with the function of valve  64 . For some applications, annulus-marking device  672  comprises a tubular stent. For some applications, annulus-marking device  672  comprises a radiopaque balloon, e.g., a nylon balloon. As successive portions of annuloplasty structure  674  are extended from within its delivery tool and are positioned along successive portions of annulus  68 , annulus-marking device  672  guides the successive portions of structure  674  under imaging as annulus-marking device  672  comprises a radiopaque material (e.g., nitinol or stainless steel). 
     Annulus-marking device  672  is at least partly stiff, and provides resistance, which facilitates positioning of structure  674 . Annulus-marking device  672  can also provide tactile feedback to the operating physician. 
     Annuloplasty structure  674  comprises a body portion which comprises a flexible material and has a longitudinal axis that runs along the length of the body portion (e.g., when the body portion is straightened). The body portion comprises radiopaque markings to aid in imaging for accurate delivery of anchors  676  to annulus  68  in order to anchor structure  674  to tissue of annulus  68 . 
     In addition to providing tactile feedback, annulus-marking device  672  can also facilitate positioning of annuloplasty structure  674  by facilitating imaging (e.g., fluoroscopy) and mechanical guidance. For example, the presence of annulus-marking device  672  and/or the shape thereof (e.g., bending due to being pressed against an atrial wall) is visible in fluoroscopic imaging, and can be used to facilitate identification of the position and angle of annuloplasty structure  674  with respect to tissues. Additionally, annulus-marking device  672  ensures that the delivery tool used to deliver structure  674  is positioned at an external perimeter of annulus-marking device  672  such that it is positioned between annulus-marking device  672  and atrial wall  122 . That is, the delivery system used to deliver structure  674  is guided mechanically by the presence of the device  672 . 
     Annulus-marking device  672  can be removed by being pulled and constrained within tool  671  in order to be retrieved and removed from the body of the subject. 
     For some applications, as shown, annulus-marking device  672  is delivered toward valve  64  in a delivery tool  671  that is separate from the delivery tool used to deliver the implant. For some applications, annulus-marking device  672  and the implant can be delivered from the same delivery tool. 
     Reference is now made to  FIGS. 3A-B  and  48 . Annulus-marking device  672  can be coupled to a plurality of elements or filaments  99  and can be shaped in any suitable shape. 
     Device  672  can be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter. Device  672  can comprise a soft and compliant braid which enables mapping of the anatomy of the atrium, atrial wall, heart valve, annulus, and ventricle. For some applications, in its expanded state, device  672  contacts the atrial wall as it is configured to expand to a diameter that is greater than a dimension of the atrium. Since the material of device  672  is compliant, it does not change the natural shape of the anatomy of the atrium. Additionally, device  672  is made from radiopaque material to facilitate fluoroscopic visualization. For some applications, tissue of valve annulus  68  and tissue coupled thereto is viewed using device  672 . Additionally, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  672  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing device  672  placed against the tissue. For some applications, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  672  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of device  672  responsively to movement of the tissue. 
     It is to be noted that although system  670  is shown on mitral valve  64 , system  670  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. It is to be further noted that although device  672  is shown as being delivered via the ventricle, device  672  may be delivered to the valve using any suitable delivery method into the atrium, e.g., transvascularly or using a minimally-invasive approach. 
     Reference is now made to  FIGS. 49A-B , which are schematic illustrations of a system  680  comprising an annulus-marking device  682  for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications. Device  682  comprises a scaffolding  683  that is collapsible and expandable. Scaffolding  683 , is configured, when expanded, to laterally push against tissue of valve  64  (e.g., leaflet  123 , annulus  68 , or a commissure). Scaffolding  683  is radiopaque and comprises a plurality of radiopaque elements  699  which are flexible and shaped as bulbs by way of illustration and not limitation. For some applications of the present invention, radiopaque elements  699  can comprise filaments  99  described hereinabove. 
     In some applications, scaffolding  683  comprises a central rod  692 , an upper laterally-expandable element  694  configured to expand laterally away from central rod  692 , a lower laterally-expandable element  696  configured to expand laterally away from central rod  692 , and at least one flexible wire  698  coupled to and extending between upper and lower laterally-expandable elements  694  and  696 . For some applications, wire  698  comprises a vertical element. When scaffolding  683  is expanded, flexible wires  698  are configured to push against the tissue of valve  64  as is described hereinbelow. It is to be noted that scaffolding  683  comprises four flexible wires  698  by way of illustration and not limitation. For some applications, scaffolding  683  can comprise any number of wires  698 . For some applications, scaffolding  683  can comprise a single wire  698 . For some applications, scaffolding  683  can comprise two wires  698 . Wires  698  comprise a flexible, radiopaque material, e.g., nitinol. A tension of wires  698  is increased by distancing upper and lower laterally-expandable elements  694  and  696  from each other. A tension of wires  698  is decreased by drawing closer upper and lower laterally-expandable elements  694  and  696 . Upper and lower laterally-expandable elements  694  and  696  are moveable longitudinally proximally and distally with respect to central rod  692  to control a tension of the at least one flexible wire  698 . The operating physician is able to discern whether wire  698  comes in contact with tissue of the heart (e.g., leaflet, commissure, or annulus) by observing deformation of wire  698  responsively to the presence of tissue and the force applied to wire  698  by the tissue. 
     When scaffolding  683  is expanded, upper laterally-expandable element  694  is configured to be disposed in an atrium of the heart, and lower laterally-expandable element  696  is configured to be disposed in a ventricle of the heart. 
     For some applications, scaffolding  683  comprises two wires  698  to help center device  682  and/or tool  690  as each wire  698  pushes against the tissue. It is to be noted that any suitable number of wires  698  can be coupled to elements  694  and  696 . Scaffolding  683  helps stabilize device  682  in valve  64 . 
     Upper and lower laterally-expandable elements  694  and  696  each comprise a respective expandable and collapsible ring. Wires  698  are coupled at corresponding locations circumferentially along the rings of laterally-expandable elements  694  and  696 . When scaffolding  683  is expanded, the first and second rings are in an expanded state. Upper and lower laterally-expandable elements  694  and  696  each comprise a respective expandable and collapsible cross-beam  697  that extends laterally away from central rod  692 . For some applications, wires  698  are coupled at corresponding locations along cross-beams  697 . When scaffolding  683  is expanded, cross-beams  697  are in an expanded state. For some applications, elements  694  and  696  expand to a fixed radius. For some applications, elements  694  and  696  may expand to abut tissue of the heart such as atrial wall  122 . 
     When scaffolding  683  is expanded and wires  698  are pulled into a tense state, wires  698  of scaffolding  683  are configured to push against tissue of the heart in order to provide an indication of the presence of the tissue. For some applications, wires  698  push against tissue of valve  64  at the commissures. For some applications, wires  698  push against tissue of leaflet  123  of valve  64 . For some applications, as each wire  698  pushes against tissue of leaflet  123 , wire  698  creates a bicuspidization of the leaflet in a manner in which leaflet  123  assumes two subcusps. 
     Annulus-marking device  682  is configured help visualize the placement of an implant  684  (e.g., an annuloplasty structure, as shown) configured for placement along annulus  68  of valve  64  of the subject. 
     Annulus-marking device  682  is coupled to a delivery tool  690  and is collapsible within a lumen of tool  690  during delivery of device  682  within valve  64 . Annulus-marking device  682  is retrievable upon removal of delivery tool  690  from the subject. 
     Scaffolding  683  comprises radiopaque material (e.g., nitinol or stainless steel) and is flexible. A plurality of radiopaque elements, such as radiopaque filaments  99 , (not shown) can be coupled to scaffolding  683  at any suitable portion thereof. The plurality of radiopaque elements or filaments  99  function as additional annulus-marking devices. Annulus-marking device  682  is configured for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications. The steering procedure is performed with the aid of imaging, such as fluoroscopy, transesophageal echo, and/or echocardiography. 
     Device  682  may be delivered percutaneously, thoracoscopically through the chest, or using open heart surgical techniques. If delivered percutaneously, device  682  may be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter and subsequently self-expand into the desired shape and tension when released from the catheter. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein under image guidance (e.g., fluoroscopic, ultrasonic, magnetic resonance, computed tomography, or combinations thereof). For some applications, device  682  comprises a wire. 
     Device  682  enables mapping of the anatomy of the atrium, atrial wall, heart valve, annulus, and ventricle. Additionally, device  682  is made from radiopaque material to facilitate fluoroscopic visualization. For some applications, tissue of valve annulus  68  and tissue coupled thereto is viewed using device  682 . Additionally, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  682  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing device  682  placed against the tissue. For some applications, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  682  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of device  682  responsively to movement of the tissue. 
     Subsequently to implanting of implant  684 , annulus-marking device  682  is retrieved. Since device  682  is flexible and compressible, device  682  is constrained within the tool during the retrieval of device  682  and subsequent removal of device  682  from the body of the subject. That is, device  682  does not function as an implant for such embodiments and is used only to guide implantation of implant  684 ; rather, device  682  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of implant  684 . 
     It is to be noted that although system  680  is shown on mitral valve  64 , system  680  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 50A-B , which are schematic illustrations of a system  700  comprising an annulus-marking device  702  for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications. Device  702  comprises a scaffolding  705  that is collapsible and expandable. Scaffolding  705 , is configured, when expanded, to laterally push against tissue of valve  64  (e.g., leaflet  123 , annulus  68 , or a commissure). Scaffolding  705  comprises a radiopaque material (e.g., nitinol or stainless steel) and comprises a wire or rod  704 . For some applications, rod  704  comprises a vertical element. Typically, when scaffolding  705  is expanded, rod  704  assumes a vertical orientation and extends from an atrial surface of valve  64  toward a ventricular surface of valve  64 . 
     For some applications, as rod  704  pushes against tissue of leaflet  123 , rod  704  creates a bicuspidization of the leaflet in a manner in which leaflet  123  assumes two subcusps  123   a  and  123   b , as shown in  FIG. 50B . 
     Annulus-marking device  702  is coupled to a delivery tool  706  and is collapsible within a lumen of tool  706  during delivery of device  702  within valve  64 . Annulus-marking device  702  is retrievable upon removal of delivery tool  706  from the subject. Rod  704  is coupled to a steerable shaft  707  that is disposed and slidable within the lumen of tool  706 . Shaft  707  is steerable to move rod  704  along annulus  68 . Rod  704  is moved with each implantation of a tissue anchor used to implant the implant (e.g., annuloplasty structure). That is, after one anchor is implanted, rod  704  is moved to a different location of valve  64  in order to indicate a position of annulus  68  at the location such that visual feedback is provided to the operating physician as to the next location to deploy another tissue anchor. 
     It is to be noted that device  702  can comprise any suitable number of rods  704  and shafts  707 . 
     For some applications of the present invention, as shown, rod  704  is coupled to a plurality of radiopaque elements (e.g., radiopaque filaments  99 , etc.) described hereinabove. The plurality of radiopaque elements or filaments  99  function as additional annulus-marking devices. The radiopaque elements or filaments  99  comprise radiopaque material (e.g., nitinol or stainless steel) and can be configured to be extremely flexible. In some embodiments, filaments  99  project away from rod  704 . For some applications, filaments  99  sway with movement of the blood. For some applications, filaments  99  press against tissue of annulus  68  and tissue coupled thereto (as shown in  FIG. 50B ), such as tissue of an atrial wall as well as tissue of leaflets  123  of the native valve. Elements or filaments  99  thus provide enhanced imaging of tissue of valve  64 . For example, in some embodiments, when filaments  99  appear bent or pressed, this imaging detects annulus tissue, while when filaments  99  are straight, this could indicate the orifice of the valve. 
     In some embodiments, filaments  99  disposed above leaflet  123  remain static, while filaments  99  disposed at leaflets  123  move and pulse with leaflet movement. A boundary between the moving and static filaments can be observed using fluoroscopy in order to indicate the root, or base, or leaflet  123 . 
     Annulus-marking device  702  is configured for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications. The steering procedure is performed with the aid of imaging, such as fluoroscopy, transesophageal echo, and/or echocardiography. Device  702  may be delivered percutaneously, thoracoscopically through the chest, or using open heart surgical techniques. If delivered percutaneously, device  702  can be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter and subsequently self-expand into the desired shape and tension when released from the catheter. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein under image guidance (e.g., fluoroscopic, ultrasonic, magnetic resonance, computed tomography, or combinations thereof). For some applications, device  702  comprises a wire. 
     Device  702  can be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter. Device  702  enables mapping of the anatomy of the atrium, atrial wall, heart valve, annulus, and ventricle. Additionally, device  702  is made from radiopaque material to facilitate fluoroscopic visualization. For some applications, tissue of valve annulus  68  and tissue coupled thereto is viewed using device  702 . Additionally, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  702  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing device  702  placed against the tissue. For some applications, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  702  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of device  702  responsively to movement of the tissue. 
     Subsequently to implanting of the implant, annulus-marking device  702  is retrieved. Since device  702  is flexible and compressible, device  702  is constrained within the tool during the retrieval of device  702  and subsequent removal of device  702  from the body of the subject. That is, device  702  does not function as an implant for such embodiments and is used only to guide implantation of the implant; rather, device  702  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of the implant. 
     It is to be noted that although system  700  is shown on mitral valve  64 , system  700  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 51A-C , which are schematic illustrations of a system  710  comprising an annulus-marking device  712  for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications. Device  712  comprises a scaffolding  713  that is collapsible and expandable. Scaffolding  713 , is configured, when expanded, to laterally push against tissue of valve  64  (e.g., leaflet  123 , annulus  68 , or a commissure). Scaffolding  713  comprises a radiopaque material (e.g., nitinol or stainless steel) and comprises a basket  714  shapes so as to define a plurality of vertical elements  716 , e.g., rods as described hereinabove with reference to  FIGS. 50A-B  or other elongate members, wires, tubes, loops, etc. Typically, scaffolding  713  expands circumferentially with respect to valve  64  in a manner in which elements  716 , or the rods, are disposed circumferentially with respect valve  64 . When scaffolding  713  is expanded, elements  716  assume vertical orientations and extends from an atrial surface valve  64  toward a ventricular surface of valve  64 . A radius of expansion of basket  714  is controlled by movement of structural elements  715  toward or away from each other along a central rod  717 . As shown in  FIG. 51A , elements  715  can be distanced from each other such that basket  714  assumes a narrower configuration. The closer elements  715  are toward each other, the more expanded and wider basket  714  is ( FIG. 51B ). For some applications, basket  714  is manually expanded. For some applications, basket  714  is configured to self-expand. 
     For some applications, as wire elements  716  push against tissue of leaflet  123 , elements  716  create a multi-cuspidization of the leaflet in a manner in which leaflet  123  assumes subcusps, as shown in  FIG. 51C . As shown, device  712  can be positioned within valve  64 . For some applications, device  712  may rest atop valve  64 . For some applications, device  712  comprises two or more leaflets in order to regulate blood flow while device  712  is positioned in valve  64 . 
     Annulus-marking device  712  is coupled to a delivery tool  718  and is collapsible within a lumen of tool  718  during delivery of device  712  within valve  64 . Annulus-marking device  712  is retrievable upon removal of delivery tool  718  from the subject. 
     It is to be noted that device  712  can comprise any suitable number of elements  716 . For some applications, scaffolding  713  and/or basket  714  comprises 3 vertical elements  716 . For some applications, scaffolding  713  and/or basket  714  comprises 5-8 vertical elements  716 . 
     For some applications of the present invention, as shown, each vertical element  716  is coupled to a plurality of radiopaque elements, e.g., radiopaque filaments  99 , etc., described hereinabove. The plurality of radiopaque elements or filaments  99  function as additional annulus-marking devices. (While often described as filaments herein, other types of radiopaque materials, markers, wires, extensions, beads, etc. can also or alternatively be used.) Elements or filaments  99  comprise radiopaque material (e.g., nitinol or stainless steel) and can be configured to be extremely flexible. In some applications, elements or filaments  99  project away from rod  704 . For some applications, filaments  99  sway with movement of the blood. For some applications, filaments  99  press against tissue of annulus  68  and tissue coupled thereto (as shown in  FIGS. 51B-C ), such as tissue of an atrial wall as well as tissue of leaflets  123  of the native valve. Radiopaque elements or filaments  99  thus provide enhanced imaging of tissue of valve  64 . For example, in some applications, when filaments  99  appear bent or pressed, this imaging detects annulus tissue, while when filaments  99  are straight, this could indicate the orifice of the valve. 
     Filaments  99  disposed above leaflet  123  remain static, while filaments  99  disposed at leaflets  123  move and pulse with leaflet movement. A boundary between the moving and static filaments can be observed using fluoroscopy in order to indicate the root, or base, or leaflet  123 . 
     Annulus-marking device  712  is configured for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications. The steering procedure is performed with the aid of imaging, such as fluoroscopy, transesophageal echo, and/or echocardiography. Device  712  can be delivered percutaneously, thoracoscopically through the chest, or using open heart surgical techniques. If delivered percutaneously, device  712  can be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter and subsequently self-expand into the desired shape and tension when released from the catheter. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein under image guidance (e.g., fluoroscopic, ultrasonic, magnetic resonance, computed tomography, or combinations thereof). For some applications, device  712  comprises a wire. 
     Device  712  enables mapping of the anatomy of the atrium, atrial wall, heart valve, annulus, and ventricle. Additionally, device  712  is made from radiopaque material to facilitate fluoroscopic visualization. For some applications, tissue of valve annulus  68  and tissue coupled thereto is viewed using device  712 . Additionally, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  712  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing device  712  placed against the tissue. For some applications, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  712  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of device  712  responsively to movement of the tissue. 
     Subsequently to implanting of the implant, annulus-marking device  712  is retrieved. Since device  712  is flexible and compressible, device  712  is constrained within tool  718  during the retrieval of device  712  and subsequent removal of device  712  from the body of the subject. That is, device  712  does not function as an implant for such embodiments and is used only to guide implantation of the implant; rather, device  712  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of the implant. 
     It is to be noted that although system  710  is shown on mitral valve  64 , system  710  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 52A-B , which are schematic illustrations of a system  730  comprising an annulus-marking device  732  that comprises at least one marker or radiopaque loop  734 , but in some applications comprises a plurality of radiopaque markers or radiopaque loops  734 , for facilitating imaging of cardiac tissue during implantation of a cardiac implant. Device  732  comprises a flexible, radiopaque material, e.g., nitinol or stainless steel, which facilitates expanding and compressing of device  732 . For some applications, device  732  comprises 1-10 markers  734 , e.g., 4 markers  734 , 5 markers  734 , etc. For some applications, device  732  itself is radiopaque. In some implementations, one or more markers or loops  734  can include one or more additional radiopaque elements thereon, e.g., radiopaque filaments, radiopaque wires, radiopaque extensions, radiopaque markers, radiopaque beads, etc., and one or more locations around the marker(s) or loop(s)  734 . Further, while shown largely in the same plane in  FIG. 52A-52B , in some implementations, one or more of the plurality of markers or loops  734  can be offset rotationally relative to other markers or loops  734  such that it is in a different plane (e.g., a plane rotated between 20-160 degrees from each other, such as 90 degrees or 120 degrees, etc.). 
     In some embodiments, the plurality of radiopaque markers  734  juxtapose each other at a given distance from each other and are each deformable by tissue at different intervals indicating proximity of tissue to the implant. The plurality of radiopaque markers can be sized differently from each other. As shown by way of illustration and not limitation, markers  734  comprise concentric loops which fan out distally and laterally from a proximal portion of device  732 . For some applications, the plurality of radiopaque markers  734  can comprise and/or be configured as petals or loops which fan out distally and laterally from a proximal portion of device  732 . In some embodiments, the petals or loops are concentric. For some applications, the plurality of radiopaque markers  734  comprise a plurality of radiopaque strips which fan out distally and laterally from a proximal portion of device  732 . For some applications, the plurality of radiopaque markers  734  comprise a wire. For some applications, each one of the plurality of radiopaque markers  734  comprises a radiopaque sail extending therefrom in order to increase radiopacity. For some applications, each one of the plurality of radiopaque markers  734  comprises a radiopaque element (e.g., filament  99  or other element described hereinabove) extending therefrom. 
     Since each one of the plurality of markers  734  contacts tissue of valve  64  at different times, the physician is able to determine the position of device  732  with respect to the tissue and determine the position of implant, e.g., annuloplasty structure  738  with respect to device  732  and thereby with respect to tissue of valve  64 . That is, the longest marker  734  is configured to contact tissue first, followed by the next longest. Each of markers  734  are stacked around each other. 
     For some applications, a largest marker  734  of the plurality of markers is configured for placement in between leaflets  123  of valve  64 , e.g., at the commissure by way of illustration and not limitation. 
     Device  732  is delivered using a delivery tool  736 . It is to be noted that device  732  is discrete from the implant and, for some applications, implant is delivered using a delivery tool other than delivery tool  736 . For some applications, the tool is shaped to define a lumen through which the implant is delivered. The implant is implanted while annulus-marking device  732  guides implantation thereof and ensures that the implant is implanted at the correct location of annulus  68  of valve  64 . Since device  732  is radiopaque, implantation of the implant can be guided under fluoroscopy only, e.g., using two angles. 
     For some applications, during delivery of device  732 , device  732  is in a compressed state within an external catheter. Once inside the atrium, the external catheter is retracted and device  732  is allowed to expand to assume a shape because of its shape-memory material. The tool is then advanced toward annulus  68  and using tactile feedback, device  732  is pressed against annulus  68 . Under fluoroscopy, device  732  is imaged in order to determine whether the tool is appropriately positioned along annulus  68 . That is, if the physician sees that a number of markers  734  of device  732  are bent, the physician determines that the tool is in the right place along annulus  68 , e.g., on annulus  68 , against an atrial wall, and/or at the hinge. If the physician detects movement of any number of markers  734 , the physician determines that the tool is positioned at least partially along leaflet  123  of valve  64 . 
     If the physician detects some markers  734  bent and some straight and moving, the physician can determine that tool is positioned partially on the annulus and partially on the leaflet. 
     For some applications, the physician compares an overall configuration of device  732  and its markers  734  to an ideal configuration of device  732  which is indicative of correct positioning of the tool at annulus  68  in order to facilitate proper positioning of the implant along annulus  68 . The physician can determine proximity of the tool to the atrial wall. For some applications, device  732  is used to measure the height of annulus  68 . 
     It is to be noted that although system  730  is shown on mitral valve  64 , system  730  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is made to  FIGS. 53A-B , which are schematic illustrations of a navigational-based guidance system  740 , which employs one or more longitudinal guides configured to facilitate guidance of an implant  750  to specific portions of annulus  68  by the guides contacting a surface of the valve (e.g., the annulus, commissure, and/or leaflets of the valve), in accordance with some applications. The longitudinal guide comprises an annulus marking device  742  which comprises a plurality of radiopaque filaments  746  that are coupled to a distal end portion of an elongate radiopaque element  744 . For some applications, elongate radiopaque element  744  comprises a flexible wire. For some applications, elongate radiopaque element  744  comprises a flexible rod. Elongate radiopaque element  744  slides with respect to body portion  752  along the longitudinal axis in order to move the plurality of radiopaque filaments  746  with respect to implant  750 . The guide comprising device  742  and element  744  comprises a flexible material (e.g., a flexible metal such as nitinol or stainless steel), and each guide is radiopaque. A plurality of eyelets  756  are disposed along a lateral outer surface of a body portion  752  of implant  750 , and each guide (e.g., a distal portion thereof) is disposed within at least some of the eyelets (e.g., the guide is threaded through the eyelets). Eyelets  756  can comprise suture or fabric. 
     In some applications, eyelets  756  are arranged in a longitudinal row along the length of body portion  752 , and the guide is disposed within the eyelets of a respective row. For some applications body portion  752  comprises a plurality of radiopaque markers  753 , which are positioned along the body portion at respective longitudinal sites. For some applications the eyelets are disposed at the same longitudinal site as a corresponding radiopaque marker. Though, optionally, the eyelets can be disposed between radiopaque markers. 
     For some applications, the distal end portion of elongate radiopaque element  744  protrudes longitudinally outward from body portion  752 . Such protruding may confer a desired behavior on annulus-marking device  742 , e.g., during distal movement of device  742 . For example, when the device  742  is moved distally against tissue, the protrusion may facilitate splaying of device  742  over the tissue. 
     Body portion  752  of implant  750  is configured to be advanced distally out of a delivery tool  748  and anchored to annulus  68  using anchors. Elongate radiopaque element  744  is disposed within a lumen of and slidable with respect to delivery tool  748 . For some applications, as shown, device  742  follows a path that extends distally from a distal end of delivery tool  748 , touches annulus  68 , and projects distally along leaflet  123  and toward the ventricle, as shown in  FIG. 53B . In such a manner, device  742  functions as vertical elements  716 , e.g., rods as described hereinabove with reference to  FIGS. 51A-C . Optionally, for some applications, device  742  extends longitudinally along body portion  752  of implant  750 . 
     As implant  750  is delivered within delivery tool  748 , device  742  is disposed in alignment with, e.g., parallel to, body portion  752  of implant  750 . As implant  750  is disposed in a linear configuration as shown in  FIG. 53A , device  742  is moved linearly along and/or in parallel with a longitudinal axis of body portion  752 . Once implant  750  is deployed along annulus  68 , implant  750  curves, and device  742  may passively extend away from the path along which implant  750  extends. That is, device  742  extends distally from the distal end of delivery tool  748 , at a nonzero angle, e.g., perpendicularly, with respect to the plane of annulus  68 . As implant  750  is deployed from within delivery tool  748 , it extends distally while device  742  remains in place. Alternatively, or in succession, device  742  is pulled proximally with respect to body portion  752  by pulling on elongate radiopaque element  744  with respect to body portion  752 . In either application, during relative movement of device  742  and body portion  752 , radiopaque filaments  746  collapse and pass through eyelets  756 . During movement of filaments  746  with respect to eyelets  756 , friction is generated which provides the operating physician with tactile feedback in addition to the imaging. 
     Annulus-marking device  742  is placed (e.g., pushed) against tissue of the valve, e.g., by virtue of being already disposed distally to a distal end of body portion  752 , or by being advanced distally after the distal end of the body portion has itself been placed against tissue of the valve. Device  742  thereby comprises a tissue-engaging portion that is configured to be placed in contact with tissue of the subject. 
     In one or more ways, the behavior of device  742  in response to being placed against the tissue of the valve facilitates guidance by viewing of body portion  752  (e.g., positioning of the body portion on the annulus). For example: 
     Resistance of device  742  being pushed further distally may indicate that the device is in contact with tissue that resists forces applied by the guide. For example, the distal end of the device may be abutting annulus  68  and/or a wall of the atrium. Conversely, lack of resistance of device  742  to being pushed further distally may indicate that the distal end of the device is not in contact with tissue that resists forces applied by the guide. For example, the distal end of the device may be moving between leaflets  123  of the valve (e.g., at a commissure), and/or may be pushing a leaflet  123  downward (e.g., into the ventricle). Such resistance (or lack thereof) can be detected mechanically (e.g., as tactile feedback to the operating physician and/or by an extracorporeal control unit). Since device  742  comprise radiopaque material, such resistance (or lack thereof) can be detected via imaging (e.g., fluoroscopically). 
     Similarly, the position, orientation and/or shape of device  742  (e.g., with respect to body portion  752  of implant  750 , tissue of the valve, etc.) may indicate against what, if anything, the device  742  is disposed. Imaging techniques such as fluoroscopy can be used to identify this position, orientation and/or shape of the device. For example, if the distal end of device  742  is positioned at the same height (i.e., at the same place on a superior-inferior axis of the subject) as the distal end of body portion  752 , this may indicate that body portion  752  and device  742  abut the same surface (e.g., annulus  68 ). Conversely, if the distal end of device  742  is positioned lower than body portion  752 , this may indicate that the body portion  752  is disposed against annulus  68 , while device  742  has passed toward or into the ventricle. Movement (e.g., beating) of the device  742  may indicate that the guide is disposed against a leaflet of the valve, and that the leaflet is moving the device as the heart beats. Such imaging may be facilitated by one or more components comprising radiopaque markings. For some applications, each device  742  comprises radiopaque filaments  746 , so as to facilitate identification during imaging. 
     Filaments  746  comprise radiopaque material (e.g., nitinol or stainless steel) and can be configured to be extremely flexible. Filaments  746  project away from elongate radiopaque element  744 . For some applications, filaments  746  sway with movement of the blood. For some applications, filaments  746  press against tissue of annulus  68  and tissue coupled thereto (as shown in  FIG. 53B ), such as tissue of an atrial wall as well as tissue of leaflets  123  of the native valve. Filaments  746  thus provide enhanced imaging of tissue of valve  64 . That is, when filaments  746  appear bent or pressed, this imaging detects annulus tissue, while when filaments  746  are straight, this could indicate the orifice of the valve. 
     Filaments  746  disposed above leaflet  123  remain static, while filaments  746  disposed at leaflets  123  move and pulse with leaflet movement. A boundary between the moving and static filaments can be observed using fluoroscopy in order to indicate the root, or base, or leaflet  123 . 
       FIG. 53B  shows body portion  752  having been placed against annulus  68  of the subject in a vicinity of left fibrous trigone. Device  742  is disposed distally to body portion  752 , and has splayed across annulus  68 , e.g., due to resistance of the annulus. As described hereinabove, this can be detected mechanically and/or by imaging. The position, orientation and/or shape of device  742 , alone and/or in combination with the other elements indicates that the portion of body portion  752  is positioned against firm tissue that is close to the commissure, which for some applications is the preferred position for anchoring of the portion of body portion  752 . Identification (e.g., mechanically and/or by imaging) of which guide is in which position can further indicate the rotational orientation of body portion  752 . 
     Once the desired position has been identified, an anchor (e.g., a first anchor) is used to anchor body portion  752 . For some applications, device  742  and elongate radiopaque element  744  can be withdrawn slightly proximally before anchoring, e.g., so as to reduce a likelihood of inadvertently anchoring the guide to the tissue. As element  744  is withdrawn, filaments  746  are collapsible as they pass through each one of the plurality of eyelets  756  ( FIG. 53A ) and expandable subsequently to passing through each eyelet  756  as filaments  746  have shape-memory. Subsequently, additional portions of body portion  752  are anchored to annulus  68 . In some applications, device  742  is moved proximally with respect to body portion  752 . This process can be repeated for each anchor until implant  750  is fully implanted. 
     Following implantation of implant  750 , device  742  is removed from the body of the subject. Device  742  is removed from within the body by pulling proximally on elongate radiopaque element  744 . Filaments  746  collapse within a lumen of delivery tool  748 . 
     It is to be noted that although system  740  is shown on mitral valve  64 , system  740  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is made to  FIGS. 54A-B , which are schematic illustrations of a navigational-based guidance system  760 , which employs one or more longitudinal guides configured to facilitate guidance of an implant  770  to specific portions of annulus  68  by the guides contacting a surface of the valve (e.g., the annulus, commissure, and/or leaflets of the valve), in accordance with some applications. The longitudinal guide comprises an annulus marking device  762  which comprises a plurality of radiopaque filaments  766  that are coupled to a distal end portion of an elongate radiopaque element  764 . For some applications, elongate radiopaque element  764  comprises a flexible wire. For some applications, elongate radiopaque element  764  comprises a flexible rod. The guide comprising device  762  and element  764  comprises a flexible material (e.g., a flexible metal such as nitinol or stainless steel), and each guide is radiopaque. A proximal end of element  764  is coupled to a tube  768  which surrounds body portion  772  of implant  770  and slides with respect to body portion  772  to move the plurality of radiopaque filaments  766  with respect to implant  770 . 
     For some applications, the distal end portion of each elongate radiopaque element  764  protrudes longitudinally outward from body portion  772 . Such protruding may confer a desired behavior on annulus-marking device  762 , e.g., during distal movement of device  762 . For example, when the device  762  is moved distally against tissue, the protrusion may facilitate splaying of device  762  over the tissue. 
     Body portion  772  of implant  770  is configured to be advanced distally out of a delivery tool  771  and anchored to annulus  68  using anchors. Elongate radiopaque element  764  is disposed within a lumen of and slidable with respect to delivery tool  771 . For some applications, as shown, device  762  follows a path that extends distally from a distal end of delivery tool  771 , touches annulus  68 , and projects distally along leaflet  123  and toward the ventricle, as shown in  FIG. 54B . In such a manner, device  762  functions as vertical elements  716 , e.g., rods as described hereinabove with reference to  FIGS. 51A-C . Optionally, for some applications, device  762  extends longitudinally along body portion  772  of implant  770 . 
     As implant  770  is delivered within delivery tool  771 , device  762  is disposed in alignment with, e.g., parallel to, body portion  772  of implant  770 . As implant  770  is disposed in a linear configuration as shown in  FIG. 54A , device  762  is moved linearly along and/or in parallel with a longitudinal axis of body portion  772 . Once implant  770  is deployed along annulus  68 , implant  770  curves, and device  762  may passively extend away from the path along which implant  770  extends. That is, device  762  extends distally from the distal end of delivery tool  771 , at a nonzero angle, e.g., perpendicularly, with respect to the plane of annulus  68 . As implant  770  is deployed from within delivery tool  771 , it extends distally while device  762  remains in place. Alternatively, or in succession, device  762  is pulled proximally with respect to body portion  772  by pulling on elongate radiopaque element  764  and/or pulling on tube  768  with respect to body portion  772 . 
     Annulus-marking device  762  is placed (e.g., pushed) against tissue of the valve, e.g., by virtue of being already disposed distally to a distal end of body portion  772 , or by being advanced distally after the distal end of the body portion has itself been placed against tissue of the valve. Device  762  thereby comprises a tissue-engaging portion that is configured to be placed in contact with tissue of the subject. 
     In one or more ways, the behavior of device  762  in response to being placed against the tissue of the valve facilitates guidance by viewing of body portion  772  (e.g., positioning of the body portion on the annulus). For example: 
     Resistance of device  762  being pushed further distally may indicate that the device is in contact with tissue that resists forces applied by the guide. For example, the distal end of the device may be abutting annulus  68  and/or a wall of the atrium. Conversely, lack of resistance of device  762  to being pushed further distally may indicate that the distal end of the device is not in contact with tissue that resists forces applied by the guide. For example, the distal end of the device may be moving between leaflets  123  of the valve (e.g., at a commissure), and/or may be pushing a leaflet  123  downward (e.g., into the ventricle). Such resistance (or lack thereof) can be detected mechanically (e.g., as tactile feedback to the operating physician and/or by an extracorporeal control unit). Since device  762  comprise radiopaque material, such resistance (or lack thereof) can be detected via imaging (e.g., fluoroscopically). 
     Similarly, the position, orientation and/or shape of device  762  (e.g., with respect to body portion  772  of implant  770 , tissue of the valve, etc.) may indicate against what, if anything, the device  762  is disposed. Imaging techniques such as fluoroscopy can be used to identify this position, orientation and/or shape of the device. For example, if the distal end of device  762  is positioned at the same height (i.e., at the same place on a superior-inferior axis of the subject) as the distal end of body portion  772 , this may indicate that body portion  772  and device  762  abut the same surface (e.g., annulus  68 ). Conversely, if the distal end of device  762  is positioned lower than body portion  772 , this may indicate that the body portion  772  is disposed against annulus  68 , while device  762  has passed toward or into the ventricle. Movement (e.g., beating) of the device  762  may indicate that the guide is disposed against a leaflet of the valve, and that the leaflet is moving the device as the heart beats. Such imaging may be facilitated by one or more components comprising radiopaque markings. For some applications, each device  762  comprises radiopaque filaments  766 , so as to facilitate identification during imaging. 
     Filaments  766  comprise radiopaque material (e.g., nitinol or stainless steel) and can be configured to be extremely flexible. Filaments  766  project away from elongate radiopaque element  764 . For some applications, filaments  766  sway with movement of the blood. For some applications, filaments  766  press against tissue of annulus  68  and tissue coupled thereto (as shown in  FIG. 54B ), such as tissue of an atrial wall as well as tissue of leaflets  123  of the native valve. Filaments  766  thus provide enhanced imaging of tissue of valve  64 . That is, when filaments  766  appear bent or pressed, this imaging detects annulus tissue, while when filaments  766  are straight, this could indicate the orifice of the valve. 
     Filaments  766  disposed above leaflet  123  remain static, while filaments  766  disposed at leaflets  123  move and pulse with leaflet movement. A boundary between the moving and static filaments can be observed using fluoroscopy in order to indicate the root, or base, or leaflet  123 . 
       FIG. 54B  shows body portion  772  having been placed against annulus  68  of the subject in a vicinity of left fibrous trigone. Device  762  is disposed distally to body portion  772 , and has splayed across annulus  68 , e.g., due to resistance of the annulus. As described hereinabove, this can be detected mechanically and/or by imaging. The position, orientation and/or shape of device  762 , alone and/or in combination with the other elements indicates that the portion of body portion  772  is positioned against firm tissue that is close to the commissure, which for some applications is the preferred position for anchoring of the portion of body portion  772 . Identification (e.g., mechanically and/or by imaging) of which guide is in which position can further indicate the rotational orientation of body portion  772 . 
     Once the desired position has been identified, an anchor (e.g., a first anchor) is used to anchor body portion  772 . For some applications, device  762  and elongate radiopaque element  764  can be withdrawn slightly proximally before anchoring, e.g., so as to reduce a likelihood of inadvertently anchoring the guide to the tissue. Subsequently, additional portions of body portion  772  are anchored to annulus  68 . In some applications, device  762  is moved proximally with respect to body portion  772 . This process can be repeated for each anchor until implant  770  is fully implanted. 
     Following implantation of implant  770 , device  762  is removed from the body of the subject. Device  762  is removed from within the body by pulling proximally on elongate radiopaque element  764 . Filaments  766  collapse within a lumen of delivery tool  771 . 
     It is to be noted that although system  760  is shown on mitral valve  64 , system  760  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is made to  FIGS. 55A-C , which are schematic illustrations of a navigational-based guidance system  780 , which employs one or more longitudinal guides configured to facilitate guidance of an implant  788  (e.g., an annuloplasty structure) to specific portions of annulus  68  by the guides contacting a surface of the valve (e.g., the annulus, commissure, and/or leaflets of the valve), in accordance with some applications. The longitudinal guide comprises an annulus marking device  782  which comprises a radiopaque wire extension  784  and at least one radiopaque distal curved tip  785  that is disposed at a nonzero angle with respect to wire extension  784 . A radiopaque element, e.g., a bead or bulb  786  is disposed at the end of tip  785  and functions to increase radiopacity of device  782 . It is to be noted that device  782  can comprise radiopaque elements or filaments  99  described hereinabove. Further, the system  780  can include features, components, elements, etc. from other systems and embodiments herein. For example, the system  780  can include and/or be used with a scaffolding  713  and/or basket  714  (e.g., as shown in  FIG. 51A-C ), and the scaffolding  713  and/or basket  714  can include any of the features described above, e.g., radiopaque elements, filaments, etc. As another example, the system  780  can include and/or be used with one or more radiopaque markers or loops  734  (e.g., as shown in  FIG. 52A-B ), and the markers or loops  734  can include any of the features described above, e.g., radiopaque elements, filaments, etc. spaced there around. Other combinations are also possible. 
     Device  782  comprises a tissue-engaging portion that is configured to be placed in contact with tissue of the subject. For some applications, device  782  comprises a flexible wire. For some applications, device  782  comprises a flexible rod, tube, line, etc. The device  782  comprises a flexible material (e.g., a flexible metal such as nitinol or stainless steel). A tube surrounds device  782 , and device  782  slides with respect to the tube. In some embodiments, as shown, the device  782  can exit the tube at a distal end opening. In some embodiments, the device  782  can exit the tube at a side opening in a side of the tube. The tube and device  782  are delivered to valve  64  using a delivery tool  783 . 
     As shown in  FIG. 55A , tool  783  places device  782  at a first angle of delivery α (alpha) with respect to a planar surface of leaflet  123 . As shown in  FIG. 55B , tool  783  moves device  782  along leaflet  123  such that distal curved tip  785  moves incrementally along leaflet  123  toward annulus  68  and toward the base of leaflet  123 . As tool  783  moves device  782 , the angle of delivery of annulus-marking device  782  with respect to the planar surface of leaflet  123  changes. For example, in  55 A, the first angle of delivery α is narrower than a second angle of delivery β (beta) shown in  FIG. 55B . For some applications of the present invention, the operating physician moves device  782 . For some applications of the present invention, movement of leaflet  123  moves device  782 . Movement of device  782  is visualized using fluoroscopy. Once curved tip  785  stops moving, the operating physician determines that tip  785  is at the base of leaflet  123 . 
     For some applications of the present invention, curved tip  785  is curved because it has shape-memory. For some applications of the present invention, curved tip  785  is curved because it presses against tissue of the subject. 
     In one or more ways, the behavior of device  782  in response to being placed against the tissue of the valve facilitates guidance by viewing of implant  788  (e.g., positioning of the implant on the annulus). 
     Resistance of device  782  being pushed further distally may indicate that device  782  is in contact with tissue that resists forces applied by device  782 . For example, tip  785  of device  782  may be abutting annulus  68  and/or a wall of the atrium. Conversely, lack of resistance of device  782  to being pushed further distally may indicate that the distal end of the device is not in contact with tissue that resists forces applied by the guide. For example, the distal end of the device may be moving between leaflets  123  of the valve (e.g., at a commissure), and/or may be pushing a leaflet  123  downward (e.g., into the ventricle). Such resistance (or lack thereof) can be detected mechanically (e.g., as tactile feedback to the operating physician and/or by an extracorporeal control unit). Since device  782  comprise radiopaque material, such resistance (or lack thereof) can be detected via imaging (e.g., fluoroscopically). In some embodiments, the device  782  can be used under the annulus, e.g., similar to the position of wire  906  and magnet  908  shown in  FIG. 65 ). In some embodiments, the device  782  can be configured and used similarly to wire  906  and/or bead/bulb  786  can be magnetic, include a magnet (e.g., magnet  908  or the like), or include a magnetic or ferrous material. 
     In some embodiments, the position, orientation and/or shape of device  782  (e.g., with respect to the body portion implant  788 , tissue of the valve, etc.) may indicate against what, if anything, the device  782  is disposed. Imaging techniques such as fluoroscopy can be used to identify this position, orientation and/or shape of the device. For example, if the distal end of device  782  is positioned at the same height (i.e., at the same place on a superior-inferior axis of the subject) as implant  788 , this may indicate that implant  788  and device  782  abut the same surface (e.g., annulus  68 ). Conversely, if the distal end of device  782  is positioned lower than implant  788 , this may indicate that implant  788  is disposed against annulus  68 , while device  782  has passed toward or into the ventricle. Movement (e.g., beating) of the device  782  may indicate that the guide is disposed against a leaflet of the valve, and that the leaflet is moving the device as the heart beats. Such imaging can be facilitated by one or more components comprising radiopaque markings. 
     Once the desired position has been identified, an anchor  789  (e.g., a first anchor) is used to anchor implant  788 . For some applications, device  782  can be withdrawn slightly proximally before anchoring, e.g., so as to reduce a likelihood of inadvertently anchoring the guide to the tissue. Subsequently, additional portions of implant  788  are anchored to annulus  68  using device  782  as a guide for each anchor implantation. 
     Following implantation of implant  788 , device  782  is removed from the body of the subject. Device  782  is removed from within the body by pulling proximally device  782  within a lumen of delivery tool  783 . 
     It is to be noted that although system  780  is shown on mitral valve  64 , system  780  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 56A-C , which are schematic illustrations of a system  790  comprising an annulus-marking device  792  that is positioned in the orifice of valve  64  in advance of an implant, e.g., an annuloplasty structure  795 , in accordance with some applications. For some applications, annulus-marking device  792  is placed in advance of the implant such that device  792  guides the implantation of the implant. Annulus-marking device  792  extends from within a delivery tool  791 . For some applications, annulus-marking device  792  comprises a stent-like expandable radiopaque braided mesh  794 , e.g., a fabric or metal mesh, that is positioned partially within the orifice of valve  64  and does not significantly interfere with function of valve  64 . For some applications, annulus-marking device  792  comprises a conical stent. As successive portions of annuloplasty structure  795  are extended from within its delivery tool and are positioned along successive portions of annulus  68 , annulus-marking device  792  guides the successive portions of structure  795  under imaging as annulus-marking device  792  comprises a radiopaque material (e.g., nitinol or stainless steel). For some applications, annulus-marking device  792  comprises a balloon made of nylon that is wholly or partially radiopaque and/or is coupled to radiopaque elements. 
     Device  792  is delivered within tool  791  in a collapsed state. Once deployed from within tool  791 , device  792  expands to assume a frustoconical shape. Device  792  comprises two or more, e.g., three as shown, pull wires  793  extending along a perimeter of mesh  794 . Pull wires  793  are pullable to transition braided mesh  794  from the frustoconical shape in  FIG. 56A  to a shape shown in  FIGS. 56B-C  in which mesh  794  assumes (1) a sloped upper portion  796  configured for positioning within the atrium, (2) a bulging ledge portion  798  configured for positioning above valve  64 , (3) a narrow portion  799  for positioning within valve  64 , and (4) a trumpet portion  797  configured for expanding within the ventricle. Pull wires  793  are pulled by the operating physician. For some applications, sloped upper portion  796  and bulging ledge portion  798  collectively assume a pear shape. 
     For some applications of the present invention, alternatively or additionally to pull wires  793 , mesh  794  is manufactured in a manner in which mesh  794  has a variable pitch when assuming the shapes as shown in  FIGS. 56B-C . That is the mesh at each of (1) sloped upper portion  796 , (2) bulging ledge portion  798 , (3) narrow portion  799 , and (4) a trumpet portion  797  is woven to have a different pitch which provides each portion with a diameter that is different from the other portions. 
     For some applications, as shown in  FIG. 56B , bulging ledge portion  798  has a greater diameter than the other portions  796 ,  797 , and  799  of the annulus-marking device  792 . For some applications, as shown in  FIG. 56C , trumpet portion  797  has a greater diameter than the other portions  796 ,  798 , and  799  of the annulus-marking device  792 . 
     Annulus-marking device  792  provides an indication of a specific section of annulus  68  immediately preceding the placement of the successive portion of structure  795  along annulus  68 . Bulging ledge portion  798  rests atop annulus  68 . Annulus-marking device  792  is at least partly stiff, and provides resistance, which facilitates positioning of structure  795 . For some applications, structure  795  slides along sloped upper portion  796 . In such applications, device  792  provides fluoroscopic and mechanical guidance of implantation of structure  795 . Sloped upper portion  796  is narrow at its proximal end in order to facilitate ease of positioning of the delivery tool used to deliver annuloplasty structure between device  792  and atrial wall  122 . Annulus-marking device  792  can also provide tactile feedback to the operating physician. Annulus-marking device  792  assumes a distinct shape in  FIGS. 56B-C  which helps the operating physician discern anatomy of the subject. For some applications, narrow portion  799  and/or bulging ledge portion  798  can comprise additional radiopaque markers to help calculate a height of annulus  68 . 
     For some applications, mesh  794  comprises two or more leaflets in order to regulate blood flow while device  792  is positioned in valve  64 . 
     Annuloplasty structure  795  comprises a body portion which comprises a flexible material and has a longitudinal axis that runs along the length of the body portion (e.g., when the body portion is straightened). The body portion comprises radiopaque markings at sites along the length of the body portion to aid in imaging for accurate delivery of anchors to annulus  68  in order to anchor structure  795  to tissue of annulus  68 . 
     In addition to providing tactile feedback, annulus-marking device  792  can also facilitate positioning of the annuloplasty structure  795  by facilitating imaging (e.g., fluoroscopy) and mechanical guidance. For example, the presence of annulus-marking device  792  and/or the shape thereof (e.g., bending due to being pressed against an atrial wall) is visible in fluoroscopic imaging, and can be used to facilitate identification of the position and angle of annuloplasty structure  795  with respect to tissues. Additionally, annulus-marking device  792  ensures that the delivery tool used to deliver structure  795  is positioned at an external perimeter of annulus-marking device  792  such that it is positioned between device  792  and atrial wall  122 . 
     Annulus-marking device  792  can be removed by being pulled and constrained within tool  791  in order to be retrieved and removed from the body of the subject. 
     In some applications, as shown, annulus-marking device  792  is delivered toward valve  64  in a delivery tool  791  that is separate from a delivery tool used to deliver the implant. For some applications, annulus-marking device  792  and the implant may be delivered from the same delivery tool. 
     It is to be noted that annulus-marking device  792  can be coupled to a plurality of radiopaque elements or filaments  99  and can be shaped in any suitable shape. 
     It is to be noted that although system  790  is shown on mitral valve  64 , system  790  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 57A-B , which are schematic illustrations of a system  800  comprising an annulus-marking device  802  that is positioned in the orifice of valve  64  in advance of an implant, e.g., an annuloplasty structure  812 , in accordance with some applications. For some applications, annulus-marking device  802  is placed in advance of the implant such that device  802  guides the implantation of the implant. Annulus-marking device  802  extends from within a delivery tool  801 . For some applications, annulus-marking device  802  comprises a stent-like, woven, expandable radiopaque braided mesh  804 , e.g., a fabric or metal mesh, that is positioned partially within the orifice of valve  64  and does not significantly interfere with function of valve  64 . For some applications, annulus-marking device  802  comprises a flexible, radiopaque material, e.g., nitinol or stainless steel. For some applications, annulus-marking device  802  comprises a conical stent. As successive portions of annuloplasty structure  812  are extended from within its delivery tool and are positioned along successive portions of annulus  68 , annulus-marking device  802  guides the successive portions of structure  812  under imaging as annulus-marking device  802  comprises a radiopaque material (e.g., nitinol or stainless steel). For some applications, annulus-marking device  802  comprises a balloon made of nylon that is wholly or partially radiopaque and/or is coupled to radiopaque elements. 
     Device  802  is delivered within tool  801  in a collapsed state. Once deployed from within tool  801 , device  802  expands to assume a frustoconical shape or a generally bulbous or generally spherical shape. In the expanded state, mesh assumes (1) a sloped upper portion  808  configured for positioning within the atrium, and (2) an asymmetrical portion  810  for positioning within valve  64 . Mesh  804  of device  802  is manufactured such that it defines a curved portion  806  of asymmetrical portion  810  of device  802  that is meant to rest against the aortic valve of the heart and begins above the aortic valve in a manner in which device  802  does not interfere with or add any pressure to the aortic valve. That is, mesh  804  is manufactured such that the braid is shorter at a given distal portion (i.e., curved portion  806 ) of mesh  804 . Mesh  804  curves upward in curved portion  806  designated for implantation against the aortic valve. 
     For some applications, mesh  804  comprises a trumpet portion (not shown) as described hereinabove with reference to  FIGS. 56B-C , that is disposed distally to asymmetrical portion  810 . For some applications, the trumpet portion has a greater diameter than the other portions  808  and  810  of the annulus-marking device  802 . 
     Annulus-marking device  802  provides an indication of a specific section of annulus  68  immediately preceding the placement of the successive portion of structure  812  along annulus  68 . Sloped upper portion  808  rests atop annulus  68 . Annulus-marking device  802  is at least partly stiff, and provides resistance, which facilitates positioning of structure  812 . For some applications, structure  812  slides along sloped upper portion  808 . In such applications, device  802  provides fluoroscopic and mechanical guidance of implantation of structure  812 . Sloped upper portion  808  is narrow at its proximal end in order to facilitate ease of positioning of the delivery tool used to deliver annuloplasty structure between device  802  and atrial wall  122 . Annulus-marking device  802  may also provide tactile feedback to the operating physician. Annulus-marking device  802  assumes a distinct shape in  FIGS. 57A-B  which helps the operating physician discern anatomy of the subject. For some applications, asymmetrical portion  810  can comprise additional radiopaque markers to help calculate a height of annulus  68 . 
     For some applications, mesh  804  comprises two or more leaflets in order to regulate blood flow while device  802  is positioned in valve  64 . 
     Annuloplasty structure  812  comprises a body portion which comprises a flexible material and has a longitudinal axis that runs along the length of the body portion (e.g., when the body portion is straightened). The body portion comprises radiopaque markings to aid in imaging for accurate delivery of anchors to annulus  68  in order to anchor structure  812  to tissue of annulus  68 . 
     In addition to providing tactile feedback, annulus-marking device  802  may also facilitate positioning of the annuloplasty structure  812  by facilitating imaging (e.g., fluoroscopy) and mechanical guidance. For example, the presence of annulus-marking device  802  and/or the shape thereof (e.g., bending due to being pressed against tissue) is visible in fluoroscopic imaging, and can be used to facilitate identification of the position and angle of annuloplasty structure  812  with respect to tissues. Additionally, annulus-marking device  802  ensures that the delivery tool used to deliver structure  812  is positioned at an external perimeter of annulus-marking device  802  such that it is positioned between device  802  and atrial wall  122 . 
     Annulus-marking device  802  can be removed by being pulled and constrained within tool  801  in order to be retrieved and removed from the body of the subject. 
     In some applications, as shown, annulus-marking device  802  is delivered toward valve  64  in a delivery tool  801  that is separate from a delivery tool used to deliver the implant. For some applications, annulus-marking device  802  and the implant may be delivered from the same delivery tool. 
     It is to be noted that annulus-marking device  802  can be coupled to a plurality of radiopaque elements or filaments  99  and can be shaped in any suitable shape. 
     It is to be noted that although system  800  is shown on mitral valve  64 , system  800  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 58A-B , which are schematic illustrations of a system  820  comprising an annulus-marking device  822  that is positioned in the orifice of valve  64  in advance of an implant, e.g., an annuloplasty structure  812 , in accordance with some applications. Except as described hereinbelow, annulus-marking device  822  can be the same as or generally similar to annulus-marking device  802 , described hereinabove with reference to  FIGS. 57A-B  and like reference numerals refer to like parts. Delivery tool  801  is used to deliver annulus-marking device  822 . For some applications, annulus-marking device  822  comprises a flexible, radiopaque material, e.g., nitinol or stainless steel. A stabilizing rod  823  is coupled at a distal end to a tissue anchor  824  that is configured to be reversibly anchored to cardiac tissue. Annulus-marking device  822  slides around stabilizing rod  823  longitudinally. Stabilizing rod  823  is configured to stabilize and guide positioning of the annulus-marking device  822 . For some applications, stabilizing rod  823  is semi-rigid. For some applications, stabilizing rod  823  functions as a track for positioning an expandable mesh  804 . For some applications, annulus-marking device  822  is placed in advance of the implant such that device  822  guides implantation of the implant. Annulus-marking device  822  extends from within delivery tool  801 . For some applications, annulus-marking device  822  comprises a stent-like, woven, expandable radiopaque braided mesh  804 , e.g., a fabric or metal mesh, that is positioned partially within the orifice of valve  64  and does not significantly interfere with function of valve  64 . For some applications, annulus-marking device  822  comprises a conical stent. As successive portions of annuloplasty structure  812  are extended from within its delivery tool and are positioned along successive portions of annulus  68 , annulus-marking device  822  guides the successive portions of structure  812  under imaging as annulus-marking device  822  comprises a radiopaque material (e.g., nitinol or stainless steel). For some applications, annulus-marking device  822  comprises a balloon made of nylon that is wholly or partially radiopaque and/or is coupled to radiopaque elements. 
     Mesh  804  of device  822  is manufactured such that it defines a curved portion  806  of asymmetrical portion  810  of device  822  that is meant to rest against the aortic valve of the heart and begins above the aortic valve in a manner in which device  822  does not interfere with or add any pressure to the aortic valve. That is, mesh  804  is manufactured such that the braid is shorter at a given distal portion (i.e., curved portion  806 ) of mesh  804 . Mesh  804  curves upward in curved portion  806  designated for implantation against the aortic valve. 
     For some applications, mesh  804  comprises a trumpet portion (not shown) as described hereinabove with reference to  FIGS. 56B-C , that is disposed distally to asymmetrical portion  810 . For some applications, the trumpet portion has a greater diameter than the other portions  808  and  810  of the annulus-marking device  822 . 
     Annulus-marking device  822  can be removed by being pulled and constrained within tool  801  in order to be retrieved and removed from the body of the subject. 
     It is to be noted that annulus-marking device  822  can be coupled to a plurality of radiopaque elements or filaments  99  and can be shaped in any suitable shape. 
     It is to be noted that although system  820  is shown on mitral valve  64 , system  820  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 59A-B , which are schematic illustrations of a system  830  comprising an annulus-marking device  832  that is positioned in the orifice of valve  64  in advance of an implant, e.g., an annuloplasty structure  812 , in accordance with some applications. Except as described hereinbelow, annulus-marking device  832  can be the same as or generally similar to annulus-marking device  802 , described hereinabove with reference to  FIGS. 57A-B  and like reference numerals refer to like parts. Delivery tool  801  is used to deliver annulus-marking device  832 . For some applications, annulus-marking device  832  comprises a flexible, radiopaque material, e.g., nitinol or stainless steel. A plurality of expandable radiopaque elements  834  are coupled to a distal end portion of expandable radiopaque braided mesh  804  and are configured to expand radially such that the plurality of expandable radiopaque elements  834  provide an indication as to a location of heart valve annulus  68 . For some applications, the plurality of expandable elements  834  comprises a flexible, radiopaque material, e.g., nitinol or stainless steel. The plurality of expandable elements  834  form device  832  into a generally umbrella shape for facilitating imaging of cardiac tissue during implantation of the implant. For some applications, the plurality of expandable elements  834  collectively form annulus-marking device  832  into a generally spherical shape. For some applications, the plurality of expandable elements  834  collectively form annulus-marking device  832  into a partially-bulbous shape. For some applications, the plurality of expandable elements  834  comprise a plurality of woven radiopaque fibers assuming a mesh. For some applications, the plurality of expandable elements  834  comprise a plurality of curved wires. 
     For some applications, the plurality of expandable elements  834  function as plurality of expandable elements  376  described hereinabove with reference to  FIGS. 17A-C . 
     For some applications, the plurality of expandable elements  834  is separate from mesh  804 . For such applications, the plurality of expandable elements  834  may be delivered to valve  64  in advance of delivery of mesh  804 . Mesh  804  can slide over the plurality of expandable elements  834  which function as a track and a guide for the positioning of mesh  804 . 
     For some applications, annulus-marking device  832  is placed in advance of the implant such that device  832  guides implantation of the implant. Annulus-marking device  832  extends from within delivery tool  801 . For some applications, annulus-marking device  832  comprises a stent-like, woven, expandable radiopaque braided mesh  804 , e.g., a fabric or metal mesh, that is positioned partially within the orifice of valve  64  and does not significantly interfere with function of valve  64 . For some applications, annulus-marking device  832  comprises a conical stent. As successive portions of annuloplasty structure  812  are extended from within its delivery tool and are positioned along successive portions of annulus  68 , annulus-marking device  832  guides the successive portions of structure  812  under imaging as annulus-marking device  832  comprises a radiopaque material (e.g., nitinol or stainless steel). For some applications, annulus-marking device  832  comprises a balloon made of nylon that is wholly or partially radiopaque and/or is coupled to radiopaque elements. 
     Mesh  804  of device  832  is manufactured such that it defines a curved portion  806  of asymmetrical portion  810  of device  832  that is meant to rest against the aortic valve of the heart and begins above the aortic valve in a manner in which device  832  does not interfere with or add any pressure to the aortic valve. That is, mesh  804  is manufactured such that the braid is shorter at a given distal portion (i.e., curved portion  806 ) of mesh  804 . Mesh  804  curves upward in curved portion  806  designated for implantation against the aortic valve. 
     For some applications, mesh  804  comprises a trumpet portion (not shown) as described hereinabove with reference to  FIGS. 56B-C , that is disposed distally to asymmetrical portion  810 . For some applications, the trumpet portion has a greater diameter than the other portions  808  and  810  of the annulus-marking device  832 . 
     Annulus-marking device  832  can be removed by being pulled and constrained within tool  801  in order to be retrieved and removed from the body of the subject. 
     It is to be noted that annulus-marking device  832  can be coupled to a plurality of radiopaque elements or filaments  99  and can be shaped in any suitable shape. 
     It is to be noted that although system  830  is shown on mitral valve  64 , system  830  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 60A-B , which are schematic illustrations of a system  840  comprising an annulus-marking device  842  that is positioned in the orifice of valve  64  in advance of an implant, e.g., an annuloplasty structure  812 , in accordance with some applications. Except as described hereinbelow, annulus-marking device  842  can be the same as or generally similar to annulus-marking device  802 , described hereinabove with reference to  FIGS. 57A-B  and like reference numerals refer to like parts. Delivery tool  801  is used to deliver annulus-marking device  842 . For some applications, annulus-marking device  842  comprises a flexible, radiopaque material, e.g., nitinol or stainless steel. A plurality of expandable snares (or hooks)  844  are coupled to a distal end portion of expandable radiopaque braided mesh  804  and are configured to expand radially such that the plurality of expandable snares  844  ensnare leaflets  123  and/or tissue of valve  64 . For some applications of the present invention, snares  844  comprises a radiopaque material, e.g., nitinol or stainless steel, and provide an indication as to a location of heart valve annulus  68 . For some applications, the plurality of expandable snares  844  are flexible. For some applications, the plurality of expandable snares  844  are rigid. For some applications, each one of the plurality of snares  844  extend distally from a distal end of expandable radiopaque braided mesh  804  and then curve proximally to a proximal tip  846 . 
     When mesh  804  is pulled proximally, snares  844  ensnare and engage the native leaflets  123  of valve  64 . By the ensnaring of leaflets  123 , snares  844  sandwich valve  64  between snares  844  and mesh  804 . Such ensnaring helps temporarily anchor device  842  to valve  64 . 
     For some applications, annulus-marking device  842  is placed in advance of the implant such that device  842  guides implantation of the implant. Annulus-marking device  842  extends from within delivery tool  801 . For some applications, annulus-marking device  842  comprises a stent-like, woven, expandable radiopaque braided mesh  804 , e.g., a fabric or metal mesh, that is positioned partially within the orifice of valve  64  and does not significantly interfere with function of valve  64 . For some applications, annulus-marking device  842  comprises a conical stent. As successive portions of annuloplasty structure  812  are extended from within its delivery tool and are positioned along successive portions of annulus  68 , annulus-marking device  842  guides the successive portions of structure  812  under imaging as annulus-marking device  842  comprises a radiopaque material (e.g., nitinol or stainless steel). For some applications, annulus-marking device  842  comprises a balloon made of nylon that is wholly or partially radiopaque and/or is coupled to radiopaque elements. 
     Mesh  804  of device  842  is manufactured such that it defines a curved portion  806  of asymmetrical portion  810  of device  842  that is meant to rest against the aortic valve of the heart and begins above the aortic valve in a manner in which device  842  does not interfere with or add any pressure to the aortic valve. That is, mesh  804  is manufactured such that the braid is shorter at a given distal portion (i.e., curved portion  806 ) of mesh  804 . Mesh  804  curves upward in curved portion  806  designated for implantation against the aortic valve. 
     For some applications, mesh  804  comprises a trumpet portion (not shown) as described hereinabove with reference to  FIGS. 56B-C , that is disposed distally to asymmetrical portion  810 . For some applications, the trumpet portion has a greater diameter than the other portions  808  and  810  of the annulus-marking device  842 . 
     Annulus-marking device  842  can be removed by being pulled and constrained within tool  801  in order to be retrieved and removed from the body of the subject. 
     It is to be noted that annulus-marking device  842  can be coupled to a plurality of radiopaque elements or filaments  99  and can be shaped in any suitable shape. 
     It is to be noted that although system  840  is shown on mitral valve  64 , system  840  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 61A-B , which are schematic illustrations of a system  850  comprising an annulus-marking device  852  that is positioned in the orifice of valve  64  in advance of an implant, e.g., an annuloplasty structure  812 , in accordance with some applications. Except as described hereinbelow, annulus-marking device  852  can be the same as or generally similar to annulus-marking device  802 , described hereinabove with reference to  FIGS. 57A-B  and like reference numerals refer to like parts. Delivery tool  801  is used to deliver annulus-marking device  852 . For some applications, annulus-marking device  852  comprises a flexible, radiopaque material, e.g., nitinol or stainless steel. An inflatable annular element  854  is coupled to a distal end portion of expandable radiopaque braided mesh  804 . Inflatable annular element  854  is configured to position and temporarily anchor expandable radiopaque braided mesh  804  within native valve  64 . For some applications, inflatable annular element  854  comprises a radiopaque material. For some applications, inflatable annular element  854  comprises a balloon, e.g., a compliant balloon. Typically, inflatable annular element  854  comprises a prosthetic valve comprising two or more leaflets  856  which regulate blood flow while device  852  is positioned in valve  64 . As shown, expandable radiopaque braided mesh  804  is positionable within valve  64 , while inflatable annular element  854  is positionable below the native heart valve, e.g., in a subannular space. 
     When mesh  804  is pulled proximally, inflatable element  854  engages with subannular tissue thereby temporarily anchoring device  852  to valve  64 . 
     For some applications, annulus-marking device  852  is placed in advance of the implant such that device  852  guides implantation of the implant. Annulus-marking device  852  extends from within delivery tool  801 . For some applications, annulus-marking device  852  comprises a stent-like, woven, expandable radiopaque braided mesh  804 , e.g., a fabric or metal mesh, that is positioned partially within the orifice of valve  64  and does not significantly interfere with function of valve  64 . For some applications, annulus-marking device  852  comprises a conical stent. As successive portions of annuloplasty structure  812  are extended from within its delivery tool and are positioned along successive portions of annulus  68 , annulus-marking device  852  guides the successive portions of structure  812  under imaging as annulus-marking device  852  comprises a radiopaque material (e.g., nitinol or stainless steel). For some applications, annulus-marking device  852  comprises a balloon made of nylon that is wholly or partially radiopaque and/or is coupled to radiopaque elements. 
     Mesh  804  of device  852  is manufactured such that it defines a curved portion  806  of asymmetrical portion  810  of device  852  that is meant to rest against the aortic valve of the heart begins above the aortic valve in a manner in which device  852  does not interfere with or add any pressure to the aortic valve. That is, mesh  804  is manufactured such that the braid is shorter at a given distal portion (i.e., curved portion  806 ) of mesh  804 . Mesh  804  curves upward in curved portion  806  designated for implantation against the aortic valve. 
     For some applications, mesh  804  comprises a trumpet portion (not shown) as described hereinabove with reference to  FIGS. 56B-C , that is disposed distally to asymmetrical portion  810 . For some applications, the trumpet portion has a greater diameter than the other portions  808  and  810  of the annulus-marking device  852 . 
     Annulus-marking device  852  can be removed by deflating inflatable element  854  and then pulling and constraining device  852  within tool  801  in order to be retrieved and removed from the body of the subject. 
     It is to be noted that annulus-marking device  852  can be coupled to a plurality of radiopaque elements or filaments  99  and can be shaped in any suitable shape. 
     It is to be noted that although system  850  is shown on mitral valve  64 , system  850  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 58A-61B . It is to be noted that any device described herein used to stabilize, temporarily anchor, and/or enhance the radiopacity of the annulus-marking devices, e.g., stabilizing rod  823 , expandable radiopaque elements  834 , expandable snares  844  and/or inflatable annular element  854  can be used in combination with any annulus-marking device described herein. 
     Reference is now made to  FIGS. 62A-B , which are schematic illustrations of a system  860  comprising an annulus-marking device  862  that is positioned in the orifice of valve  64  in advance of an implant, e.g., an annuloplasty structure  812 , in accordance with some applications. A delivery tool  861  is used to deliver annulus-marking device  862 . For some applications, annulus-marking device  862  comprises or is coupled to a flexible, radiopaque material. Device  862  comprises an inflatable temporary valve that is inflatable from a collapsed state to an inflated state. In the expanded state, the inflatable temporary valve defines (1) a proximal non-compliant cylindrical balloon  864  configured for positioning within the native heart valve  64  and partially within the atrium, and (2) a distal compliant balloon  866 , e.g., a toroidal balloon, configured for positioning in a subannular space of native heart valve  64 . Device  862  comprises two or more leaflets  868  which regulate blood flow while device  862  is positioned in valve  64 . 
     Device  862  is inflated until compliant balloon  866  cannot expand further due to resistance by the subannular tissue surrounding balloon  866 . When device  862  is pulled proximally, distal compliant balloon  866  engages with subannular tissue thereby temporarily anchoring device  862  to valve  64 . 
     For some applications, annulus-marking device  862  is placed in advance of the implant such that device  862  guides implantation of the implant. That is, the implant slides along the proximal non-compliant balloon  864 . 
     Annulus-marking device  862  can be removed by deflating balloons  864  and  866  and then pulling and constraining device  862  within tool  861  in order to be retrieved and removed from the body of the subject. 
     It is to be noted that annulus-marking device  862  can be coupled to a plurality of radiopaque elements or filaments  99  and can be shaped in any suitable shape. 
     It is to be noted that although system  860  is shown on mitral valve  64 , system  860  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIGS. 63A-B , which are schematic illustrations of a system  870  comprising an annulus-marking device  872  for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications. Device  872  comprises a scaffolding  874  that is collapsible and expandable. Scaffolding  874 , is configured, when expanded, to laterally push against tissue of valve  64  (e.g., leaflet  123 , annulus  68 , or a commissure). Scaffolding  874  is radiopaque and comprises a plurality of radiopaque elements  878  which are flexible and shaped as bulbs by way of illustration and not limitation. For some applications of the present invention, radiopaque elements  878  can comprise radiopaque elements or filaments  99  described hereinabove. 
     Scaffolding  874  comprises a central rod  873 , a first loop element  876  configured to expand laterally away from central rod  873  and a second loop element  876  configured to expand laterally away from central rod  873 . At least one curved, flexible wire  877  is coupled to and extends from central rod  873  at least within a space defined by first and second loop elements  876 . A first magnetic element  879  is coupled to an end of flexible wire  877  and is moveable by a second magnetic element that is not coupled to scaffolding  874 . That is, for some applications, the second magnetic element comprises a magnetic element  886  that is coupled to a distal end of a portion of a delivery tool  884  used to deliver a cardiac implant, e.g., annuloplasty structure  880 . 
     For some applications, wire  877  has shape-memory and curves toward the subannular groove. Wire  877  may be protractible and extendable from within rod  873 . For some applications, magnetic elements  879  and  886  comprise electromagnets. For some applications, magnetic elements  879  and  886  comprise ferromagnets. For some applications, magnetic element  879  comprises an electromagnet and magnetic element  886  comprises a ferromagnet. For some applications, magnetic element  879  comprises a ferromagnet and magnetic element  886  comprises an electromagnet. Wire  877  and magnetic element  879  are guided around the subannular groove responsively to movement of the portion of a delivery tool  884  and magnetic element  886  coupled thereto. As shown in  FIG. 63B , wire  877  and magnetic element  879  are guided toward a location along annulus  68  for implanting tissue anchor  882 . Once tissue anchor  882  is implanted, delivery tool  884  is moved to a different location along an atrial surface of annulus  68 , and wire  877  and magnetic element  879  are guided to a corresponding location in the subannular space of valve  64 . In such a manner, wire  877  and magnetic element  879  provide increased fluoroscopic visualization of the implantation procedure, because wire  877  and magnetic element  879  comprise radiopaque material. 
     First and second loop elements  876  are configured to expand laterally away from central rod  873 . For some applications, first and second loop elements  876  are configured to move longitudinally with respect to central rod  873 . When scaffolding  874  is expanded, a first half of each of first and second loop elements  876  is configured to be disposed in the atrium of the heart and a second half of each of first and second loop elements  876  is configured to be disposed in the ventricle of the heart. When scaffolding  874  is expanded, loop elements  876  are configured to push against the tissue of valve  64  as is described hereinbelow. It is to be noted that scaffolding  874  comprises two loop elements  876  by way of illustration and not limitation. For some applications, scaffolding  874  can comprise any number of loop elements  876 . Loop elements  876  comprise a flexible, radiopaque material, e.g., nitinol. The operating physician is able to discern whether loop elements  876  come in contact with tissue of the heart (e.g., leaflet, commissure, or annulus) by observing deformation of loop elements  876  responsively to the presence of tissue and the force applied to loop elements  876  by the tissue. 
     Scaffolding  874  helps stabilize device  872  in valve  64 . 
     For some applications, scaffolding  874  comprises two loop elements  876  to help center device  872  and/or its delivery tool  871  as each wire loop element  876  pushes against the tissue. It is to be noted that device  872  can comprise any suitable number of loop elements  876 . 
     A radius of expansion of scaffolding  874  is controlled by movement of structural elements  875  toward or away from each other along a central rod  873 . When elements  875  are distanced from each other, scaffolding  874  assumes a narrower configuration. The closer elements  875  are toward each other, the more expanded and wider scaffolding  87  is. For some applications, scaffolding  874  is manually expanded. For some applications, scaffolding  874  is configured to self-expand. 
     For some applications, loop elements  876  push against tissue of valve  64  at the commissures. For some applications, loop elements  876  push against tissue of leaflet  123  of valve  64 . For some applications, as each loop element  876  pushes against tissue of leaflet  123 , loop element  876  creates a bicuspidization of the leaflet in a manner in which leaflet  123  assumes two subcusps. 
     Annulus-marking device  872  is configured help visualize the placement of an implant (e.g., an annuloplasty structure  880 , as shown) configured for placement along annulus  68  of valve  64  of the subject. 
     Annulus-marking device  872  is coupled to a delivery tool  871  and is collapsible within a lumen of tool  871  during delivery of device  872  within valve  64 . Annulus-marking device  872  is retrievable upon removal of delivery tool  871  from the subject. 
     Scaffolding  874  comprises radiopaque material (e.g., nitinol or stainless steel) and is flexible. A plurality of radiopaque elements or radiopaque filaments  99  (not shown) can be coupled to scaffolding  874  at any suitable portion thereof. The plurality of radiopaque elements or radiopaque filaments  99  function as additional annulus-marking devices. Annulus-marking device  872  is configured for aiding implantation of cardiac devices under the guidance of imaging, in accordance with some applications. The steering procedure is performed with the aid of imaging, such as fluoroscopy, transesophageal echo, and/or echocardiography. 
     Device  872  may be delivered percutaneously, thoracoscopically through the chest, or using open heart surgical techniques. If delivered percutaneously, device  872  may be made from a superelastic material (e.g., nitinol or stainless steel) enabling it to be folded and collapsed such that it can be delivered in a catheter and subsequently self-expand into the desired shape and tension when released from the catheter. For example, percutaneous vascular access can be achieved by conventional methods into the femoral or jugular vein under image guidance (e.g., fluoroscopic, ultrasonic, magnetic element resonance, computed tomography, or combinations thereof). For some applications, device  872  comprises a wire. 
     Device  872  enables mapping of the anatomy of the atrium, atrial wall, heart valve, annulus, and ventricle. Additionally, device  872  is made from radiopaque material to facilitate fluoroscopic visualization. For some applications, tissue of valve annulus  68  and tissue coupled thereto is viewed using device  872 . Additionally, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  872  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing device  872  placed against the tissue. For some applications, the tissue of the native heart valve annulus  68  and tissue coupled thereto is viewed by imaging annulus-marking device  872  with respect to the tissue of the native heart valve annulus and the tissue coupled thereto by viewing movement of device  872  responsively to movement of the tissue. 
     Subsequently to implanting of annuloplasty structure  880 , annulus-marking device  872  is retrieved. Since device  872  is flexible and compressible, device  872  is constrained within the tool during the retrieval of device  872  and subsequent removal of device  872  from the body of the subject. That is, device  872  does not function as an implant for such embodiments and is used only to guide implantation of implant  684 ; rather, device  872  acts as a guide for implantation while placed temporarily within the body of the patient to be subsequently removed therefrom following the implantation of structure  880 . 
     It is to be noted that although system  870  is shown on mitral valve  64 , system  870  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 64 , which is a schematic illustration of a system  890  comprising an annulus-marking device  892  comprising at least one magnetic element, e.g., first and second magnetic elements  898  and  899 , configured to be positioned at respective cardiac tissue of or in a vicinity of valve  64  of the subject, in accordance with some applications. For some applications, first magnetic element  898  is coupled to a cross-beam  896  which extends from a central rod  894 . Cross-beam  896  is expandable within an atrium of the heart of the subject. For some applications, cross-beam  896  extends in both directions, laterally from central rod  894 . For some applications, cross-beam  896  extends in a single direction, laterally from central rod  894 . Central rod  894  is coupled at a distal end thereof to a tissue anchor  895  which is reversibly coupled to tissue of the heart (e.g., to tissue at the apex, as shown by way of illustration and not limitation). In such a manner, central rod  894  functions to stabilize device  892  during the imaging and implantation of the implant. For some applications, second magnetic element  897  is configured to be disposed in vasculature  899  surrounding valve  64  (e.g., in a right circumflex artery). For some applications, second magnetic element  897  comprises a wire that is partially ring-shaped, e.g., C-shaped. 
     For some applications, first and second magnetic elements  898  and  897  comprise electromagnets. For some applications, first and second magnetic elements  898  and  897  comprise ferromagnets. 
     For some applications, first magnetic element  898  comprises a ferromagnet while second magnetic element  897  comprises an electromagnet. For some applications, first magnetic element  898  comprises an electromagnet while second magnetic element  897  comprises a ferromagnet. 
     Device  892  is delivered to valve  64  using a delivery tool  891 , e.g., during transvascular approach or during a minimally-invasive procedure. 
     Positioning of first and second magnetic elements  898  and  897  generates a magnetic field. The magnetic field helps ensure proper positioning of device  892  with respect to tissue of valve  64 . The magnetic field helps prevent movement of device  892  with respect to tissue of valve  64 . 
     Once device  892  is positioned at valve  64 , under imaging guidance, e.g., under fluoroscopy, an implant, e.g., an annuloplasty structure, is implanted at annulus  68  of valve  64  using annulus-marking device  892  as a guide. The annuloplasty structure can be positioned between an external surface of magnetic element  898  and the atrial wall. 
     For some applications, tissue of native heart valve annulus  68  and tissue coupled thereto is viewed using device  892 , i.e., using magnetic elements  898  and  897 . The tissue is viewed by imaging annulus-marking device  892  with respect to the tissue of native heart valve annulus  68  and the tissue coupled thereto by viewing elements  898  and  897  against the tissue. Annulus-marking device  892  is imaged with respect to the tissue of native heart valve annulus  68 , tissue of at least one leaflet  123 , and tissue of the atrial wall. 
     Once the annuloplasty structure is implanted along annulus  68 , annulus-marking device  892  is retrieved. For some applications, device  892  is constrained within tool  891  and extracted from the body of the subject. 
     It is to be noted that although system  890  is shown on mitral valve  64 , system  890  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. 
     Reference is now made to  FIG. 65 , which is a schematic illustration of a system  900  comprising an annulus-marking device  902  comprising at least one magnetic element, e.g., first and second magnetic elements  908  and  899 , configured to be positioned at respective cardiac tissue of or in a vicinity of valve  64  of the subject, in accordance with some applications. For some applications, first magnetic element  908  is coupled to at least one curved, flexible wire  906  that is coupled to and extends from central tube or rod  904 . For some applications, wire  906  has shape-memory and curves toward the subannular groove. Wire  906  may be protractible and extendable from within rod  904 . In some applications, wire  906  extends and retracts from a side opening in rod or tube  904   
     In some embodiments, central rod  904  is coupled at a distal end thereof to a tissue anchor  905  which is reversibly coupled to tissue of the heart (e.g., to tissue at the apex, as shown by way of illustration and not limitation). In such a manner, central rod  904  functions to stabilize device  902  during the imaging and implantation of the implant. However, it is not required for central tube or rod  904  to extend so deep into the ventricle or to be anchored at all. For some applications, second magnetic element  897  is configured to be disposed in vasculature  899  surrounding valve  64  (e.g., in a right circumflex artery). For some applications, second magnetic element  897  comprises a wire that is partially ring-shaped, e.g., C-shaped. 
     For some applications, first and second magnetic elements  908  and  897  comprise electromagnets. For some applications, first and second magnetic elements  908  and  897  comprise ferromagnets. 
     For some applications, first magnetic element  908  comprises a ferromagnet while second magnetic element  897  comprises an electromagnet. For some applications, first magnetic element  908  comprises an electromagnet while second magnetic element  897  comprises a ferromagnet. 
     Device  902  is delivered to valve  64  using a delivery tool  901 , e.g., during transvascular approach or during a minimally-invasive procedure. 
     Positioning of first and second magnetic elements  908  and  897  generates a magnetic field. The magnetic field helps ensure proper positioning of device  902 , in particular wire  906 , with respect to tissue of valve  64 . The magnetic field helps prevent movement of device  902  with respect to tissue of valve  64 . 
     Once device  902  is positioned at valve  64 , under imaging guidance, e.g., under fluoroscopy, an implant, e.g., an annuloplasty structure, is implanted at annulus  68  of valve  64  using annulus-marking device  902  as a guide. The annuloplasty structure can be positioned between an external surface of magnetic element  908  and the atrial wall. 
     For some applications, tissue of native heart valve annulus  68  and tissue coupled thereto is viewed using device  902 , i.e., using magnetic elements  908  and  897 . The tissue is viewed by imaging annulus-marking device  902  with respect to the tissue of native heart valve annulus  68  and the tissue coupled thereto by viewing elements  908  and  897  placed against the tissue. Annulus-marking device  902  is imaged with respect to the tissue of native heart valve annulus  68 , tissue of at least one leaflet  123 , and tissue of the atrial wall. 
     Once the annuloplasty structure is implanted along annulus  68 , annulus-marking device  902  is retrieved. For some applications, device  902  is constrained within tool  901  and extracted from the body of the subject. 
     It is to be noted that although system  900  is shown on mitral valve  64 , system  900  can be used on any cardiac valve, e.g., a tricuspid valve, or any other tissue of the subject. Further, the system  900  can include features, components, elements, etc. from other systems and embodiments herein. For example, the system  900  can include and/or be used with a scaffolding  713  and/or basket  714  (e.g., as shown in  FIG. 51A-C ), and the scaffolding and/or basket  714  can include any of the features described above, e.g., radiopaque elements, filaments, etc. As another example, the system  900  can include and/or be used with one or more radiopaque markers or loops  734  (e.g., as shown in  FIG. 52A-B ), and the markers or loops  734  can include any of the features described above, e.g., radiopaque elements, filaments, etc. spaced there around. Other combinations are also possible. 
     It is to be noted that, whereas some techniques known in the art comprise selecting an annuloplasty structure based on a target (e.g., desired, calculated, and/or physiological) circumference of the posterior portion of the annulus, applications of the present invention comprise selecting an annuloplasty structure based on an existing (e.g., pathological) circumference of the annulus or a portion thereof (e.g., a posterior portion of the annulus). 
     Reference is made to  FIGS. 1-65 . Following implantation of the annuloplasty structures described herein, the dimensions of the annuloplasty structures can be adjusted remotely and while the patient is not on a cardio-pulmonary bypass pump (i.e., with a beating heart), under fluoroscopy and/or echo guidance. 
     Systems  20 ,  30 ,  40 ,  60 ,  90 ,  100 ,  120 ,  140 ,  160 ,  170 ,  180 ,  190 ,  200 ,  220 ,  230 ,  250 ,  270 ,  280 ,  300 ,  320 ,  330 ,  340 ,  350 ,  360 ,  370 ,  380 ,  390 ,  400 ,  410 ,  420 ,  430 ,  440 ,  450 ,  460 ,  470 ,  480 ,  490 ,  500 ,  510 ,  520 ,  530 ,  540 ,  560 ,  570 ,  580 ,  590 ,  600 ,  610 ,  620 ,  630 ,  640 ,  650 ,  670 ,  680 ,  700 ,  710 ,  730 ,  740 ,  760 ,  780 ,  790 ,  800 ,  820 ,  830 ,  840 ,  850 ,  860 ,  870 ,  890 , and  900  can be advanced using (1) a trans-septal procedure in which the system is advanced through vasculature of the patient at any suitable access location (e.g., femoral vein), (2) a minimally-invasive transapical approach (as shown in  FIG. 28 ), (3) a minimally-invasive transatrial approach (e.g., an intercostal approach), and/or (4) a surgical, open-heart approach. Furthermore, for some applications, the systems described herein are not steerable and can comprise straight elements (e.g., in a surgical, open-heart procedure). 
     Systems  20 ,  30 ,  40 ,  60 ,  90 ,  100 ,  120 ,  140 ,  160 ,  170 ,  180 ,  190 ,  200 ,  220 ,  230 ,  250 ,  270 ,  280 ,  300 ,  320 ,  330 ,  340 ,  350 ,  360 ,  370 ,  380 ,  390 ,  400 ,  410 ,  420 ,  430 ,  440 ,  450 ,  460 ,  470 ,  480 ,  490 ,  500 ,  510 ,  520 ,  530 ,  540 ,  560 ,  570 ,  580 ,  590 ,  600 ,  610 ,  620 ,  630 ,  640 ,  650 ,  670 ,  680 ,  700 ,  710 ,  730 ,  740 ,  760 ,  780 ,  790 ,  800 ,  820 ,  830 ,  840 ,  850 ,  860 ,  870 ,  890 , and  900  for imaging and repairing a dilated annulus of the patient can be used to treat any cardiac valve of the patient, e.g., the aortic valve, the pulmonary valve, the mitral valve, and the tricuspid valve. Systems described herein for treatment of valves can be used to treat other annular muscles within the body of the patient. For example, the systems described herein can be used in order to treat a sphincter muscle within a stomach of the patient. 
     The scope of the present invention includes the use systems  20 ,  30 ,  40 ,  60 ,  90 ,  100 ,  120 ,  140 ,  160 ,  170 ,  180 ,  190 ,  200 ,  220 ,  230 ,  250 ,  270 ,  280 ,  300 ,  320 ,  330 ,  340 ,  350 ,  360 ,  370 ,  380 ,  390 ,  400 ,  410 ,  420 ,  430 ,  440 ,  450 ,  460 ,  470 ,  480 ,  490 ,  500 ,  510 ,  520 ,  530 ,  540 ,  560 ,  570 ,  580 ,  590 ,  600 ,  610 ,  620 ,  630 ,  640 ,  650 ,  670 ,  680 ,  700 ,  710 ,  730 ,  740 ,  760 ,  780 ,  790 ,  800 ,  820 ,  830 ,  840 ,  850 ,  860 ,  870 ,  890 , and  900  (or subcomponents thereof) and methods described hereinabove on any suitable tissue of the patient (e.g., stomach tissue, urinary tract, and prostate tissue). 
     Reference is now made to  FIGS. 1-37 and 39-65 . Systems  20 ,  30 ,  40 ,  60 ,  90 ,  100 ,  120 ,  140 ,  160 ,  170 ,  180 ,  190 ,  200 ,  220 ,  230 ,  250 ,  270 ,  280 ,  300 ,  320 ,  330 ,  340 ,  350 ,  360 ,  370 ,  380 ,  390 ,  400 ,  410 ,  420 ,  430 ,  440 ,  450 ,  460 ,  470 ,  480 ,  490 ,  500 ,  510 ,  520 ,  530 ,  540 ,  570 ,  580 ,  590 ,  600 ,  610 ,  620 ,  630 ,  640 ,  650 ,  670 ,  680 ,  700 ,  710 ,  730 ,  740 ,  760 ,  780 ,  790 ,  800 ,  820 ,  830 ,  840 ,  850 ,  860 ,  870 ,  890 , and  900  are designed in order to enhance fluoroscopic imaging of cardiac procedures while reducing the need to utilize other types of imaging such as echocardiographic, ultrasound, etc. However, the radiopaque features and/or other features of these systems can also be configured to be more visible under echocardiography, ultrasound, etc., such as by adding an echogenic coating and/or modifying the features in other ways to be more visible. For some applications, the radiopaque features can be replaced and or modified to adapt those features and/or the associated structures for viewing under echocardiography, ultrasound, etc. 
     Reference is again made to  FIGS. 1-65 . The annuloplasty structures described herein and the methods of delivery therefor include annuloplasty structures and methods of delivery therefor as described in PCT Patent Application PCT/IL2013/050860 to Sheps et al., entitled, “Controlled steering functionality for implant-delivery tool,” filed on Oct. 23, 2013, which published as WO/2014/064694 and which is incorporated herein by reference. For some applications, the systems described herein can be used to guide implantation of annuloplasty structure combining a flat band, e.g., a braided fabric or metal band, as described hereinabove with reference to  FIG. 6B . That is, for such applications, the annuloplasty structure can be sleeveless, e.g., not tubular, etc. 
     Additionally, the scope of the present invention includes applications described in one or more of the following: 
     U.S. patent application Ser. No. 12/435,291 to Maisano et al., entitled, “Adjustable repair chords and spool mechanism therefor,” filed on May 4, 2009, which published as US Patent Application Publication 2010/0161041; 
     U.S. patent application Ser. No. 12/437,103 to Zipory et al., entitled, “Annuloplasty ring with intra-ring anchoring,” filed on May 7, 2009, which published as US Patent Application Publication 2010/0286767; 
     U.S. patent application Ser. No. 12/548,991 to Maisano et al., entitled, “Implantation of repair chords in the heart,” filed on Aug. 27, 2009, which published as US Patent Application Publication 2010/0161042; 
     PCT Patent Application PCT/IL2009/001209 to Cabiri et al., entitled, “Adjustable annuloplasty devices and mechanisms therefor,” filed on Dec. 22, 2009, which published as PCT Publication WO 10/073246; 
     PCT Patent Application PCT/IL2010/000357 to Maisano et al., entitled, “Implantation of repair chords in the heart,” filed on May 4, 2010, which published as WO 10/128502; 
     PCT Patent Application PCT/IL2010/000358 to Zipory et al., entitled, “Deployment techniques for annuloplasty ring and over-wire rotation tool,” filed on May 4, 2010, which published as WO 10/128503; 
     PCT Patent Application PCT/IL2012/050451 to Sheps et al., entitled, “Controlled steering functionality for implant-delivery tool,” filed on Nov. 8, 2012, which published as WO/2013/069019; and/or 
     PCT Patent Application PCT/IL2013/050860 to Sheps et al., entitled, “Controlled steering functionality for implant-delivery tool,” filed on Oct. 23, 2013, which published as WO/2014/064694. 
     All of these applications are incorporated herein by reference. Techniques described herein can be practiced in combination with techniques described in one or more of these applications. Further, each of the techniques, methods, operations, steps, etc. described herein can be performed on a living animal or on a non-living simulation, such as on a cadaver, cadaver heart, simulator (e.g. with the body parts, tissue, etc. being simulated), etc. 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.