Patent Publication Number: US-2023157568-A1

Title: Probe with radiopaque tag

Description:
RELATED APPLICATION INFORMATION 
     The present application is a continuation of, and claims the benefit of, U.S. patent application Ser. No. 16/797,619, filed Feb. 21, 2020, and entitled “PROBE WITH RADIOPAQUE TAG,” which claims benefit of U.S. Provisional Patent Application No. 62/852,272 of Govari, et al., filed on May 23, 2019. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to medical instruments, and in particular, but not exclusively to, tracking positions of medical instruments. 
     BACKGROUND 
     Medical images such as CT scans are often captured prior to a medical procedure and then registered with a position tracking coordinate system of a medical instrument so that the medical instrument may be displayed together with the medical scan to aid navigation of the medical instrument in a body-part by a physician. 
     By way of example, U.S. Pat. No. 6,317,621 to Graumann, et al., describes a method and apparatus for catheter navigation in three-dimensional vascular tree exposures, particularly for intercranial application, the catheter position is detected and mixed into the 3D image of the pre-operatively scanned vascular tree reconstructed in a navigation computer and an imaging (registering) of the 3D patient coordination system ensues on the 3D image coordination system prior to the intervention using a number of markers placed on the patient&#39;s body, the position of these markers being registered by the catheter. The markers of a C-arm x-ray device for 3D angiography are detected in at least two 2D projection images, from which the 3D angiogram is calculated, and are projected back on to the imaged subject in the navigation computer and are brought into relation to the marker coordinates in the patient coordinate system, using projection matrices applied to the respective 2D projection images, these matrices already having been determined for the reconstruction of the 3D volume set of the vascular tree. 
     U.S. Pat. No. 10,441,236 of Bar-tal, et al., describes a coordinate system registration module, including radiopaque elements arranged in a fixed predetermined pattern and configured, in response to the radiopaque elements generating a fluoroscopic image, to define a position of the module in a fluoroscopic coordinate system of reference. The module further includes one or more connections configured to fixedly connect the module to a magnetic field transmission pad at a predetermined location and orientation with respect to the pad, so as to characterize the position of the registration module in a magnetic coordinate system of reference defined by the magnetic field transmission pad. 
     SUMMARY 
     There is provided in accordance with an embodiment of the present disclosure, a medical procedure system, including a medical instrument configured to be inserted into a body part of a living subject, and including position-tracking transducers configured to provide position signals, a shaft, a distal end, and at least one radiopaque marker, a position tracking sub-system configured to compute a position including at least one location and an orientation of the distal end of the medical instrument in a position-tracking sub-system coordinate frame responsively to the position signals, a fluoroscope configured to capture fluoroscopic images of an interior of the body part and the at least one radiopaque marker of the medical instrument over time, a display, and a registration sub-system configured to render, to the display, the captured fluoroscopic images including at least one marker-image of the at least one radiopaque marker, and at least one graphical representation indicative of the computed position of the distal end, receive user-alignment input aligning the at least one graphical representation with the at least one marker-image, and register the position-tracking sub-system coordinate frame with a coordinate frame of the fluoroscope responsively to the received user-alignment input. 
     Further in accordance with an embodiment of the present disclosure the distal end includes an element which is configured to extend away from an axis of the shaft, the at least one radiopaque marker includes a first radiopaque marker disposed on the shaft, and a second radiopaque marker disposed on the element which is configured to extend away from the axis of the shaft, the position tracking sub-system is configured to compute a first position including a first location and an orientation of the distal end of the medical instrument in the position-tracking sub-system coordinate frame responsively to at least one of the position signals, and a second position including a second location of the distal end of the medical instrument in the position-tracking sub-system coordinate frame responsively to at least one of the position signals, and the registration sub-system is configured to render, to the display, the captured fluoroscopic images including a first marker-image of the first radiopaque marker, a second marker-image of the second radiopaque marker, a first graphical representation indicative of the computed first position of the distal end, and a second graphical representation indicative of the computed second position of the distal end, receive user-alignment input aligning the first graphical representation with the first marker-image, and the second graphical representation with the second marker-image. 
     Still further in accordance with an embodiment of the present disclosure the position-tracking transducers include a first coil disposed coaxially with the shaft. 
     Additionally, in accordance with an embodiment of the present disclosure the first radiopaque marker includes a radiopaque cylinder. 
     Moreover, in accordance with an embodiment of the present disclosure the first coil is wound on the radiopaque cylinder. 
     Further in accordance with an embodiment of the present disclosure the position-tracking transducers includes a second coil disposed orthogonally to the first coil. 
     Still further in accordance with an embodiment of the present disclosure the element which is configured to extend away from the axis of the shaft is included in an inflatable balloon. 
     Additionally, in accordance with an embodiment of the present disclosure the first radiopaque marker includes a radiopaque cylinder, the first coil being wound on the radiopaque cylinder, and the position-tracking transducers including a second coil disposed orthogonally to the first coil. 
     Moreover, in accordance with an embodiment of the present disclosure the element which is configured to extend away from the axis of the shaft is included in an elongated element including an electrode. 
     Further in accordance with an embodiment of the present disclosure the position-tracking transducers include the electrode. 
     Still further in accordance with an embodiment of the present disclosure the second radiopaque marker is co-located with the electrode. 
     Additionally, in accordance with an embodiment of the present disclosure the at least one radiopaque marker includes a cylinder with a longitudinal gap. 
     There is also provided in accordance with another embodiment of the present disclosure, a medical procedure method, including inserting a medical instrument into a body part of a living subject, computing a position including at least one location and an orientation of a distal end of the medical instrument in a position-tracking sub-system coordinate frame responsively to position signals provided by position-tracking transducers of the medical instrument, capturing, using a fluoroscope, fluoroscopic images of an interior of the body part and at least one radiopaque marker of the medical instrument over time, rendering, to a display, the captured fluoroscopic images including at least one marker-image of the at least one radiopaque marker, and at least one graphical representation indicative of the computed position of the distal end, receiving user-alignment input aligning the at least one graphical representation with the at least one marker-image, and registering the position-tracking sub-system coordinate frame with a coordinate frame of the fluoroscope responsively to the received user-alignment input. 
     Moreover in accordance with an embodiment of the present disclosure the distal end includes an element which is configured to extend away from an axis of a shaft of the medical instrument, the at least one radiopaque marker includes a first radiopaque marker disposed on the shaft, and a second radiopaque marker disposed on the element which is configured to extend away from the axis of the shaft, the method further including computing a first position including a first location and an orientation of the distal end of the medical instrument in the position-tracking sub-system coordinate frame responsively to at least one of the position signals, computing a second position including a second location of the distal end of the medical instrument in the position-tracking sub-system coordinate frame responsively to at least one of the position signals, rendering, to the display, the captured fluoroscopic images including a first marker-image of the first radiopaque marker, a second marker-image of the second radiopaque marker, a first graphical representation indicative of the computed first position of the distal end, and a second graphical representation indicative of the computed second position of the distal end, and receiving user-alignment input aligning the first graphical representation with the first marker-image, and the second graphical representation with the second marker-image. 
     Further in accordance with an embodiment of the present disclosure the position-tracking transducers include a first coil disposed coaxially with the shaft. 
     Still further in accordance with an embodiment of the present disclosure the first radiopaque marker includes a radiopaque cylinder. 
     Additionally, in accordance with an embodiment of the present disclosure the first coil is wound on the radiopaque cylinder. 
     Moreover, in accordance with an embodiment of the present disclosure the position-tracking transducers includes a second coil disposed orthogonally to the first coil. 
     Further in accordance with an embodiment of the present disclosure the element which is configured to extend away from the axis of the shaft is included in an inflatable balloon. 
     Still further in accordance with an embodiment of the present disclosure the first radiopaque marker includes a radiopaque cylinder, the first coil being wound on the radiopaque cylinder, and the position-tracking transducers including a second coil disposed orthogonally to the first coil. 
     Additionally, in accordance with an embodiment of the present disclosure the element which is configured to extend away from the axis of the shaft is included in an elongated element including an electrode. 
     Moreover, in accordance with an embodiment of the present disclosure the position-tracking transducers include the electrode. 
     Further in accordance with an embodiment of the present disclosure the second radiopaque marker is co-located with the electrode. 
     Still further in accordance with an embodiment of the present disclosure the at least one radiopaque marker includes a cylinder with a longitudinal gap. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood from the following detailed description, taken in conjunction with the drawings in which: 
         FIG.  1    is a partly pictorial, partly block diagram view of a medical procedure system constructed and operative in accordance with an embodiment of the present invention; 
         FIG.  2    is a block diagram of a processor of the system of  FIG.  1   ; 
         FIG.  3    is a schematic view of a medical instrument of the system of  FIG.  1    being inserted into a body-part of a living subject; 
         FIG.  4    is a schematic view of the medical instrument of  FIG.  3    showing a guidewire of the medical instrument being extended into the body-part; 
         FIG.  5    is a schematic view of a balloon device being extended over the guidewire of  FIG.  4   ; 
         FIG.  6    is a schematic view of an inflatable balloon of the balloon device of  FIG.  5    being inflated; 
         FIG.  7    is a schematic view of the medical instrument of  FIG.  6    including cross-sectional views of transducers of the medical instrument; 
         FIG.  8    is a schematic view a user interface screen including a captured fluoroscopic image and graphical representations indicative of computed positions of the distal end of the medical instrument; 
         FIG.  9    is a schematic view of the user interface screen of  FIG.  8    after an initial user alignment of the graphical representations; 
         FIG.  10    is a schematic view of the user interface screen of  FIG.  9    after a further user alignment of the graphical representations; 
         FIG.  11    is a schematic view of the user interface screen of  FIG.  10    after yet further user alignment of the graphical representations; 
         FIG.  12    is a flowchart including exemplary steps in a registration method for use in the system of  FIG.  1   ; 
         FIG.  13    is a schematic view of multi-prong probe for use with the system of  FIG.  1   ; and 
         FIG.  14    is a schematic view of an alternative radiopaque marker for use in the system of  FIG.  1   . 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Overview 
     As mentioned previously, medical images such as CT scans are often captured prior to a medical procedure and then registered with a position tracking coordinate system of a medical instrument so that the medical instrument may be displayed together with the medical scan to aid navigation of the medical instrument in a body-part by a physician. 
     In some circumstances, for example, due to a medical procedure becoming more complicated than originally anticipated, or for other reasons, a physician may want to introduce a fluoroscope to capture fluoroscopic images during the medical procedure. In order to see how the body parts are moving with respect to the medical tool, the coordinate frame of the fluoroscope needs to be registered with the coordinate frame of the tracking system which is tracking the medical instrument. Mid-procedure registration may be problematic. For example, how can registration be performed quickly and accurately so as not to add delays to the medical procedure and to provide an accurate picture of the body part with respect to the moving medical instrument. 
     Embodiments of the present invention provide a medical procedure system with the capability of introducing a fluoroscope mid-procedure and quickly and accurately registering the coordinate frame of the fluoroscope with the coordinate frame of a position-tracking sub-system which is tracking the medical instrument using transducers. Once the two coordinate frames have been successfully registered, the fluoroscopic images can be displayed with a representation of the medical instrument superimposed thereon to accurately show the position of the moving medical instrument with respect to the real-time fluoroscopic images of the body part. 
     The medical instrument includes radiopaque markers which are placed on the medical instrument within a given special relationship of the position-tracking transducers of the medical instrument and enable registration of both location, orientation including roll of the two coordinate frames, as will be described below in more detail. 
     When a fluoroscope is introduced, the radiopaque markers are seen in captured fluoroscopic images. The radiopaque markers provide enough information to indicate locations and an orientation including roll of the medical instrument in the fluoroscopic images. The position of the medical instrument is also computed from position signals provided by the position-tracking transducers and provides at least one location and orientation including roll of the medical instrument in the coordinate frame of the position-tracking sub-system. 
     In one embodiment, the medical instrument includes a shaft and an inflatable balloon. The shaft includes a first coil wound upon a radiopaque cylinder which is coaxial with the shaft and a second coil disposed orthogonally to the first coil. The balloon includes a radiopaque marker disposed thereon. The radiopaque cylinder indicates a location and orientation (excluding roll) of the medical instrument in the fluoroscopic images. The radiopaque marker disposed on the balloon indicates a roll of the medical instrument in the fluoroscopic images. The first and second coils together provide signals indicative of a location and orientation (including roll) of the shaft. A cylinder defined by the location and orientation of the first coil is superimposed over one of the fluoroscopic images. An element defined by the roll computed by the signal of the second coil is also superimposed over the fluoroscopic image. A user then manipulates the superimposed cylinder and the element to align them with the image of the radiopaque cylinder and balloon radiopaque marker, respectively. The user alignment input defines the differences between the fluoroscope coordinate frame and the position-tracking sub-system coordinate frame. The user alignment input is then used to register the two coordinate frames with each other. 
     In other embodiments the coil is not wound over the radiopaque cylinder, but it wound over another portion of the medical instrument and has a known spatial relationship to the radiopaque cylinder. 
     In another embodiment, a probe having a shaft and prongs extending from the shaft may be used. The probe includes a coil wound over a radiopaque cylinder disposed in the shaft, typically coaxially with the shaft. Signals from the coil may be used to compute a location and orientation (excluding roll) of the probe. The prongs also include a plurality of electrodes. One of the electrodes may be used to determine a roll of the probe. A radiopaque marker may also be disposed on one of the prongs, for example, adjacent to, or co-located with, the electrode used to determine the roll of the probe. Alternatively, the roll may be determined using a second coil disposed orthogonally to the abovementioned coil, which is wound on the radiopaque cylinder. A cylinder defined by the location and orientation of the coaxial coil is superimposed over the fluoroscopic images. An element defined by the roll computed by the signal of the second coil is also superimposed over the fluoroscopic image. A user then manipulates the superimposed cylinder and the element to align them with the image of the radiopaque cylinder and prong radiopaque marker, respectively. The user alignment input defines the differences between the fluoroscope coordinate frame and the position-tracking sub-system coordinate frame. The user alignment input is then used to register the two coordinate frames with each other. 
     The medical instrument is not limited to including a balloon or a plurality of prongs, but may include any medical instrument having a distal end with an element which extends away from an axis of the shaft so as to provide meaningful data about the roll of the medical instrument based on the radiopaque marker disposed on the element which extends away from the axis of the shaft. The extent to which the element needs to extend away from the axis of the shaft may depend on the desired registration accuracy. 
     In some embodiments, a single radiopaque marker may be used to align location, orientation including roll. For example, the radiopaque marker may include a cylinder with a longitudinal gap. A cylinder with a similarly sized longitudinal gap defined by a location and orientation (including roll) computed from signals provided by the coaxial coil and an orthogonally placed coil is superimposed over one of the fluoroscopic images. A user then manipulates the superimposed cylinder with the image of the radiopaque cylinder aligning the cylinders including the longitudinal gaps of the cylinders. The user alignment input defines the differences between the fluoroscope coordinate frame and the position-tracking sub-system coordinate frame. The user alignment input is then used to register the two coordinate frames with each other. 
     System Description 
     Documents incorporated by reference herein are to be considered an integral part of the application except that, to the extent that any terms are defined in these incorporated documents in a manner that conflicts with definitions made explicitly or implicitly in the present specification, only the definitions in the present specification should be considered. 
     Turning now to the drawings, reference is now made to  FIG.  1   , which is a partly pictorial, partly block diagram view of a medical procedure system  20  constructed and operative in accordance with an embodiment of the present invention, and to  FIG.  2   , which is a block diagram of a processor of the system of  FIG.  1   . The medical procedure system  20  is typically used during an invasive and/or investigative procedure on a nasal sinus or another body part (such as the brain or heart) of a patient  22 . 
     For the procedure, a magnetic field radiation assembly  24  may be positioned behind and/or around the head of the patient  22 , for example by fixing the assembly  24  to a bed  23  upon which the patient  22  is lying. The magnetic field radiation assembly  24  in the pictured example comprises five magnetic field radiators  26 , which are fixed in a horseshoe shaped frame, the frame being positioned beneath or around the patient  22  so that the magnetic field radiators  26  surround the head of the patient  22 . Alternatively, smaller or larger numbers of radiators  26  may be used, in various different configurations. The magnetic field radiators  26  are configured to radiate alternating magnetic fields at respective frequencies into a region  30  where the body part is located, in proximity to the magnetic field radiation assembly  24  and which includes the head of patient  22 . 
     The alternating magnetic fields induce signals in position-tracking transducers  32 . The position-tracking transducers  32  are shown disposed on a medical instrument  28  in order to track a position of the medical instrument  28 . In some embodiments, the medical instrument  28  may include an inflatable balloon  60 . By way of example only, the medical instrument  28  may include any one or more of the following, a probe for inserting into the body-part, an endoscope, and/or a surgical tool such as an ENT tool, suction tool, microdebrider, shaver, and/or grasper. 
     The position of a distal end  62  of the medical instrument  28  may be tracked using a position-tracking sub-system  64 , which tracks position and orientation coordinates of the position-tracking transducers  32  fitted at the distal end  62 . The position-tracking transducers  32  are configured to output signals that are indicative of the position of the transducers  32 . The signals are processed by the position-tracking sub-system  64  running on processor  38  to track the position of the distal end  62  of the medical instrument  28 . In embodiments, where the position-tracking sub-system  64  is a magnetic tracking sub-system, the position-tracking transducers  32  includes at least one coil, and typically two or three orthogonally placed coils. In other embodiments, the position-tracking sub-system  64  may be an electrically-based tracking sub-system using multiple head surface electrodes to track the position of the medical instrument  28  based on a signal emitted by at least one electrode (comprised in the position-tracking transducer  32 ) of the medical instrument  28 . The position-tracking sub-system  64  may be implemented using any suitable location tracking sub-system, for example, but not limited to, an ultrasound-based tracking system where the position-tracking transducers  32  includes at least one ultrasound transducer. Using the position-tracking sub-system  64 , a physician  54  advances the distal end  62  of the medical instrument  28  in a body-part, described in more detail below. 
     As described in more detail below, position-tracking transducers  32  are affixed to the medical instrument  28 , and determination of the location and orientation of the position-tracking transducers  32  enables tracking the location and orientation of the distal end  62  (or other location) of the medical instrument  28 , that may be reversibly inserted into a body-part of the patient  22  (the living subject). 
     A system using magnetic field radiators, such as the magnetic field radiators  26 , for tracking an entity inserted into a patient is described in U.S. Pat. No. 10,772,489, of Govari et al., which is incorporated herein by reference. In addition, the Carto® system produced by Biosense Webster of 33 Technology Drive, Irvine, Calif. 92618 USA, uses a tracking system similar to that described herein for finding the location and orientation of a coil in a region irradiated by magnetic fields. 
     Elements of system  20 , including radiators  26 , may be controlled by the processor  38 , which comprises a processing unit communicating with one or more memories  42 . Typically, the elements may be connected by cables to the processor  38 , for example, radiators  26  may be connected by a cable  58  to the processor  38 . Alternatively, or additionally, the elements may be coupled wirelessly to the processor  38 . The processor  38  may be mounted in a console  50 , which comprises operating controls  51  that typically include a keypad and/or a pointing device such as a mouse or trackball. The console  50  also connects to other elements of the medical procedure system  20 , such as a proximal end  52  of the medical instrument  28  via a cable  19 . A physician  54  uses the operating controls  51  to interact with the processor  38  while performing the procedure, and the processor  38  may present results produced by system  20  on a display  56 . 
     In some embodiments, prior to performing the medical procedure, CT images of the patient  22  are acquired. The CT images are stored in the memory  42  for subsequent retrieval by the processor  38 . In  FIG.  1   , the display  56  is shown displaying a view  59  of a previous CT scan (or other suitable scan) which may be used as an aid for the physician  54  to guide the medical instrument  28  in the body-part. The CT images may be registered with the magnetic coordinate system so that a representation of the medical instrument  28  may be displayed with the CT images on the display  56 . 
     In practice, some or all of these functions of the processor  38  may be combined in a single physical component or, alternatively, implemented using multiple physical components. These physical components may comprise hard-wired or programmable devices, or a combination of the two. In some embodiments, at least some of the functions of the processor may be carried out by a programmable processor under the control of suitable software. This software may be downloaded to a device in electronic form, over a network, for example. Alternatively, or additionally, the software may be stored in tangible, non-transitory computer-readable storage media, such as optical, magnetic, or electronic memory. 
     The medical procedure system  20  may also include a fluoroscope  40  for capturing fluoroscopic images. The medical procedure system  20  also includes a registration sub-system  44  running on the processor  38 . The fluoroscope  40  and the registration sub-system  44  are described in more detail below with reference to  FIG.  714   . 
     Reference is now made to  FIG.  3   , which is a schematic view of the medical instrument  28  of the system  20  of  FIG.  1    being inserted into a body-part of a living subject. The distal end  62  of the medical instrument  28  is shown being inserted into a sinus cavity of the patient  22 .  FIG.  3    shows that the medical instrument  28  includes a guide  46 . The guide  46  may be fixed in multiple configurations ranging from straight to a number of curved formations. 
     Reference is now made to  FIG.  4   , which is a schematic view of the medical instrument  28  of  FIG.  3    showing a guidewire  48  of the medical instrument  28  being extended into the body-part. The guidewire  48  is extended from within the guide  46  into the body-part. The guidewire  48  generally extends in the direction in which the distal end of the guide  46  was fixed. 
     Reference is now made to  FIG.  5   , which is a schematic view of a balloon device  66  of the medical instrument  28  being extended over the guidewire  48  of  FIG.  4   . In  FIG.  5   , the balloon device  66  is shown with its balloon deflated. The balloon device  66  is described in more detail with reference to  FIGS.  6  and  7    below. 
     Reference is now made to  FIG.  6   , which is a schematic view of the inflatable balloon  60  of the balloon device  66  of  FIG.  5    being inflated. The inflatable balloon  60  may be inflated for any suitable medical procedure. In the example of  FIG.  6    the inflatable balloon  60  is inflated to perform a sinus dilation. The inflatable balloon  60  may be inflated using any suitable method for example, using air or a liquid such as saline. A suitable base design for the medical instrument  28  may be based on the RELIEVA SCOUT® Multi-Sinus Dilation System produced by Acclarent, Inc., of Irvine, Calif., USA. The medical instrument  28  has been described herein as an ENT tool with the inflatable balloon  60 . The medical instrument  28  may be any suitable medical instrument for use in any suitable body-part of a living subject. 
     Reference is now made to  FIG.  7   , which is a schematic view of the medical instrument  28  of  FIG.  6    including cross-sectional views  72  of transducers  32  of the medical instrument  28 .  FIG.  7    includes two cross-sectional views  72 , labeled  72 - 1  and  72 - 2 . Both of the cross-sectional views  72 - 1  are longitudinal cross-sectional views, showing the various layers of the medical instrument  28 . 
     The medical instrument  28  includes two position-tracking transducers  32  (labeled  32 - 1  and  32 - 2 ). The position-tracking transducer  32 - 1  may be a coil disposed coaxially with a shaft  68  of the medical instrument  28 . The position-tracking transducer  32 - 2 , which may be a coil, is disposed orthogonally to the position-tracking transducer  32 - 1 . The position-tracking transducers  32 - 1 ,  32 - 2  together provide signals which may be used to compute a location and orientation including roll of the distal end  62  of the medical instrument  28 . The position-tracking transducers  32  are shown as being disposed at a distal end of the inflatable balloon  60 . In some embodiments, the position-tracking transducers  32  may be disposed at any suitable location on the medical instrument  28 , for example, any suitable location of the shaft  68 . 
     The medical instrument  28  includes a radiopaque marker  70  disposed on the shaft  68 . In some embodiments, the radiopaque marker  70  includes a radiopaque 
     cylinder as shown in  FIG.  7   . In some embodiments, the position-tracking transducer  321  is a coil, which is wound on the radiopaque cylinder. 
     The medical instrument  28  also includes a radiopaque marker  74  disposed on an element (e.g., comprised in the inflatable balloon  60 ) of the distal end  62  that is configured to extend away from the axis of the shaft  68 . 
     The radiopaque markers  70 ,  74  may be comprised of any suitable radiopaque material, for example, but not limited to, stainless steel or iron, which is suitably coated with a biocompatible material. 
     The cross-sectional views  72 - 1 ,  72 - 1  shows that the guidewire  48  is disposed in the guide  46 , and the guide  46  is surrounded by an inner layer  76  and an outer layer  78  of the balloon device  66  with the position-tracking transducers  32  sandwiched between the inner layer  76  and the outer layer  78 . 
     The cross-sectional view  72 - 1  shows radiopaque marker  70  (e.g., the radiopaque cylinder) surrounding the inner layer  76  with the coil of the position-tracking transducer  32 - 1  being wound on the radiopaque marker  70  coaxially with the shaft  68 . The cross-sectional view  72 - 2  shows the coil of the position-tracking transducer  32 - 2  wound orthogonally to the coil of the position-tracking transducer  321   
     The radiopaque marker  70  may be conveniently disposed in the same location as the position-tracking transducer  32 - 1  for easier computation of the registration described below. In some embodiments, the position-tracking transducer  32 - 1  may be disposed in a different location from the radiopaque marker  70  and compensation between the given spatial relationship between the radiopaque marker  70  and the position-tracking transducer  32 - 1  is taken into account when performing the registration computations. 
     Reference is now made to  FIG.  8   , which is a schematic view a user interface screen  80  including a captured fluoroscopic image  82  and graphical representations  84  (labeled  84 - 1  and  84 - 2 ) indicative of computed positions of the distal end  62  of the medical instrument  28 . Reference is also made to  FIG.  7   . The captured fluoroscopic image  82  includes a marker-image  86  of the radiopaque marker  70  and a marker-image  88  of the radiopaque marker  74 . The positioning of the graphical representation  84 - 1  in the user interface screen  80  corresponds to a computed location and orientation of the radiopaque marker  70  based on the signals provided by the position-tracking transducer  32 - 1 . The positioning of the graphical representation  84 - 2  in the user interface screen  80  corresponds to a computed location of the radiopaque marker  74  based on the signals provided by the position-tracking transducer  32 - 2 . The location of the radiopaque marker  74  may be computed based on a position around a circumference of the inflatable balloon  60  according to the computed roll of the position-tracking transducer  32 - 2  wherein the circumference also passes through the radiopaque marker  74 . 
     As part of the registration process, the physician  54  manipulates the graphical representations  84  using the operating controls  51  so that the graphical representation  84 - 1  and the graphical representation  84 - 2  are aligned with the marker-image  86  and the marker-image  88 , respectively. The physician  54  may manipulate the graphical representations  84  by moving them across the user interface screen  80  in any suitable direction (up, down, left, right, diagonal etc.), moving them backwards, moving them forwards, and changing the orientation and rotation (roll) of the graphical representations  84 . As the graphical representations  84  correspond to different positions (e.g., the radiopaque marker  70  and the radiopaque marker  74 ) on the same item (e.g., the medical instrument  28 ), the graphical representations  84  are automatically moved together maintaining a same spatial relationship between the graphical representations  84 . Nevertheless, it may be more intuitive for the physician  54  to try to align the cylinders (e.g., the graphical representation  84 - 1  with the marker-image  86 ) initially and then align the graphical representation  84 - 2  with the marker-image  88  afterwards. 
     Reference is now made to  FIG.  9   , which is a schematic view of the user interface screen  80  of  FIG.  8    after an initial user alignment of the graphical representations  84 .  FIG.  9    shows that the graphical representations  84  have been moved closer to the marker-image  86  and the marker-image  88 . 
     Reference is now made to  FIG.  10   , which is a schematic view of the user interface screen  80  of  FIG.  9    after a further user alignment of the graphical representations  84 .  FIG.  10    shows that the graphical representation  84 - 1  has been rotated clockwise and is almost aligned with the marker-image  86 , but the graphical representation  84 - 2  is still misaligned with the marker-image  88 . 
     Reference is now made to  FIG.  11   , which is a schematic view of the user interface screen  80  of  FIG.  10    after yet further user alignment of the graphical representations  84 . The graphical representations  84  have been rotated around the axis of the graphical representation  84 - 1  until the graphical representation  84 - 2  is almost aligned with the marker-image  88 . Once the graphical representations  84 - 1 ,  84 - 2  are aligned with the marker-image  86  and the marker-image  88 , respectively, the user alignment inputs are used to register the coordinate frame of the fluoroscope  40  with the coordinate frame of the position-tracking sub-system  64  ( FIG.  2   ). 
     Reference is now made to  FIG.  12   , which is a flowchart  90  including exemplary steps in a registration method for use in the system  20  of  FIG.  1   . The position-tracking sub-system  64  ( FIG.  2   ) is configured to compute (block  92 ) a position including at least one location and an orientation (including roll) of the distal end  62  of the medical instrument  28  in a position-tracking sub-system coordinate frame responsively to the position signals provided by the position-tracking transducers  32  ( FIG.  7   ). 
     In some embodiments, the position tracking sub-system  64  is configured to compute: a first position including a first location and an orientation (e.g., of the radiopaque marker  70 ) of the distal end  62  of the medical instrument  28  in the position-tracking sub-system coordinate frame responsively to the position signal provided by the position-tracking transducer  32 - 1 ; and a second position including a second location (e.g., of the radiopaque marker  74 ) of the distal end  62  of the medical instrument  28  in the position-tracking sub-system coordinate frame responsively to the position signal provided by the position-tracking transducer  32 - 2 . 
     The fluoroscope  40  is introduced and is configured to capture (block  94 ) fluoroscopic images of an interior of the body part and the radiopaque markers  70 ,  74  of the medical instrument  28  over time. 
     The registration sub-system  44  is configured to render (block  96 ), to the display  56 , the captured fluoroscopic images including the marker-image  86  of the radiopaque marker  70  and the marker-image  88  of the radiopaque marker  74 , and superimpose over the fluoroscopic images, the graphical representations  84  at positions indicative of the computed position(s) of the distal end  62  (e.g., the graphical representation  84 - 1  at the first computed position and the graphical representations  842  at the second computed position). 
     The registration sub-system  44  is configured to receive (block  98 ) user-alignment input aligning the respective graphical representations  84  with the respective ones of the marker-images  86 ,  88  (e.g., aligning the graphical representation  84 - 1  with the marker-image  86  and the graphical representation  84 - 2  with the marker-image  88 . The registration sub-system  44  is configured to move (block  100 ) the graphical representations  84  with respect to the marker-images  86 ,  88  on the user interface screen  80  ( FIGS.  8 - 11   ) according to the received user-alignment input. The steps of blocks  98  and  100  may repeated to allow for multiple updates to the movement of the graphical representations  84  on the user interface screen  80 . 
     The registration sub-system  44  is configured to register (block  102 ) the position-tracking sub-system coordinate frame with a coordinate frame of the fluoroscope responsively to the received user-alignment input which brought the graphical representations  84  from their original position to their final position aligned with the marker-images  86 ,  88 . 
     Once the position-tracking sub-system coordinate frame is registered with the coordinate frame of the fluoroscope  40 , the processor  38  is configured to render to the display  56  the fluoroscopic images with a representation of the medical instrument  28  thereon. 
     Reference is now made to  FIG.  13   , which is a schematic view of multi-prong probe  104  for use with the system  20  of  FIG.  1   . The multi-prong probe  104  includes a shaft  106  and multiple prongs  108 , each of the prongs  108  including a plurality of electrodes  110 . Only some of the electrodes  110  have been labeled for the sake of simplicity. An example of the multi-prong probe  104  is the CARTO PENTARAY® catheter produced by Biosense Webster of 33 Technology Drive, Irvine, Calif. 92618 USA. The multi-prong probe  104  includes a coil  112  wound on a radiopaque cylinder marker  114  which is disposed on the shaft  106  and is coaxial with the shaft  106 . The coil  112  is covered with an outer layer  116 . The coil  112  is configured as a position-tracking transducer and provides a position signal to the position-tracking sub-system  64  ( FIG.  2   ) in order to compute a location and orientation of the radiopaque cylinder  114 . 
     One of the electrodes  110 , an electrode  110 - 1 , of one of the prongs  108 , a prong  108 - 1  is disposed over a radiopaque layer marker  118  over the prong  108 - 1  and is therefore co-located with the radiopaque layer marker  118 . The prong  108 - 1  is an elongated element which is configured to extend away from the axis of the shaft  106 . The electrode  110 - 1  may be used as a position-tracking transducer to provide a position signal to the position-tracking sub-system  64  in order to compute a position (e.g., location) of the radiopaque layer  118  which allows a roll of the multi-prong probe  104  to be registered. 
     Reference is now made to  FIG.  14   , which is a schematic view of an alternative radiopaque marker  120  for use in the system  20  of  FIG.  1   . In some embodiments, the single radiopaque marker  120  replaces the cylindrical radiopaque marker of  FIG.  7   , and may be used to align location, orientation including roll. For example, the single radiopaque marker  120  may include a cylinder with a longitudinal gap  122 . A cylinder with a similarly sized longitudinal gap defined by a location and orientation (including roll) computed from signals provided by the coaxial coil  32 - 1  ( FIG.  7   ) and the orthogonally placed coil  32 - 2  ( FIG.  7   ) is superimposed over one of the fluoroscopic images in the user interface screen  80  ( FIGS.  8 - 11   ). The physician  54  then manipulates the superimposed cylinder with respect to the marker-image of the radiopaque cylinder in order to align the cylinders including the longitudinal gaps of the cylinders. The user alignment input defines the differences between the fluoroscope coordinate frame and the position-tracking sub-system coordinate frame with respect to location and orientation including roll. The user alignment input is then used to register the two coordinate frames with each other. 
     Various features of the invention which are, for clarity, described in the contexts of separate embodiments may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment may also be provided separately or in any suitable sub-combination. 
     The embodiments described above are cited by way of example, and the present invention is not limited by what has been particularly shown and described hereinabove. Rather the scope of the invention includes both combinations and subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.