Abstract:
A thrombosis filter which can be securely affixed at a selected location in the vascular system of a patient and removed when no longer required. An embodiment of the thrombosis filter includes a plurality of struts formed of a shape memory material. A change in temperature can cause the struts to extend and engage a wall of a blood vessel.

Description:
This is a continuation of Application Ser. No. 09/500,209, filed Feb. 8, 2000, now U.S. Pat. No. 6,540,767. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to filters for use inside blood vessels. More particularly, the present invention relates to thrombus filters which can be securely affixed at a selected location in the vascular system and removed when no longer required. 
     BACKGROUND OF THE INVENTION 
     There are a number of situations in the practice of medicine when it becomes desirable for a physician to place a filter in the vascular system of a patient. One of the most common applications for vascular filters is the treatment of Deep Venous Thrombosis (DVT). Deep Venous Thrombosis patients experience clotting of blood in the large veins of the lower portions of the body. These patients are constantly at risk of a clot breaking free and traveling via the inferior vena cava to the heart and lungs. This process is known as pulmonary embolization. Pulmonary embolization can frequently be fatal, for example when a large blood clot interferes with the life-sustaining pumping action of the heart. If a blood clot passes through the heart it will be pumped into the lungs and may cause a blockage in the pulmonary arteries. A blockage of this type in the lungs will interfere with the oxygenation of the blood causing shock or death. 
     Pulmonary embolization may be successfully prevented by the appropriate placement of a thrombus filter in the vascular system of a patient&#39;s body. Placement of the filter may be accomplished by performing a laparotomy with the patient under general anesthesia. However, intravenous insertion is often the preferred method of placing a thrombus filter in a patient&#39;s vascular system. 
     Intravenous insertion of a thrombus filter is less invasive and it requires only a local anesthetic. In this procedure, the thrombus filter is collapsed within a delivery catheter. The delivery catheter is introduced into the patients vascular system at a point which is convenient to the physician. The delivery catheter is then fed further into the vascular system until it reaches a desirable location for filter placement. The thrombus filter is then released into the blood vessel from the delivery catheter. 
     In the treatment of Deep Venous Thrombosis, a thrombus filter is placed in the inferior vena cava of a patient. The inferior vena cava is a large vessel which returns blood to the heart from the lower part of the body. The inferior vena cava may be accessed through the patient&#39;s femoral vein. 
     Thrombus filters may be placed in other locations when treating other conditions. For example, if blood clots are expected to approach the heart and lungs from the upper portion of the body, a thrombus filter may be positioned in the superior vena cava. The superior vena cava is a large vessel which returns blood to the heart from the upper part of the body. The superior vena cava may by accessed through the jugular vein, located in the patient&#39;s neck. 
     Once placed inside a blood vessel, a thrombus filter acts to catch and hold blood clots. The flow of blood around the captured clots allows the body&#39;s lysing process to dissolve the clots. 
     SUMMARY OF THE INVENTION 
     The present invention pertains to a thrombosis filter which can be securely affixed at a selected location in the vascular system of a patient and removed when no longer required. In a first embodiment, the thrombosis filter includes a strut formation, a wire formation, and a body portion. The body portion includes a plurality of apertures. The strut formation includes a plurality of struts each having a fixed end and a free end. The fixed ends of the struts are each fixably attached to the body portion of the thrombus filter inside the apertures; one strut radiating from each aperture. 
     The wire formation is comprised of a plurality of wires. Each wire has a fixed end and a free end. The fixed ends of the wires are fixably attached to the body portion of the thrombus filter. The struts radiate away from the proximal end of the body portion in a proximal direction such that the strut formation is generally conical in shape. Likewise, the wires radiate away from the distal end of the body portion in a distal direction such that the wire formation is generally conical in shape. 
     When the thrombosis filter is disposed in a blood vessel, the wire formation acts to capture blood clots. The generally conical shape of the wire formation serves to urge captured blood clots toward the center of the blood flow. The flow of blood around the captured clots allows the body&#39;s natural lysing process to dissolve the clots. The struts are formed of a shape memory material. At about body temperature, the struts assume an extended shape and engage the walls of the blood vessel. At a selected temperature, other than body temperature, the struts assume a contracted shape. This contracted shape causes the struts to contract inside the apertures of the body portion. 
     Various techniques can be used to alter the temperature of the struts causing them to retract. Suitable techniques for warming the thrombosis filter include applying electromagnetic energy to a portion of the thrombosis filter (e.g. laser light delivered by an optical fiber), and inducing an electrical current through a portion of the thrombosis filter. In a preferred embodiment, the struts are cooled by introducing a relatively cool fluid into the blood vessel proximate the thrombosis filter. After the struts are retracted, the thrombosis filter can be readily pulled into the lumen of a removal catheter. 
     A second embodiment of the thrombosis filter includes a generally cylindrical anchoring portion and a generally conical filtering portion terminating at a body member. The filtering portion includes a plurality of elongated strands. The strands of the filtering portion are arranged in an interwoven pattern to create cells. The interwoven pattern of strands enables the filtering portion to trap or capture blood clots. The conical shape of the filtering portion urges captured blood clots toward the center of the blood flow. The flow of blood around the captured blood clots allows the body&#39;s natural lysing process to dissolve the clots. 
     The strands extend beyond the filtering portion to create the anchoring portion. The strands are formed from a shape memory alloy. The shape memory alloy construction of the thrombosis filter allows it to change shape in response to a change in temperature. At about body temperature, the thrombosis filter assumes an extended shape. At a selected temperature other than body temperature, the thrombosis filter assumes a contracted shape. When the thrombosis filter assumes a contracted shape the anchor portion of the thrombosis filter disengages the walls of the blood vessel. When it is desirable for the thrombosis filter to be removed from a blood vessel, a physician may selectively heat or cool the thrombosis filter causing it to assume the contracted shape. Various techniques can be used to change the temperature of the thrombosis filter. In a preferred embodiment, the thrombosis filter is cooled by introducing a relatively cold fluid into the blood vessel proximate the thrombosis filter. Once the thrombosis filter assumes a contracted shape, it may be pulled in the lumen of a removal catheter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a plan view of a thrombus filter with struts in an extended position; 
         FIG. 2  is a plan view of a thrombus filter with struts in a contracted position; 
         FIG. 3  is a plan view illustrating the removal of a thrombus filter from a blood vessel; 
         FIG. 4  is a plan view of an alternate embodiment of a thrombus filter, 
         FIG. 5  is a plan view of the thrombus filter of  FIG. 4 ; 
         FIG. 6  is a plan view of an additional embodiment of a thrombosis filter in accordance with the present invention; 
         FIG. 7  is a plan view of the thrombus filter of  FIG. 6  in an expanded state; 
         FIG. 8  is a diagrammatic view illustrating a process which may be used to remove a thrombus filter from the body of a patient, the diagrammatic view including an exemplary embodiment of a thrombus filter, and an exemplary embodiment of a removal catheter; 
         FIG. 9  is a diagrammatic view of the apparatus illustrated in  FIG. 8 , the thrombus filter being in a contracted state; 
         FIG. 10  is a diagrammatic view illustrating an additional process which may be used to remove a thrombus filter from the body of a patient, the diagrammatic view including an exemplary embodiment of a thrombus filter, and an exemplary embodiment of a removal catheter; 
         FIG. 11  is a diagrammatic view of the apparatus illustrated in  FIG. 10 , the thrombus filter being in a contracted state. 
         FIG. 12  is a perspective view of an additional embodiment of a thrombosis filter; 
         FIG. 13  is a plan view of an additional embodiment of a thrombosis filter; 
         FIG. 14  is a plan view of an additional embodiment of a thrombosis filter; 
         FIG. 15  is a plan view of an additional embodiment of a thrombosis filter; p  FIG. 16  is a plan view of an additional embodiment of a thrombosis filter; and  5   
         FIG. 17  is a perspective view of an additional embodiment of a thrombosis filter. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following detailed description should be read with reference to the drawings, in which like elements in different drawings are numbered identically. The drawings which are not-necessarily to scale, depict selected embodiments and are not intended to limit the scope of the invention. 
     Examples of constructions, materials, dimensions, and manufacturing processes are provided for selected elements. All other elements employ that which is known to those of skill in the field of the invention. Those skilled in the art will recognize that many of the examples provided have suitable alternatives which may be utilized. 
     Reference is now made to the drawings, in which like numbers refer to like elements throughout.  FIG. 1  is a plan view of a thrombosis filter  20  positioned in a lumen  21  of a blood vessel  22 . Blood vessel  22  includes walls  23  which define lumen  21 . The main components of thrombosis filter  20  are a body portion  24 , a strut formation  26  and a wire formation  28 . 
     Body portion  24  includes a plurality of apertures  30 . Strut formation  26  includes a plurality of struts  32  each having a fixed end  34 , and a free end  36 . Fixed ends  34  of struts  32  are each fixedly attached to body portion  24  inside apertures  30 ; one strut  32  radiating from each aperture  30 . 
     Wire formation  28  is comprised of a plurality of wires  37 . Each wire  37  has a fixed end  38  and a free end  40 . Fixed ends  38  of wires  37  are fixedly attached to body portion  24 . 
     Wire  37  may include a plurality of bends  46  disposed between free end  40  and fixed end  38 . Free end  40  of each wire  37  includes an anchor  50 . Likewise, each strut  32  includes an anchor  52 . In  FIG. 1 , anchors  50  and  52  are pictured as sharp projections or barbs. It should be understood that anchors  50  and  52  may be comprised of other means for anchoring without departing from the spirit or scope of this invention. 
     Body portion  24  includes a proximal end  60  and a distal end  62 . A coupling member  64  is fixedly attached to proximal end  60  of body portion  24 . 
     Struts  32  radiate away from proximal end  60  of body portion  24  in a proximal direction. Such that strut formation  26  is generally conical in shape. Likewise, wires  37  radiate away from distal end  62  of body portion  24  in a distal direction such that wire formation  28  is generally conical in shape. 
     When thrombosis filter  20  is disposed in a blood vessel, wire formation  28  acts to trap, or capture blood clots. The generally conical shape of wire formation  28  serves to urge captured blood clots toward the center of the blood flow. The flow of blood around the captured blood clots allows the body&#39;s natural lysing process to dissolve the clots. 
     Struts  32  act as opposing wall contacting members and serve to position thrombosis filter  20  in the center of lumen  21  of blood vessel  22  shown with hidden lines in FIG.  1 . Likewise, wires  37  act as opposing wall contacting members and serve to position thrombosis filter  20  in the center of lumen  21  of blood vessel  22 . Anchors  52  of struts  32  generally oppose anchors  50  of wires  37 . These opposing anchors  50  and  52  serve to maintain the position of thrombosis filter  20 , preventing it from migrating upstream or downstream in blood vessel  22 . In the embodiment shown in  FIG. 1  anchors  50  and  52  include a plurality of sharp projections which penetrate the walls of blood vessel  22 . 
     Struts  32  and wires  37  may all be fabricated from wire with a circular, rectangular or other cross section. For example, straight wires  37  may be comprised of 0.018″ diameter wire. Stainless steel, titanium, and nickel titanium alloy have all been found to be acceptable materials for wires  37 . 
     Struts  32  are formed from a shape-memory material. The shape-memory material of struts  32  may be a shape-memory polymer, or a shape-memory alloy. Suitable shape memory materials are commercially available from Memry Technologies (Brookfield, Conn.), TiNi Alloy Company (San Leandro, Calif.), and Shape Memory Applications (Sunnyvale, Calif.). In a preferred embodiment, struts  32  are comprised of an alloy of titanium and nickel known in the art as Nitinol. 
     The shape-memory material construction of struts  32  enable struts  32  to change shape in response to a change in temperature. At about body temperature, struts  32  assume an extended shape as shown in FIG.  1 . At a selected temperature other than body temperature, struts  32  assume a contracted shape as shown in FIG.  2 . 
     In  FIG. 2 , struts  32  have partially contracted inside apertures  30  of body portion to  24 . As a result of the contraction of struts  32 , anchors  52  have retracted from walls  23  of blood vessel  22 . 
     Various techniques can be used to alter the temperature of struts  32 . Suitable techniques for warming struts  32  include applying electromagnetic energy to body portion  24  (e.g. laser light delivered by an optical fiber), and applying electrical energy to thrombosis filter  20  (e.g. inducing a current through struts  32 ). 
     A process which may be used to remove thrombosis filter  20  from lumen  21  of blood vessel  22  is illustrated in  FIG. 3. A  removal catheter  110  with a lumen  112  and a distal end  114  is disposed in lumen  21  of blood vessel  22 . Removal catheter  10  enters the patients vascular system at a point which is readily accessible to the physician. Once in the vascular system, removal catheter  110  is urged forward until distal end  114  is proximate thrombosis filter  20 . For example, if thrombosis filter  20  is located in the inferior vena cava of a patients vascular system, removal catheter  110  may enter the vascular system at the femoral vein. Alternately, if thrombosis filter  20  is located in the superior vena cava of a patients vascular system, removal catheter  110  may enter the vascular system at the jugular vein. In either case, the filter removal procedure is minimally invasive, and does not require general anesthesia. 
     An elongated retrieval member  116  including a distal end  118  and a proximal end  120  (not shown) is disposed in lumen  112  of removal catheter  110 . In  FIG. 3 , distal end  118  of retrieval member  116  has been releasibly mated to coupling member  64  of thrombosis filter  20 . Proximal end  120  of elongated retrieval member  116  protrudes beyond the proximal end of removal catheter  110 . Both removal catheter  110  and retrieval member  116  extend outside the body of the patient. 
     When distal end  114  of removal catheter  110  reaches a position proximate thrombosis filter  20 , the temperature of struts  32  is altered, causing them to retract. With struts  32  in a retracted position, thrombosis filter  20  may be readily pulled into lumen  112  of removal catheter  110  by applying a pulling force to proximal end  120  of retrieval member  116 . This pulling force is transferred via retrieval member  116  to thrombosis filter  20 . The pulling force applied to retrieval member  116  of thrombosis filter  20  pulls anchors  50  of wires  37  out of blood vessel  22 . 
     As shown if  FIG. 3 , pulling thrombosis filter  20  into lumen  112  of removal catheter  110  causes wires  37  to collapse causing wire formation  28  to transform from a generally conical shape toward a generally cylindrical shape. With wires  37  in a collapsed position, thrombosis filter  20  may be pulled completely into lumen  112  of removal catheter  110 . Once thrombosis filter  20  is inside lumen  112 ; removal catheter  110  may be withdrawn from blood vessel  22 . 
       FIG. 4  is a plan view of a second embodiment of a thrombosis filter  400 , disposed in a blood vessel  450 . Blood vessel  450  includes a lumen  452  defined by blood vessel walls  454 . Thrombosis filter  400  includes a generally cylindrical anchoring portion  402 , and a generally conical filtering portion  404  terminating at a body member  406 . Filtering portion  404  includes a plurality of elongated struts or strands  410 . Strands  410  of filtering portion  404  are arranged in an interwoven pattern to create cells  412 . The interwoven pattern of strands  410  enables filtering portion  404  to trap, or capture blood clots. The conical shape of filtering, portion  404  urges captured blood clots toward the center of the blood flow. The flow of blood around the captured blood clots allows the to body&#39;s natural lysing process to dissolve the clots. 
     Strands  410  extend beyond filtering portion  404  into anchoring portion  402 . Strands  410  are formed from a shape-memory material. The shape-memory material of strands  410  may be a shape-memory polymer or a shape memory metal. Suitable shape memory materials are commercially available from Memry Technologies (Brookfield, Conn.), TiNi Alloy Company (San Leandro, Calif.), and Shape Memory Applications (Sunnyvale, Calif.). In a preferred embodiment, strands  410  are comprised of an alloy of titanium and nickel known in the art as Nitinol. 
     The term “strands”, as used in describing strands  410  should not be mistaken as limiting strands  410  to elements having a circular cross section. The cross section of strands  410  may be any number of shapes. For example, the cross section of strands  410  could be rectangular, elliptical, etc. Embodiments of the present invention have been envisioned in which strands  410  are comprised of laser cut elements. 
     The shape-memory alloy construction of strands  410  enable thrombosis filter  400  to change shape in response to a chance in temperature. In  FIG. 4 , thrombosis filter  400  is shown in an extended shape  420 . Thrombosis filter  400  assumes extended shape  420  when strands  410  are generally at about body temperature. A contracted shape  430  is shown with phantom lines in FIG.  4 . Thrombosis filter  400  assumes contracted shape  430  when strands  410  are at a selected temperature other than body temperature. 
     When it is desirable for thrombosis filter  400  to be removed from a blood vessel, a physician may selectively heat or cool thrombosis filter  400  causing it to assume contracted shape  430 . When thrombosis filter  400  assumes contracted shape  430 , anchoring portion  402  retracts away from walls  454  of blood vessel  450 . 
     Various techniques may be utilized to change the temperature of thrombosis filter  400 . Suitable techniques for warming thrombosis filter  400  include applying electromagnetic energy to body member  406  (e.g. laser light delivered by an optical fiber), and applying electrical energy to thrombosis filter  400  (e.g. inducing a current through strands  410 ). In a preferred cooling method, the thrombosis filter is cooled by introducing a relatively cold fluid into the body proximate the thrombosis filter. 
     Thrombosis filter  400  may be removed from lumen  452  of blood vessel  450  utilizing a method similar to the one described for the previous embodiment. A removal catheter is positioned in lumen  452  of blood vessel  450  so that the distal end of the removal catheter is proximate thrombosis filter  400 . 
     Embodiments of the present invention are possible in which portions of the thrombosis filter are coated with a coating material. Embodiment of the present invention have been envisioned in which the coating material prevents tissue growth proximate the filter to facilitate subsequent disengagement of the filter. Embodiment of the present invention have also been envisioned in which the coating material comprises a non-stick material to facilitate subsequent disengagement of the filter. These envisioned coating materials may be utilized with the various embodiments disclosed herein. 
     The removal catheter may enter the patients vascular system at a point which is readily accessible to the physician. Once in the vascular system, the removal catheter is urged forward until its distal end is proximate thrombosis filter  400 . For example, if thrombosis filter  400  is located in the inferior vena cava of a patients vascular system, the removal catheter may enter the vascular system at the femoral vein. Alternately, if thrombosis filter  400  is located in the superior vena cava of a patients vascular system, the removal catheter may enter the vascular system at the jugular vein. In either case, the filter removal procedure is minimally invasive, and usually does not require general anesthesia. 
     An elongated retrieval member is disposed in the lumen of the retrieval catheter. The distal end of the elongated retrieval member is releasably mated to a coupling member  440  which is fixedly attached to body member  406  of thrombosis filter  400 . 
     A presently preferred method includes the step of altering the temperature of strands  410 . When the temperature of strands  410  is altered, they change shape, causing thrombosis filter  400  to retract from extended position  420  to contracted position  430 . The change in shape causes anchor portion  402  to disengage walls  454  of blood vessel  450   
     With anchor portion  402  disengaged from walls  454  of blood vessel  450 , thrombosis filter  400  may be readily pulled into the lumen of the retrieval catheter. The pulling force is applied to thrombosis filter  400  by pulling on the proximal end of the elongated retrieval member which has been joined to coupling member  440 . 
       FIG. 5  is a plan view illustrating thrombosis filter  400  taken from line A—A shown in FIG.  4 . Thrombosis filter  400  is disposed in lumen  452  of blood vessel  450 . Thrombosis filter  400  includes filtering portion  404 . Filtering portion  404  includes strands  410  which are arranged in an interwoven pattern to create cells  412 . The interwoven pattern of strands  410  enables filtering portion  404  to trap, or capture blood clots. The conical shape of filtering portion  404  urges captured blood clots toward the center of the blood flow. The flow of blood around the captured blood clots allows the body&#39;s natural lysing process to dissolve the clots. 
       FIG. 6  is a plan view of an additional embodiment of a thrombosis filter  500 . In the embodiment of  FIG. 6 , thrombus filter  500  includes a body portion  502  and a plurality of spokes  506 . Spokes  506  each have a joined end  508  and a free end.  510 . Joined end  508  of each spoke  506  is fixedly attached to body portion  502 . Spokes  506  radiate outwardly from body portion  502  such that thrombus filter  500  is generally conical in shape. An anchor member  512  is disposed proximate the free end  510  of each spoke  506 . 
     Thrombosis filter  500  also includes a ring  520  which is disposed proximate free ends  510  of spokes  506 . In the embodiment of  FIG. 6 , each spoke  506  is fixed to ring  520 . Those of skill in the art will appreciate that many methods may be used to fix ring  520  to Spokes  506 . Examples of methods which may be Suitable in some applications include welding, brazing, soldering, and the use of adhesives. Other embodiments of thrombus filter  500  are possible, in which ring  520  mechanically engages spokes  506 . For example, spokes  506  may include holes, slots, or eyes. In this exemplary embodiment, ring  520  may be threaded through the holes, slots, or eyes of spokes  506 . 
     As shown in  FIG. 6 , ring  520  of thrombus filter  500  includes a plurality of bends  522 . In a presently preferred embodiment, ring  520  is comprised of a shape memory alloy. Suitable shape memory alloys are commercially available from Memry Technologies (Brookfield, Conn.), TiNi Alloy Company (San Leandro, Calif.), and Shape Memory Applications (Sunnyvale, Calif.). In a presently most preferred embodiment, ring  520  is comprised of an alloy of titanium and nickel known in the art as Nitinol. 
     When thrombus filter  500  is released in a blood vessel, spokes  506  expand outward so that free ends  510  of spokes  506  contact the walls of the blood vessel. The geometry of anchor members  512  results in localized contact between the thrombus filter and the blood vessel walls. Anchor members  512  become imbedded in the walls of the blood vessel proximate these points of initial contact. 
       FIG. 7  is a plan view of a thrombus filter  500  in an expanded state. Thrombus filter  500  of the embodiment shown in  FIGS. 6 and 7  includes an insulating layer  524  substantially covering thrombus filter  500  including body portion  502 , spokes  506 , and anchor members  512 . A number of materials have been found to be suitable for use in insulating layer  524 , these materials include fluoropolytetrafluoroethylene (PTFE), polyethylene (PE), polypropylene (PP), polyvinylchloride (PVC), and polyurethane. A number of manufacturing processes may be used to create insulating layer  524 . For example, a portion of insulating layer  524  may be made up of sections of shrink tubing. The shrink tubing sections may be positioned over the spokes then shrunk by the application of heat. A spray process may also be used to apply insulating layer  524  to thrombus filter  500 . For example, spraying PTFE solids in a suitable solvent carrier is a process which has been found suitable for this application. 
     Another material which may be used to fabricate insulating layer  524  is a thermoplastic generically known as parylene. There are a variety of polymers based on para-xylylene. These polymers are typically placed onto a substrate by vapor phase polymerization of the monomer. Parylene N coatings are produced by vaporization of a di(P-xylylene)dimer, pyrollization, and condensation of the vapor to produce a polymer that is maintained at comparatively lower temperature. In addition to parylene-N, parylene-C is derived from di(monochloro-P-xylylene) and parylene-D is derived from di(dichloro-P-xylylene). There are a variety of known ways to apply parylene to substrates. 
     It should be understood that insulating layer  524  may include apertures, when these apertures are necessary to create an electrical circuit. The significance of these apertures and insulating layer  524  will be made clear in the discussion which follows. 
       FIG. 8  is a diagrammatic view illustrating a process which may be used to remove a thrombus filter  600  from the body of a patient. In the embodiment of  FIG. 7 , thrombus filter  600  includes a body portion  602  and a plurality of spokes  606 . Spokes  606  each have a joined end  608  and a free end  610 . Joined end  608  of each spoke  606  is fixedly attached to body portion  602 . In a presently preferred embodiment, body portion  602  is comprised of a non-conductive material so that body portion  602  does not form a path for electric current between spokes  606 . 
     Thrombosis filter  600  also includes a ring  620  which is disposed proximate free ends  610  of spokes  606 . In a presently preferred embodiment, ring  620  is electrically coupled to spokes  606 . Also in a presently preferred embodiment, ring  620  is comprised of a shape memory alloy. Suitable shape memory alloys are commercially available from Memry Technologies (Brookfield, Conn.), TiNi Alloy Company (San Leandro, Calif.), and Shape Memory Applications (Sunnyvale, Calif.). In a presently most preferred embodiment, ring  620  is comprised of an alloy of titanium and nickel known in the art as Nitinol. 
     In  FIG. 8 , thrombus filter  600  is disposed within a lumen  632  of a blood vessel  630 . A removal catheter  640  is also disposed within lumen  632  of blood vessel  630 . A distal end  644  of removal catheter  640  is disposed proximate thrombus filter  600 . Removal catheter also includes a lumen  642  and a proximal end  646 . 
     A first electrical conductor  650  and a second electrical conductor  660  are disposed inside lumen  642  of removal catheter  640 . First electrical conductor  650  includes a proximal end  654  and a distal end  652 . Second electrical conductor  660  includes a proximal end  664  and a distal end  662 . 
     As in the previous embodiment, thrombus filter  600  includes a insulating layer  624 . In the embodiment of  FIG. 8 , distal end  652  of first electrical conductor  650  has penetrated insulating layer  624  of thrombus filter  600  to form an electrical connection with a first spoke  616 . Likewise, distal end  662  of second electrical conductor  660  has penetrated insulating layer  624  of thrombus filter  600  to form an electrical connection with a second-spoke  618 . 
     A number of methods may be suitable for forming the electrical connection between the distal ends of the electrical conductors and the spokes. For example, a needle electrode may be disposed at distal ends  652 ,  662  of electrical conductors  650 ,  660  respectively. The needle electrodes could penetrate insulating layer  524  and make electrical contact with the spokes. An easily deformable material such as silicone rubber or foam rubber could be disposed around the needle electrode to insulate the electrical connection. 
     Proximal end  654  of first electrical conductor  650  and proximal end  664  of second electrical conductor  660  are both electrically coupled to a power supply  670 . Power supply  670  is used to selectively apply a voltage differential between first electrical conductor  650  and second electrical conductor  660 . 
     In the embodiment of  FIG. 8 , a circuit path between first spoke  616  and second spoke  618  comprises ring  620 . In a presently preferred embodiment, current must travel through ring  620  in order to pass from first spoke  616  to second spoke  618 . The voltage differential created by power supply  670  induces a current flow through ring  620 . The flow of current through ring  620  causes the temperature of ring  620  to be altered. When the temperature of ring  620  is altered, ring  620  assumes a contracted position as shown in FIG.  9 . 
       FIG. 9 , is a diagrammatic view of the thrombus filter of  FIG. 8  with ring  620  in a contracted position. As shown in  FIG. 9 , the contraction of ring  620  causes anchors  612  to disengage the walls of blood vessel  630 . Once anchors  612  are disengaged from the walls of blood vessel  630 , thrombus filter  600  may be pulled into lumen  642  of removal catheter  640 . 
       FIG. 10  is a diagrammatic view illustrating an additional process which may be used to remove a thrombus filter  700  from the body of a patient. In the embodiment of  FIG. 10 , thrombus filter  700  includes a body portion  702  and a plurality of spokes  706 . Spokes  706  each have a joined end  708  and a free end  710 . Joined end  708  of each spoke  706  is fixedly attached to body portion  702 . In a presently preferred embodiment, of thrombus filter  700 , body portion  702  is electrically insulated from the plurality of spokes  706  with the exception of a first spoke  0 . 716 . In this presently preferred embodiment, body portion  702  is electrically coupled to first spoke  716 . 
     Thrombosis filter  700  also includes a ring  720  which is disposed proximate free ends  710  of spokes  706 . In a presently preferred embodiment, ring  720  is electrically coupled to first spoke  716 . Also in a presently preferred embodiment, ring  720  is comprised of a shape memory alloy. Suitable shape memory alloys are commercially available from Memry Technologies (Brookfield, Conn.), TiNi Alloy Company (San Leandro, Calif.), and Shape Memory Applications (Sunnyvale, Calif.). In a presently most preferred embodiment, ring  720  is comprised of an alloy of titanium and nickel known in the art as Nitinol. 
     In  FIG. 10 , thrombus filter  700  is disposed within a lumen  732  of a blood vessel  730 . A removal catheter  740  is also disposed within lumen  732  of blood vessel  730 . A distal end  744  of removal catheter  740  is disposed proximate thrombus filter  700 . Removal catheter also includes a lumen  742 , a proximal end  746 , and a ring electrode  780  disposed proximate the distal end thereof. 
     A first electrical conductor  750  and a second electrical conductor  760  are disposed inside lumen  742  of removal catheter  740 . First electrical conductor  750  includes a proximal end  754  and a distal end  752 . Second electrical conductor  760  includes a proximal end  764  and a distal end  762 . 
     As shown in  FIG. 10 , distal end  762  of second electrical conductor  760  is coupled to ring electrode  780 . Distal end  752  of first electrical conductor  750  is coupled to body portion  702  of thrombus filter  700 . As in the previous embodiment, thrombus filter  700  includes a insulating layer  724 . 
     In the embodiment of  FIG. 10 , distal end  752  of first electrical conductor  750  has penetrated insulating layer  724  of thrombus filter  700  to form an electrical connection with body portion  702 . Also in the embodiment of  FIG. 10 , insulating layer  724  includes an aperture  790 . Aperture  790  allows a portion of thrombus filter  700  to make electrical contact with the body of the patient. Those of skill in the art will appreciate that a number of embodiments of aperture  790  are possible without deviating from the spirit and scope of the present invention. 
     Proximal end  754  of first electrical conductor  750  and proximal end  764  of second electrical conductor  760  are both electrically coupled to a power supply  770 . 
     Power supply  770  is used to selectively apply a voltage differential between first electrical conductor  750  and second electrical conductor  760 . 
     In the embodiment of  FIG. 10 , a circuit path between first electrical conductor  750  and second electrical conductor  760  comprises body portion  702 , first spoke  716 , ring  720 , aperture  790 , ring electrode  780 , and the body of the patient. Those of skill in the art will appreciate that many embodiments of the present invention are possible in which current flows through the body of the patient. For example, current may flow between ring electrode  780  and aperture  790  through the blood. By way of a second example, embodiments of the present invention have been envisioned in which ring electrode  780  is replaced with a conductive patch which may be applied to an area of exposed skin on the patients body. In this envisioned embodiment, the path of current flow through the patient will include tissue. 
     The voltage differential created by power supply  770  induces a current flow through ring  720 . The flow of current through ring  720  causes the temperature of ring  720  to be altered. When the temperature of ring  720  is altered, ring  720  assumes a contracted position as shown in FIG.  11 . 
       FIG. 11 , is a diagrammatic view of the thrombus filter of  FIG. 10  with ring  720  in a contracted position. As shown in  FIG. 11 , the contraction of ring  720  causes anchors  712  to disengage the walls of blood vessel  730 . Once anchors  712  are disengaged from the walls of blood vessel  730 , thrombus filter  700  may be pulled into lumen  742  of removal catheter  740 . 
       FIG. 12  is a perspective view of an additional embodiment of a thrombosis filter  800 . Thrombus filter  800  includes a first hub  802 , a second hub  804 , and a plurality of ribs  806  extending between first hub  802  and second hub  804 . In the embodiment of  FIG. 12 , thrombus filter  800  is shown in an expanded state. When thrombus filter  800  is in an expanded state, each rib  806  forms one or more bends  808 . 
       FIG. 13  is a plan view of thrombosis filter  800  of FIG.  12 . First hub  802  and ribs  806  are visible in FIG.  13 . In  FIG. 13  it may be appreciated that ribs  806  extend radially away from first hub  802  when thrombosis filter  800  is in an expanded state. 
       FIG. 14  is a plan view of thrombosis filter  800  in a contracted state. In  FIG. 14  it may be appreciated that ribs  806  are substantially flush with first hub  802  and second hub  804  when thrombosis filter  800  is in a contracted state. Thrombosis filter  800  may be formed by laser cutting a section of tubing to form ribs  806 . Methods in accordance with the present invention may be utilized to cause thrombosis filter  800  to contract from the expanded shape shown in  FIGS. 12 and 13  to the contracted shape shown in FIG.  14 . 
       FIG. 15  is a plan view of an additional embodiment of a thrombosis filter  820 . Thrombosis filter  820  includes a base portion  822  and a plurality of branches  824 . In the embodiment of  FIG. 15 , thrombosis filter  820  is shown in an expanded state. It may be appreciated that branches  824  radiate away from base portion  822  when thrombosis filter  820  is in an expanded state. 
       FIG. 16  is a plan view of thrombosis filter  820  in a contracted state. In  FIG. 16  it may be appreciated that branches  824  do not appreciably extend in a radial direction beyond base portion  822  when thrombosis filter  820  is in a contracted state. Methods in accordance with the present invention may be utilized to cause thrombosis filter  820  to contract from the expanded shape shown in  FIG. 15  to the contracted shape shown in FIG.  16 . Thrombosis filter  820  may be formed by laser cutting a section of tubing to form branches  824 . 
       FIG. 17  is a perspective view of an additional embodiment of a thrombosis filter  840 . Thrombosis filter  840  includes a body portion  842 . A plurality of legs  844  radiate away from body portion  842  forming a generally conical portion  846  of thrombosis filter  840 . Thrombosis filter  840  also includes a plurality of arms  848 . A portion of each arm is fixed to body portion  842 . Each arm extends radially away from body portion  842 . In the embodiment of  FIG. 17  each arm includes a curve  150 . In the embodiment of  FIG. 17 , thrombosis filter  840  is shown in an expanded state. Methods in accordance with the present invention may be utilized to cause thrombosis filter  840  to contract from the expanded shape shown in  FIG. 17  to a contracted shape. 
     Numerous advantages of the invention covered by this document have been set forth in the foregoing description. It will be understood, however, that this disclosure is, in many respects, only illustrative. Changes may be made in details, particularly in matters of shape, size, and arrangement of parts without exceeding the scope of the invention. The inventions&#39;s scope is, of course, defined in the language in which the appended claims are expressed.