Patent Publication Number: US-2007123975-A1

Title: Medical device with magnetic resonance visibility enhancing structure

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is a continuation (and claims the benefit of priority under 35 USC 120) of U.S. application Ser. No. 10/359,970, filed Feb. 6, 2003. The disclosure of the prior application is considered part of (and is incorporated by reference in) the disclosure of this application. 
    
    
     BACKGROUND  
      The present invention relates generally to devices for use in vascular treatments. More particularly the present invention relates to devices used in vascular. treatments that incorporate a magnetic resonance visibility enhancing structure, the devices being adapted for use in magnetic resonance imaging.  
      Vascular stents are known medical devices used in various vascular treatments of patients. Stents commonly include a tubular member that is moveable from a collapsed, low profile, delivery configuration to an expanded, deployed configuration. In the expanded configuration, an outer periphery of the stent frictionally engages an inner periphery of a lumen. The deployed stent then maintains the lumen such that it is substantially unoccluded and flow therethrough is substantially unrestricted. However, various stent designs render the stent substantially invisible during a Magnetic Resonance Imaging procedure.  
      Magnetic Resonance Imaging, (MRI) is a non-invasive medical procedure that utilizes magnets and radio waves to produce a picture of the inside of a body. An MRI scanner is capable of producing pictures of the inside of a body without exposing the body to ionizing radiation (X-rays). In addition, MRI scans can see through bone and provide detailed pictures of soft body tissues.  
      A typical MRI scanner includes a magnet that is utilized to create a strong homogeneous magnetic field. A patient is placed into or proximate the magnet. The magnetic field causes a small majority of the atoms with a net magnetic moment, also referred to as spin, to align in the same direction as the magnetic field. When a radiowave is directed at the patient&#39;s body, atoms precessing in the magnetic field with a frequency equal to the radiowave are able to adapt the radiowave energy, which causes them to “tumble over” and align in the opposite direction of the magnetic field. The frequency at which atoms with a net spin precess in a magnetic field is also referred to as the Larmor frequency. The opposing alignment is at a higher energy level compared to the original orientation. Therefore, after removing the radiowave, atoms will return to the lower energetic state. As the atoms return to the lower energetic state, a radio signal is sent at the Lamor frequency. These return radio waves create signals (resonance signals) that are detected by the scanner at numerous angles around the patient&#39;s body. The signals are sent to a computer that processes the information. and compiles an image or images. Typically, although not necessarily, the images are in the form of 2-dimensional “slice” images.  
      An ability to effectively view areas proximate a stent during an MRI procedure is desirable. In particular, viewing areas inside and proximate a tubular member of a stent may be desirable both during deployment and after deployment of the stent in a patient. However, various current stent designs prevent adequate imaging of the area surrounding the stent. Instead, the images are distorted and thus cannot be used.  
     SUMMARY  
      Embodiments of the present invention relate to medical devices that reduce the distortion of medical resonance images taken of the devices. In particular, various structures are utilized to enhance visibility proximate and inside of a tubular member of a stent. In one particular embodiment, the stent does not contain electrically conductive loops. In another embodiment, ring portions in the stent are arranged such that current in one ring portion is opposed by current in another connected ring portion.  
      The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.  
    
    
     DESCRIPTION OF DRAWINGS  
       FIG. 1  is a partial block diagram of an illustrative magnetic resonance imaging system.  
       FIG. 2  is an illustration of a coil in a changing magnetic field.  
       FIG. 3A  is a side perspective view of a stent.  
       FIG. 3B  is a cross-section of the stent illustrated in  FIG. 3A .  
       FIG. 3C  is an alternative embodiment of a portion of a cross-section of the stent illustrated in  FIG. 3A .  
       FIG. 4  is a top view of a portion of a stent that has been unfolded.  
       FIG. 5  is a top view of a stent that has been unfolded.  
       FIG. 6A  is a side perspective view of a stent.  
       FIG. 6B  is a cross-section of a portion of the stent illustrated in  FIG. 6A .  
       FIG. 7  is an illustration of two connected ring portions.  
       FIG. 8  is a perspective view of a stent having electrically opposed ring portions.  
       FIG. 9A  is a perspective view of an alternative embodiment of a stent having electrically opposed rings.  
       FIG. 9B  is a portion of the stent in  FIG. 9A . 
    
    
      Like reference symbols in the various drawings indicate like elements.  
     DETAILED DESCRIPTION  
       FIG. 1  is a partial block diagram of an illustrative magnetic resonance imaging system. In  FIG. 1 , subject  100  on support table  110  is placed in a homogeneous magnetic field generated by magnetic field generator  120 . Magnetic field generator  120  typically comprises a cylindrical magnet adapted to receive subject  100 . Magnetic field gradient generator  130  creates magnetic field gradients of predetermined strength in three mutually orthogonal directions at predetermined times. Magnetic field gradient generator  130  is illustratively comprised of a set of cylindrical coils concentrically positioned within magnetic field generator  120 . A region of subject  100  into which a device  150 , shown as a stent, has been inserted, is located in the body of subject  100 .  
      RF source  140  radiates pulsed radio frequency energy into subject  100  and stent  150  at predetermined times and with sufficient power at a predetermined frequency to influence nuclear magnetic spins in a fashion known to those skilled in the art. The influence on the atoms causes them to resonate at the Larmor frequency. The Larmor frequency for each spin is directly proportional to the absolute value of the magnetic field experienced by the atom. This field strength is the sum of the static magnetic field generated by magnetic field generator  120  and the local field generated by magnetic field gradient generator  130 . In an illustrative embodiment, RF source  140  is a cylindrical external coil that surrounds the region of interest of subject  100 . Such an external coil can have a diameter sufficient to encompass, the entire subject  100 . Other geometries, such as smaller cylinders specifically designed for imaging the head or an extremity can be used instead. Non-cylindrical external coils such as surface coils may alternatively be used.  
      External RF receiver  160  illustratively detects RF signals emitted by the subject in response to the radio frequency field created by RF source  140 . In an illustrative embodiment, external RF receiver  160  is a cylindrical external coil that surrounds the region of interest of subject  100 . Such an external coil can have a diameter sufficient to encompass the entire subject  100 . Other geometries, such as smaller cylinders specifically designed for imaging the head or an extremity can be used instead. Non-cylindrical external coils, such as surface coils, may alternatively be used. External RF receiver  160  can share some or all of its structure with RF source  140  or can have a structure entirely independent of RF source  140 . The region of sensitivity of RF receiver  160  is larger than that of the stent  150  and can encompass the entire subject  100  or a specific region of subject  100 . The RF signals detected by external RF receiver  160  are sent to imaging and tracking controller unit  170  where they are analyzed. Controller  170  displays signals received by RF receiver  160  on visual display  190 .  
      Establishing a homogenous, or uniform, magnetic field with magnetic field generator  120  in addition to switched linear gradient magnetic fields activated in various sequences as well as timely switching the RF radiowave in various sequences, as known in the art, enables the production of internal images of subject  100 . It is common that the magnetic field surrounding stent  150  is distorted, which causes distortion of images obtained proximate stent  150 . This is because devices that include ferromagnetic materials will generally distort magnetic fields. For example, it is common for the material and structure of stent  150  to affect the magnetic field around stent  150 . Such effects reduce the influence that magnetic field generator  120 , gradient generator  130  and RF source  140  have on the nuclear magnetic spins in subject  100 . In particular, the spins inside a tubular member of a stent are commonly not excited during an MRI and thus no image is detected.  
      One embodiment of the present invention includes using non-ferromagnetic materials in stent  150  to reduce this distortion. Such materials include, by way of example, platinum, iridium, tantalum, titanium, gold, niobium, hafnium alloys exhibiting non-ferromagnetic properties, and other non-ferromagnetic materials. Combinations of non-ferromagnetic materials can also be utilized without departing from the scope of the present invention. Another effect that commonly distorts the magnetic field around an intravascular device is associated with Faraday&#39;s Law. Faraday&#39;s Law simply states that any change in a magnetic environment of a coil will cause a voltage (emf) to be “induced” in the coil. Stent  150  can act as a coil when implanted in a subject during an MDRI process. The change in magnetic environment is caused either by stent  150  moving within a magnetic field, or by changes in the magnetic field proximate stent  150 . For example, stent  150  may move due to the heart beating or magnetic field changes may be induced by gradient generator  130  or RF Source  140 .  
      According to Faraday&#39;s Law, the induced emf in a coil is equal to the negative of the rate of change of magnetic flux through the coil times the number of turns in the coil. When an emf is generated by a change in magnetic flux, the polarity of the induced emf produces a current creating a magnetic field that opposes the change which produces it. Accordingly, the induced magnetic field inside any loop of wire acts to keep the magnetic flux inside the loop constant.  
       FIG. 2  further illustrates this effect. Coil  200  has been placed in a magnetic field produced by magnet  202 . The magnetic field is represented by a vector B. Any change in magnetic field B, herein represented as .DELTA.B, causes a current, represented as arrow  204 , to be produced in coil  200 . Current  204  causes a magnetic field B.sub.I to be induced, which opposes the change .DELTA.B.  
      When attempting to produce an image of stent  150  inside subject  100 , the stent acts as a coil or, depending on the structure of the stent, as multiple coils. During various phases of an MRI process to influence the nuclear spins, a change in the magnetic field inside the stent is generated. For example, gradient generator  130  may generate a pulse in order to influence spins to be analyzed by controller  170 . The gradient generator  130  thus changes the magnetic field and accordingly a change in magnetic field proximate the stent is opposed by Faraday&#39;s Law. As a result, spins proximate the stent are not excited and images of the stent show a lack of signal.  
      In order to reduce the effect of Faraday&#39;s Law on spins inside the stent, various stent designs have been made in accordance with embodiments of the present invention. In one embodiment, the creation of electrical loops within a stent structure is avoided. In yet another embodiment, a structure is used wherein current moving in one direction is opposed by a parallel current moving in the opposite direction. Using these designs, the visibility of a stent during an MRI process is enhanced.  
      Stent  320  is illustrated in  FIG. 3A  according to one embodiment of the present invention. Stent  320  includes a plurality of bands or rings  322 , wrapped around a central axis  323  to form a generally tubular structure  321 . Rings  322  can be made of a material that is substantially non-ferromagnetic. Illustratively, rings  322  are equally spaced about axis  323  and flexible to allow bending of tubular structure  321 . The flexibility of tubular structure  321  allows stent  320  to be placed in various lumens of different shapes and sizes. Rings  322  frictionally engage an inner periphery of a lumen when tubular structure  321  is open to allow fluid flow therethrough. Each of the rings  322  extend axially from a first end  324  of the stent  320  to a second end  326  of the stent  320  and terminate at each end to prevent the formation of electrical loops.  
      An insulating material has been applied to each of rings  322  prior to assembly of the tubular structure  321 . The insulating material could be applied to rings  322  in various ways, such as coating and depositing, for example. Thus, each of the rings  322  is spaced apart from the others by the insulating material. Accordingly, each of the rings  322  is electrically insulated from each other ring in order to prevent electrical loops from forming in tubular structure  321 . Various insulating materials may be used including, as examples, polymeric and ceramic materials. As appreciated by those skilled in the art, other stent structures such as mesh or woven structures may also be used.  
      As illustrated, various rings  322  of tubular structure  321  intersect at an angle to form a braided structure. For example, rings  328  and  330  intersect at an angle.  FIG. 3B  illustrates a cross section of an intersection between rings  328  and  330 . At the point of intersection, insulating material  332  coating ring  328  engages insulating material  334  coating ring  330 . Thus, rings  328  and  330  are electrically insulated from each other and spaced apart by their respective insulating materials.  
      It will be appreciated that not all of the rings in a stent need to be coated with an insulating material. The insulating material is only needed to prevent any electrically conducting loops. For example, coating may be applied to only one of the rings at an intersection point with another ring. Additionally, intersecting rings may be made of differing materials, such as ring  328  being electrically conductive and ring  330  made of an insulating material.  FIG. 3C  illustrates an intersection point wherein a ring  330  contacts insulating material  332  coated over ring  328 . Ring  330  does not include an insulating coating. The coating of insulating material needs only to be present at the point of intersection and may be applied prior to or after assembly of rings  328  and  330 . In one embodiment, a heat shrink tube comprised of polytetrafluoroethylene (PTFE) is used to coat one of the rings. It will further be appreciated that the coating does not have to encompass the total circumference of the ring, but only a section of the ring in order to avoid electrical contact between two rings. For example, a flat wire may be coated with insulating material on one side, wherein the side with the insulating coating intersects with another ring.  
       FIG. 4  illustrates an unfolded plan view, of a portion of a stent  340  according, to an alternative embodiment of the present invention. Rings  342 ,  344  and  346  extend axially along stent  340 . Each of the rings  342 ,  344  and  346  are zigzag in shape and meet at various intersection points. Rings  342  and  346  are coated pith an insulating material. In another embodiment, rings  342  and  346  are made of a ceramic or polymeric material. As illustrated in  FIG. 4B , at a point of intersection between ring  342  and  344 , a suitable connector  348  may be used. Connector  348  may be a ring or a tube that holds rings  342  and  344  together.  
       FIG. 5  illustrates an unfolded plan view of a stent  350  according to another embodiment of the present invention. Stent  350 , when wrapped around a central axis  352 , forms a generally tubular structure. Stent  350  includes an undulating ring  354  formed of a plurality of peaks  356  and troughs  358 . If desired, ring  354  may be coated with an insulating material.  
      A pattern in undulating ring  354  is comprised of semi-circular elements that support a greater surface area of a lumen. Other types of patterns may also be used. Ring  354  also includes free ends  360  and  362  that terminate at opposite ends of the stent and prevent the formation of electrical loops within tubular member  351 .  
      Also, ring  354  is wound such that a plurality of rows  366  are formed in tubular member  351 . In order to enhance the structural integrity of stent  350 , connectors  364  are provided between rows  366  of ring  354 . In order to prevent electrical loops from forming in tubular member  351 , the connectors  364  are illustratively made of an insulating, material, such as a polymer or a ceramic. Alternatively, connector  364  may be a metal wire coated with an insulating material and connected between rows  366  so as to not make an electrical connection with ring  354 .  
      In some instances, a radiopaque material is used on stents in order to enhance their visibility under x-ray procedures. Typically, a radiopaque metallic layer is applied to stents made of various polymers or ceramics that are non-radiopaque. The radiopaque layer typically distorts magnetic resonance images as discussed earlier. In order to prevent the formation of electrical loops in the radiopaque layer, a stent  370  similar to that shown in  FIG. 6A  may be used. Stent  370  forms a generally tubular structure  371  when in a deployed position, formed by a plurality of coil-shaped members  372 . For each of the plurality of coil-shaped members  372 , an insulating material, illustratively a polymeric or a ceramic material, is used to separate portions of a radiopaque layer to prevent electrical loops from being formed within the stent.  
       FIG. 6B  illustrates a cross section of a portion designated  6 B of the stent in  FIG. 6A . A base layer  376  is comprised of polymer or ceramic material. The radiopaque layer  378  is then applied to the polymer or ceramic connector. However, gaps  380  in the radiopaque layer may be made such that a polymer or ceramic top layer  382  can coat the radiopaque layer  378  and prevent any connections between portions of the radiopaque layer  378  by filling gaps  380  with insulating material. Thus, portions of the radiopaque layer  378  are spaced apart from each other by the insulating material in gaps  380 . Gaps  380  can be made in the radiopaque layer by a masking procedure during a coating process or by laser ablation after the radiopaque layer has been deposited. Illustratively, each of the plurality of coil-shaped members includes at least one gap  380  formed in radiopaque layer  378 , the gap being filled with insulating material.  
      In an alternative embodiment, a design is chosen wherein rings of a stent are twisted in such a way that a current in one direction is counteracted by a current in the opposite direction. This is explained with regard to  FIG. 7 .  FIG. 7  illustrates a first ring portion  400  having spaced apart ends  401  and  402  and a second ring portion  404  including spaced apart ends  405  and  406 .  
      Each of the ring portions are connected to each other via connectors  408  and  410 . Connector  408  connects end  401  of ring portion  400  to end  405  of ring portion  404 . Connector  410  connects end  402  of ring portion  400  to end  406  of ring portion  404 . Collectively, ring portions  400  and  404  are connected together to form a ring  411  having a gap  412  along a periphery of the ring  411 .  
      Accordingly, when ring portions  400  and  404  are subject to a changing magnetic field represented as .DELTA.B, current flowing in each of the ring portions  400  and  404  will be opposed, which is represented by arrows  414  and  416 . This allows spins, for example spin  418 , to be excited by RF source  140  and gradient generator  130 . When used in a stent, a plurality of rings similar to ring  411  allows spins inside a tubular member of the stent to be excited.  
       FIG. 8  illustrates a plurality of rings  411  connected together in a stent  420 . The rings  411  include ring portions  400  and  404  as previously described. The ring portions  400  and  404  have spaced apart ends  401 ,  402  and  405 ,  406 , respectively. Collectively, the ring portions of rings  411  define gaps  412  along an outer periphery of each of the rings  411 . In order to improve the structural integrity of stent  420 , gaps  412  can be spaced apart radially about the circumference of the tubular structure of the stent. In one embodiment, at least one of the gaps  412  is spaced apart radially from at least one of the other gaps  412  about the circumference of stent  420 .  
      Other embodiments having electrically opposed rings may be used. For example, a stent as shown in  FIG. 9A  may be used in magnetic resonance imaging. Since each of the currents generated in the rings are offset by a corresponding opposite current, the inside of the stent is visible during magnetic resonance imaging from various angles. Stent  450  includes a plurality of rings  452  similar to ring  411  illustrated, in  FIG. 7 . Each of the rings  452  has a corresponding gap  454  along a periphery of the stent  450  in an axial direction with respect to central axis  455 . Accordingly, spins inside stent  450  are excited and the MRI visibility inside stent  450  is enhanced. The plurality of rings  452  are attached to each other by connectors  456 . Illustratively, connectors  456  are made of an insulating material.  
       FIG. 7B  illustrates a portion designated  8 B of the stent in  FIG. 7A . Ring  460  includes a first ring portion  462  and a second ring portion  464 . Ring portion  462  has spaced apart ends  466  and  468  and ring portion  464  has spaced apart ends  470  and  472 . Connector  474  connects end  466  to end  470  while connector  476  connects end  468  to end  472 . Ring portions  462  and  464  also define a gap  478  along the outer periphery of ring  460 . Each of the other rings  452  are constructed similarly to ring  460 .  
      Insulating materials within the stents in the above examples can be various polymeric or ceramic materials. One such material is ePTFE (expanded polytetrafluoroethylene). Various ePTFE fibers, films and tubes can be purchased from Zeus Industrial Products of Orangeburg, S.C.; International Polymer Engineering of Tempe, Ariz.; and W.L. Gore &amp; Associates, Inc. of Elkton, Md. The ePTFE materials are soft, microporous (herein various pore sizes of 0.2-3 microns), flexible and exhibit dielectric properties, strength and biocompatibility. Flexible films or fibers can be fabricated into connector stent connections and then heated to 372.degree. C. for approximately 10 minutes. Consequently, the ePTFE connections are adhered together to form stent connectors. The heat treatments can be varied and are generally conducted 10.degree. C. below the melting or degrading temperature of PTFE. The treatments increase the tensile strength of the ePTFE films, tubes or fibers. There are various other ways to fabricate ePTFE. For example, stent connectors can be connected by multiple layer tubes, then subjected to heat treatments. The ePTFE film can also be wrapped around stent connectors to make the connections.  
      Although the present invention has been described with reference to illustrative embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.  
      A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention.