Patent Publication Number: US-2010114275-A1

Title: Implantable medical lead including winding for improved mri safety

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application contains subject matter that is related to the following copending U.S. patent applications: 1) Ser. No. 12/110,150, filed Apr. 25, 2008, titled “Implantable Medical Lead Configured for Improved MRI Safety” (Attorney Docket A08P1014); 2) Ser. No. 11/932,030, filed Oct. 31, 2007, titled “Implantable Medical Lead Configured for Improved MRI Safety” (Attorney Docket A07P1164); and 3) Ser. No. 12/197,957, filed Aug. 25, 2008, titled “MRI Compatible Lead” (Attorney Docket A08P1034). The entire disclosures of these related applications are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to medical methods and apparatus. More specifically, the present invention relates to implantable medical leads and methods of manufacturing and utilizing such leads. 
     BACKGROUND OF THE INVENTION 
     Existing implantable medical leads for use with implantable pulse generators, such as neurrostimulators, pacemakers, defibrillators or implantable cardioverter defibrillators (“ICD”), are prone to heating and induced current when placed in the strong magnetic (static, gradient and RF) fields of a magnetic resonance imaging (“MRI”) machine. The heating and induced current are the result of the lead acting like an antenna in the magnetic fields generated during an MRI. Heating and induced current in the lead may result in deterioration of stimulation thresholds or, in the context of a cardiac lead, even increase the risk of cardiac tissue damage and perforation. 
     Over fifty percent of patients with an implantable pulse generator and implanted lead require, or can benefit from, an MRI in the diagnosis or treatment of a medical condition. MRI modality allows for flow visualization, characterization of vulnerable plaque, non-invasive angiography, assessment of ischemia and tissue perfusion, and a host of other applications. The diagnosis and treatment options enhanced by MRI are only going to grow over time. For example, MRI has been proposed as a visualization mechanism for lead implantation procedures. 
     There is a need in the art for an implantable medical lead configured for improved MRI safety. There is also a need in the art for methods of manufacturing and using such a lead. 
     SUMMARY 
     Disclosed herein is an implantable medical lead for coupling to an implantable pulse generator and configured for improved MRI safety. In particular, embodiments disclosed herein may improve MRI safety by reducing or even canceling induced currents in medical leads. Such reduction and/or canceling may reduce or even eliminate risks of stimulation and/or heating resulting from exposure of medical leads to magnetic and/or electrical fields. 
     In one embodiment, the lead may include a tubular body, a first electrode and a first electrical coil conductor. The first electrode may be operably coupled to the tubular body near the distal end. The first electrical coil conductor may extend distally through the body from the proximal end and may electrically connect to the first electrode. The first coil conductor may also include at least one transition in which the first coil conductor changes from being helically coiled in a first direction to being coiled in a second opposite direction. 
     Disclosed herein is a method of forming an implantable medical lead configured for improved MRI safety. In particular, embodiments disclosed herein may produce a lead that improves MRI safety by reducing or even canceling induced currents in medical leads. 
     In one embodiment, the method may include: helically coiling at least a portion of a first electrical coil conductor by winding the first coil conductor in a first direction, and winding the first coil conductor in a second direction opposite the first direction so as to form a transition in the first coil conductor in which the first coil conductor changes from being helically coiled in the first direction to being helically coiled in the second direction. The method may also include forming an implantable medical lead including the helically coiled first coil conductor. 
     While multiple embodiments are disclosed, still other embodiments of the present invention will become apparent to those skilled in the art from the following detailed description, which shows and describes illustrative embodiments of the invention. As will be realized, the invention is capable of modifications in various aspects, all without departing from the spirit and scope of the present invention. Accordingly, the drawings and detailed description are to be regarded as illustrative in nature and not restrictive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an isometric view of a lead and a pulse generator for connection thereto. 
         FIG. 2  is an isometric view of a longitudinal segment of the lead tubular body in the vicinity of arrow A in  FIG. 1 , wherein the coil conductor is formed to include a transition according to one embodiment. 
         FIGS. 3A-C  illustrate induced currents in a portion of the coil conductor near the transition in the helical coiling of the coil conductor that may result from exposing the coil conductor to electrical and/or magnetic fields. 
         FIG. 4  is an isometric view of a longitudinal segment wherein the coil conductor is formed to include two transitions according to another embodiment. 
         FIG. 5  is an isometric view of a longitudinal segment wherein the coil conductor is formed to include a plurality of transitions according to another embodiment. 
         FIG. 6  an isometric view of a longitudinal segment wherein two coil conductors are formed to include a plurality of transitions according to one embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Disclosed herein is an implantable medical lead  10  configured for improved MRI safety. In various embodiments, the lead  10  may include conductors and/or conductor arrangements configured to reduce, if not totally eliminate, the potential for MRI induced currents and heating in conductors extending through the lead body to electrodes, such as those used for pacing, sensing and/or defibrillation. 
     For an overview discussion regarding an embodiment of a lead  10  configured for improved MRI safety, reference is made to  FIG. 1 , which is an isometric view of such a lead  10  and a pulse generator  15  for connection thereto. As shown in  FIG. 1 , the pulse generator  15 , which may be a neurrostimulator, pacemaker, defibrillator or ICD, may include a housing  31  and a header  32 . The housing  31  may enclose the electrical components of the pulse generator  15 . The header  32  may be mounted on the housing  31  and may include lead receiving receptacles  33  for connecting one or more leads  10  to the pulse generator  15 . 
     As illustrated in  FIG. 1 , in one embodiment, the lead  10  may include a proximal end  20 , a distal end  25  and a tubular body  30  extending between the proximal and distal ends. The proximal end  20  may include a lead connective end  35  having a pin contact  40 , a first ring contact  45 , a second ring contact  46 , which is optional, and sets of spaced-apart radially projecting seals  50 . In other embodiments, the lead connective end  35  may include a greater or lesser number of contacts and may include the same or different types of seals. The lead connective end  35  may be received in one of the lead receiving receptacles  33  of the pulse generator  15  such that the contacts  40 ,  45 ,  46  electrically contact corresponding electrical terminals within the respective receptacle  33  and the seals  50  prevent the ingress of body fluids into the respective receptacle  33 . 
     As depicted in  FIG. 1 , in one embodiment, the lead distal end  25  may include a distal tip  55 , an anchor  60 , a tip electrode  65 , and a ring electrode  70 . The anchor  60  may be extendable from an orifice in the distal tip  75 . The tip electrode  65  may form the distal tip  75  of the lead body  30 , and the ring electrode  70  may extend about the circumference of the lead body  30  proximal of the tip electrode  65 . In other embodiments, there may be a greater or lesser number of electrodes  65 ,  70  in similar or different configurations. Also, the anchor  60  may or may not have other configurations and may or may not also serve as an electrode. 
     As indicated in  FIG. 1 , the lead  10  may include an optional defibrillation coil  80 , which may extend about the circumference of the lead body  30 . The defibrillation coil  80  may be located proximal of the ring electrode  70 . 
     In one embodiment, the tip electrode  65  may be in electrical communication with the pin contact  40  via electrical conductors, the ring electrode  70  may be in electrical communication with the ring contact  45  via other electrical conductors, and the defibrillation coil  80  may be in electrical communication with the second ring contact  46  via yet other conductors. The various conductors may extend through the lead body  30  and are described later in this Detailed Description. 
     But for the novel conductor configurations discussed below, the conductors could act as an antenna in the magnetic field of an MRI. As a result, current could be induced in the conductors, causing the conductors and the electrodes connected thereto to stimulate and/or heat and potentially damage the lead and/or tissue contacting the electrodes. 
     For a discussion of an embodiment of the lead  10  configured to reduce, if not totally eliminate, current induction and heating caused in lead conductors subjected to MRI, reference is made to FIGS.  2  and  3 A-C.  FIG. 2  is an isometric view of a longitudinal segment of the lead tubular body  30  in the vicinity of arrow A in  FIG. 1 , wherein the body  30  is represented schematically only for the sake of reference, without regard to the layers of the body  30 , to illustrate a coil conductor arrangement  110 .  FIGS. 3A-C  illustrate induced currents that may result from exposure of the lead  10  to a MRI. 
     As shown in  FIG. 2 , the coil conductor arrangement  110  may comprise an electrical coil conductor  112 . The coil conductor  112  may be formed of an electrically conductive material, such as MP35N, silver-cored MP35N, tantalum, etc. The coil conductor  112  may be formed of a single filar or multiple filars, for example, having a diameter of between approximately 0.002″ and approximately 0.01″. Also, the coil conductor  112  may be insulated or not. For example, the lead body  30  may include one or more insulation layers, as appropriate or desired. The insulation may be of any suitable material, such as silicone rubber, polyurethane, silicone rubber-polyurethane-copolymer (“SPC”), etc. In general, the coil conductor  112  may be any known or hereafter developed conductor that is suitable for use in an implantable medical lead. 
     As shown in  FIG. 2 , the coil conductor  112  may be helically wound or coiled for a portion of its length. For example, the coil conductor  112  may be helically wound for one or more discrete portions of its length or over its entire length, as appropriate or desired. Only a portion of the coil conductor is shown in  FIG. 2  to illustrate particular inventive features. However, it should be understood that the coil conductor extends the length of the body  30 , for example, to provide electrical communication between one or more of the contacts  40 ,  45 ,  46  and one or more of the electrodes  65 ,  70 ,  80 . 
     The coil conductor  112  may include one or more transitions  114  at which the direction of the helical coiling changes direction. For example, a first portion  112   a  of the coil conductor  112  may be coiled counterclockwise as viewed axially from the right side of  FIG. 2 . A second portion  112   b  of the coil conductor  112  after the transition  114  may be coiled in an opposite direction, that is, clockwise as viewed axially from the right side of  FIG. 2 . It should be understood that the direction of coiling for the first and second portions  112   a,    112   b  may be reversed from that shown. In general, each transition  114  in the coil conductor  112  is a region in which the direction of the coiling changes. Thus, as illustrated for other embodiments discussed herein, the coil conductor  112  may include multiple transitions  114  in which the direction of coiling changes from clockwise to counter clockwise and vice versa. The coil conductor configurations with the transitions  114  disclosed herein may be applied to various lead configurations such as coaxial leads, co-radial leads, and etc. 
     Referring to  FIGS. 3A-C , the transition  114  may reduce, if not totally eliminate, current induction and heating caused in the coil conductor  112  by exposure to electrical and/or magnetic fields, such as with a MRI. For example, as indicated in  FIG. 3A , an electrical field E along the conductors or, more particularly, the component of the electrical field E extending along the axis of the lead may cause induced currents J 1  and J 2  to flow in the coil conductor in opposite directions on each side of the transition  114 , with the direction of the helical coiling determining the direction of the respective induced current. If the lengths of the first and second portions  112   a,    112   b  (shown in  FIG. 2 ) are equal or substantially equal, then the induced currents may be equal or substantially equal and may cancel each other entirely. 
     Similarly, a magnetic field B in a direction perpendicular to the axis of the lead, as shown in  FIG. 3B , or a magnetic field B in an axial direction, as shown in  FIG. 3C , may cause induced currents J 1  and J 2  to flow in the coil conductor in opposite directions on each side of the transition  114 , again with the direction of the helical coiling determining the direction of the respective induced current. Thus, the induced currents J 1  and J 2  may substantially, if not entirely, cancel each other. 
     Referring back to  FIG. 2 , a reinforcement ring  116  may be disposed over the transition  114 . The reinforcement ring  116  may be made of a metal material or a non-metal material such as PEEK, etc. The reinforcement ring  116  may be imbedded in or form a portion of the lead body jacket layers extending over the conductors and forming the outer surface of the lead body. The reinforcement ring  116  may be positioned to extend over the transition  114  and for a distance to either side of the transition to reinforce the lead body in the vicinity of the transition. This reinforcement may or may not be desirable depending on the extent to which the transition changes the mechanical properties of the lead body as compared to the regions of the lead body not having a transition. It should be understood that the transition  114  may be defined as a region in which the change from coiling in one direction to another occurs. As conductor may have a single transition  114  or multiple transitions  114  as it extends longitudinally along the lead. While a transition  114  may be substantially formed of two oppositely wound coils or loops intersecting each other to provide a reverse in the winding direction, some transitions  114  may further include a substantially axially extending portion that is not coiled in either direction, thereby forming the junction or intersection between the two oppositely wound coils. The use of a reinforcement ring  116  may provide a mechanical structure for the transition point protecting breakage of the lead or conductor of the coil during motion or deflection of the lead. The width of the ring  116  may be 1-5 mm and thickness may be in a range of 4-10 mils. 
     As can be understood from  FIG. 2 , in one embodiment, the first portion  112   a  of the coil conductor  112  is helically coiled at a first angle a relative to a normal projection from the Z axis. The second portion  112   b  of the coil conductor is helically coiled at a second angle β relative to a normal projection from the Z axis. In relation to the X axis projection in  FIG. 2 , the first angle α is a negative angle and the second angle β is a positive angle. Thus, as can be understood from  FIG. 2 , the helical angle of the coil conductor  112  changes in the transition  114  from negative to positive or vice versa. Depending on the embodiment, the magnitude of the helical angle may remain constant or may change moving across the transition  114 . 
     As discussed above, the coil conductor may include more than one transition  114 .  FIG. 4  illustrates an embodiment in which two transitions  114   a  and  114   b  are provided. In this example, the transitions  114  are spaced apart by less than five windings or complete turns. The spacing between adjacent transitions  114  may determine the length of the portions of the coil conductor before and after each transition  114  and thus the magnitude of the current induced in the portions of the conductor  112  before and after each transition. Conversely, the length of the coil conductor  112  (or the length of coiled portions of the coil conductor  112 ) may determine the suitable number of transitions  114  and/or spacing between adjacent transitions  114 . The spacing between adjacent transitions  114  may be wavelength dependent. The wavelength along the wire may depend on parameters such as, for example, coil pitch, coil diameter, wire insulation thickness, etc. 
     Depending on the embodiment, a coil may have one, two, three or more transitions  114  between the distal and proximal ends of the coil. The spacing of the transitions along the length of the coil may be generally uniform or equal, or the non-uniform or unequal. Depending on the embodiment, reinforcement rings  116   a,    116   b  may be provided for each respective transition  114   a,    114   b,  some of the transitions or none of the transitions. 
       FIG. 5  illustrates another embodiment in which the coil conductor  112  includes a plurality of transitions  114 . As shown, the transitions  114  may be periodic, for example, with the helical pitch length and/or length of portions of the coil conductor  112  before and after each transition  114  being equal. Also, the transitions  114  may be spaced apart by less than 1.5 windings. This may help to minimize the magnitude of the induced currents prior to canceling at the transitions  114 , thereby minimizing any resulting heating. While the embodiment depicted in  FIG. 5  is shown without reinforcement rings  116 , in other such embodiments, the reinforcement rings  116  may be provided for some or all transitions  114 . Where reinforcement rings  116  are not provided, the shrink tubing or other reinforcing layers may be extended over the regions including the transitions  114  to reinforce the lead body. 
     It should be understood that the transitions may be employed in a plurality of separate coil conductors and/or a plurality of filars. For example,  FIG. 6  illustrates an embodiment in which a first coil conductor  112   a  includes a plurality of transitions  114  and a second coil conductor  112   b  includes a plurality of transitions. The first and second coil conductors  112   a,    112   b  may be coaxial or co-radial, oppositely wound wires as shown, as appropriate or desired. Further, the first and second coil conductors  112   a,    112   b  may be connected to the same or different electrodes and/or contacts. 
     As shown in  FIG. 6 , the transitions  114  of the second coil conductor  112   b  may be axially aligned or substantially axially aligned with the transitions  114  of the first coil conductor  112   a.  This may allow a same reinforcement ring (not shown in  FIG. 6 ) to be used for a transition  114  of the first coil conductor  112   a  and a transition  114  of the second coil conductor  112   b.  Alternatively, the transitions  114  of the second coil conductor  112   b  may be axially offset with respect to the transitions  114  of the first coil conductor  112   a.  This may reduce the effect of induced currents in portions of the first and second coil conductors  112   a,    112   b  that occur before being cancelled at a respective transition  114 , for example, by reducing overlapping induced currents. As discussed herein, the approach of including one or more transitions in a coil conductor may be used to reduce or even eliminate the adverse effects of currents that would otherwise be induced in leads exposed to magnetic and/or electric fields. It should be understood that the approach described herein may be combined with any one or more of the approaches described in the incorporated copending applications referenced above in the “Cross-Reference to Related Application” section of this present patent application. For example, the helical coil pitch may vary along at least a portion of the coil conductor, increasing either proximally or distally. The lead may include one or more shield layers. Further, multiple coil conductors and/or filars may be made of different electrically conductive materials, may have opposing directions of increasing/decreasing helical coil pitch, and/or may have alternating relative radial positions. For any of the embodiments described above with respect to  FIGS. 2-6 , no coil loop may short circuit to an adjacent coil loop. Thus, the wires are insulated via their own insulation jackets or via the lead body material in which they are imbedded such that no coil loop is capable of electrically shorting to an adjacent coil loop. Thus, the electricity is forced to go through the entire length of the wire forming a coiled conductor, as opposed to shorting from coil loop to coil loop of the coiled conductor. Insulation for the wires may be in the form of electrical insulation coatings or jackets, which may be of materials such as, for example, ETFE, PTFE, etc. 
     It should be understood that the different embodiments described herein are not mutually exclusive or exhaustive of the embodiments contemplated. In particular, any of the features discussed with respect to one of the embodiments may be employed in combination with any of the features discussed with respect to other embodiments. Thus, although the present invention has been described with reference to preferred embodiments, persons 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.