Abstract:
The present invention provides, among other things, means to suppress AC current propagation along elongated medical devices incorporating long conductive structures. AC currents in the frequency range from approximately 10 MHz to 3 GHz can be substantially suppressed without altering the low and DC frequency response of the medical device.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of U.S. application Ser. No. 12/117,342, filed May 8, 2008, the contents of which are incorporated herein by reference. This application also claims the benefit of U.S. Provisional Application No. 60/998,478, filed Oct. 11, 2007, and 60/998,477, filed Oct. 11, 2007, the contents of both of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to methods and devices for reducing or eliminating the effects of electromagnetic fields on long metallic structures as are typically found in medical devices such as leads, catheters, guide wires, needles and cannulars. 
       BACKGROUND 
       [0003]    Medical devices, including but not limited to guide wires, transseptal needles, cannulars, and the like, often employ conductive metals or alloys such as stainless steel, Nitinol, brass, carbon nanotubes and others in the form of solid rods or tubes because these materials have superior mechanical characteristics, including torque transfer and tensile strength. These rods and tubes present conductive surfaces that when exposed to electromagnetic fields, such as for example those present in magnetic resonance imaging (“MRI”) systems, may sustain undesired currents or voltages that interact with the surrounding blood and tissue, potentially resulting in unwanted tissue heating, nerve stimulation or other negative effects resulting in erroneous diagnosis or therapy delivery. 
         [0004]    Other medical devices, including but not limited to electrocardiographs (“ECGs”), electroencephalographs (“EEGs”), squid magnetometers, implantable pacemakers, implantable cardioverter-defibrillators (“ICDs”), neurostimulators, electrophysiology (“EP”) mapping and radio frequency (“RF”) ablation systems, and the like, consist of or commonly employ one or more conductive surfaces, often in the form of leads and catheters that either receive or deliver voltage, current or other electromagnetic pulses from or to an organ or its surrounding tissue for diagnostic or therapeutic purposes. 
         [0005]    Further, such structures commonly include bare or insulated, single or multi strand cables, rods and tubes or may include single or multi filar coils of bare or insulated wire or a combination of some or all of the above to facilitate the transfer of mechanical forces and/or the conduction of electrical signals to and from the proximal (system) end to the distal (patient) end of the device. When exposed to electromagnetic fields, such as for example those present in magnetic resonance imaging (“MRI”) systems, these conductive surfaces may sustain undesired currents and or voltages that interact with the surrounding blood and tissue, potentially resulting in unwanted tissue heating, nerve stimulation or other negative effects resulting in erroneous diagnosis or therapy delivery. 
         [0006]    An example of a typical medical device incorporating conductive surfaces in the form of rods and tubes is shown in FIG. I. The guide wire A typically consists of an atraumatic tip C and a shaft D. The tip is typically made by coiling a thin flexible wire around a tapered shaft, resulting in a very bendable tip designed to prevent injury to the vascular structure through which the guide wire is advanced towards its intended target. The shaft is typically made from stainless steel or Nitinol since both materials have superior characteristics, including tensile strength and torque transfer capability, for a given diameter, ranging from 0.006″ to 0.039″. Once in place, the actual diagnostic or therapeutic device, such as a balloon catheter or a stent delivery tool, slides over the guide wire and is advanced “over the rail,” reducing the risk of vascular puncture by the diagnostic/therapeutic tool. 
         [0007]    The atraumatic tip is connected to the guide wire shaft, a continuous tube or rod (D), through a transition region (B), predominately designed to facilitate a strong, smooth transition between the two pieces of the guide wire. The transition region may also be used to establish an electric connection between the typically conductive tip and the guide wire shaft, allowing the guide wire itself to be used as a diagnostic or therapeutic tool. The guide wire or sections thereof are sometimes covered with a thin insulating film (not shown in FIG. I) to isolate the shaft from its surroundings to, for example, maintain biocompatibility or provide electrical insulation for low frequency AC signals. The atraumatic tip and guide wire shaft can sometimes sustain currents when exposed to an electromagnetic field, such as for example, that encountered in an MRI system. These currents can, for example, induce heating or cause nerve stimulation in the tissue surrounding the device, either directly or by creating current pathways through direct contact points between the tissue and the atraumatic tip or the shaft. 
         [0008]    Illustrations of multi stranded cables such as for example used for the transfer of diagnostic and therapeutic electromagnetic signals in ICD leads and RF ablation catheters are shown in FIGS. IIa and IIb. The cables E and K each consist of three (3) layers F, G, H and L, M, N of insulated and bare wires, respectively, twisted about the longitudinal axis. The cross-section of cable E is shown in FIG. IIc. In both cable examples, E and K, additional layers I and J are utilized to provide mechanical integrity, electrical layer-to-layer isolation or shielding, depending on the conductivity of the layer, or all of the above. The conductive paths provided by single or multi stranded wires can sustain unwanted currents when exposed to an electromagnetic field, such as for example encountered in an MRI system. These currents can induce heating in the tissue surrounding the device either directly or by creating current pathways through the tissue involving electrodes attached to cables. 
         [0009]    FIG. IIIa shows a combination of multi stranded cables and a multi filar coil to transfer diagnostic and therapeutic electromagnetic signals to different electrodes of an ICD lead. The lead body consists of an insulating extrusion Q surrounded by an insulating jacketing material P. The extrusion Q contains various lumens to allow the cables R and coil S to be run through the extrusion. Coil S electrically connects a distal corkscrew shaped active fixation tip (helix) of the lead to the proximal pulse generator while at the same time allowing the transfer of torque from the proximal to the distal end during the implant procedure to facilitate the extension of the helix. In FIG. IIIb, the coil consists of four tightly bundled bare filars that are coiled at a certain, essentially constant pitch, resulting in a gap U between filar bundles. In FIG. IIIc, a smaller number of insulated filars is used, again coiled at a specific, essentially constant pitch, this time resulting in a gap Y between filar bundles. The coil consists of a set of filars. Even though the cables R are shown to be identical in FIG. IIIa, they may actually differ in diameter and construction since the electrical requirements for the cables connecting the shock electrodes to the pulse generator are more demanding than for the cable connecting the ring electrode to the pulse generator. The conductive paths provided by the cables and coil can sustain unwanted currents when exposed to an electromagnetic field, such as for example encountered in an MRI system. These currents can induce heating in the tissue surrounding the device either directly or by creating current pathways through the tissue involving electrodes attached to the cables and coil. 
         [0010]    A typical approach to reduce the current and voltage induced in the catheter and lead-like structures is the use of discrete components, often self-resonating RF chokes or LC (“tank”) circuits to block RF currents on the wires or conductors. These components “break” or interrupt the original conductor, which may affect the mechanical characteristics of the device and increase the potential for mechanical failure, clearly making this approach impractical for devices, such as guide wires, that use tubes and rods for their tensile strength and torque transfer characteristics. In addition, discrete components such as capacitors and inductors cannot be obtained in small enough sizes to allow the manufacture of small diameter multi-stranded cables, in particular if multiple blocking circuits are required. Furthermore, a large current pulse is delivered through some of the cables in an ICD lead, placing an extra burden on the discrete component specifications, typically resulting in larger components not compatible with the lead space requirements. 
       SUMMARY 
       [0011]    In some embodiments, the present invention provides a medical device having one or more elongated bodies in the form of a rod or tube comprised of a base material such as stainless steel, Nitinol, brass, carbon nanotubes, etc., and having an electrical conductivity consistent with these materials. One or multiple coaxial layers of alternating conductivity materials, that is, resistive/dielectric layers followed by highly conductive layers, are formed on top of each other. One or more of these layers, in contrast to the base material, are not continuous, but rather consist of patterns that either by themselves, through interaction with other layers, the base material and/or the surrounding environment, either directly or through a dielectric/resistive layer, form electrical structures and barriers that are substantially different in their response to AC signals at one or multiple frequencies or frequency bands than that of a medical device formed from the base material alone. In some embodiments, the electrical structures are created to present high impedances or section of high impedances at a specific frequency or frequencies or frequency bands for AC signals propagating along the rod/tube. In other embodiments, the electrical structures are created to match the AC signal propagation properties of the rod/tube to its immediate environment, such as blood or tissue, at a specific frequency or frequencies or frequency bands. 
         [0012]    In some embodiments, the electrical structures formed between one or more coaxial layers forms a string of inductors. In other embodiments, the structures form a string of low pass filters including shunt capacitances between one or more layers and series inductors formed on one or more layers. In some embodiments, the structures form parallel resonant circuits, formed by the shunt capacitance between various layers and series inductors on other layers. In some embodiments, a string of resonant circuits is created, either operating at the same frequency band or multiple frequency bands. In other embodiments, the electrical structures formed between one or more coaxial layers form a string of self-resonating inductors or a string of self-resonating inductors, either operating at the same frequency band or multiple frequency bands. 
         [0013]    In some embodiments, two coaxial layers cover at least a lengthwise portion of at least one conductive shaft, the two coaxial layers including a first, inner layer and a second, outer layer, the first layer comprising a dielectric/resistive material (e.g., PEEK or PTFE) and the second layer comprising a highly conductive material (e.g., gold, silver, copper, or other metals). Furthermore, the second layer incorporates at least one or multiple patterns partially or fully exposing the first layer to form resistive, capacitive or inductive sections or combinations thereof. 
         [0014]    Various embodiments herein suppress the propagation of alternating currents in the frequency range from approximately 10 MHz to 3 GHz. 
         [0015]    In some embodiments, the present invention provides a medical device having one or more elongated bodies in the form of a multi stranded cable and in which at least one or more layers of the cable contain a set of wires of varying conductivities. The set of wires may include bare wires, insulated wires, non-conducting wires, wires of low conductivity, and wires of high conductivity. The set of wires is twisted along the longitudinal axis to form a part of the cable. The pitch of each layer may be adjusted as needed. The cable may also incorporate coaxially wrapped thin layers of foil or tubes of varying conductivity, providing a radial separation of the wire sets and the ability to control the electrical interaction between the wire sets. The cable may also include an insulating or conducting layer to provide mechanical stability and/or to control electrical interaction with the environment exterior to the cable. 
         [0016]    In some embodiments, the present invention provides a medical device having one or more elongated bodies in the form of a multi stranded cable and in which at least one or more layers contain a set of wires of which at least one is a mechanically continuous wire including at least one or more insulated section and one or more non-insulated section. In addition, the set of wires may include bare wires, insulated wires, non-conducting wires, wires of low conductivity, and wires of high conductivity. The set of wires is twisted along the longitudinal axis to form a part of the cable. The pitch of each layer may be adjusted as needed. The cable may also incorporate coaxially wrapped thin layers of foil or tubes of varying conductivity, providing a radial separation of the wire sets and the ability to control the electrical interaction between the wire sets. The cable may also include an insulating or conducting layer to provide mechanical stability and/or control electrical interaction with the environment exterior to the cable. 
         [0017]    In addition, the present invention provides a method of controlling the current induced by an electromagnetic field on a medical device including elongated conductive structures such as single or multi stranded cables. The method includes the act of forming a single inductor of desired inductance, a string of inductors with equal or different inductance, a single self resonant circuit between the layers of the cable, and a string of self-resonant circuits at a single or multiple frequencies between the layers of the cable, wherein the cable remains mechanically continuous. The method also includes the act of using the interaction between single or multi stranded cables in the elongated conductive structure of the medical device to suppress AC propagation at a specific frequency or frequencies or over a single or multiple frequency bands. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    FIG. I is a perspective view of a typical medical device having elongated conductive pathways in the form of a tube or rod as typically found in guide wires. 
           [0019]    FIG. IIa is a perspective view of a multi stranded cable utilizing insulated wires to form the layers of the cable. Such a cable can be found in RF ablation catheters. 
           [0020]    FIG. IIb is a perspective view of a multi stranded cable utilizing bare wires to form the layers of the cable. Such a cable can be found in ICD leads. 
           [0021]    FIG. IIc is a cross section of the cable shown in FIG. IIa. 
           [0022]    FIG. IId is a cross section of the cable shown in FIG. IIa without the additional layer I. 
           [0023]    FIG. IIe is a cross section of yet another multi stranded cable configuration. 
           [0024]    FIG. IIIa shows a combination of multi stranded cables and a multi filar coil to transfer diagnostic and therapeutic electromagnetic signals to different electrodes of an ICD lead. 
           [0025]    FIG. IIIb shows a coil consisting of tightly bundled bare filars. 
           [0026]    FIG. IIIc shows a coil consisting of insulated filars. 
           [0027]      FIG. 1  is a perspective view of a medical device incorporating a tube or rod with conductive surface pattern according to some embodiments of the present invention. 
           [0028]      FIG. 1   a  is a longitudinal cross-sectional view of the area  10  marked in  FIG. 1 . 
           [0029]      FIG. 1   b  is an equivalent electrical circuit diagram of the basic, core shaft D of FIG. I. 
           [0030]      FIG. 1   c  is an equivalent electrical circuit diagram of the shaft modified according to some embodiments of the present invention. 
           [0031]      FIG. 2  is a perspective view of a medical device incorporating a tube or rod with conductive surface pattern according to yet another embodiment of the present invention. 
           [0032]      FIG. 2   a  is an equivalent electrical circuit diagram of the shaft modified according to yet another embodiment of the present invention. 
           [0033]      FIG. 3  is a perspective view of a medical device incorporating a tube or rod with conductive surface pattern according to yet another embodiment of the present invention. 
           [0034]      FIG. 4  is a perspective view of a multi stranded cable as used in medical devices in which one layer is formed according to some embodiments of the present invention. 
           [0035]      FIG. 4   a  is an equivalent electrical circuit diagram of a cable formed according to some embodiments of the present invention. 
           [0036]      FIG. 4   b  is a perspective view of the cable layer of  FIG. 4  formed according to some embodiments of the present invention. 
           [0037]      FIG. 4   c  is a perspective view of the wire set used to form the cable layer of  FIG. 4   b.    
           [0038]      FIG. 4   d  is a wire used in the wire set of  FIG. 4   c . The wire is formed according to some embodiments of the present invention. 
           [0039]      FIG. 5  is a perspective view of a multi stranded cable as used in medical devices in which one layer is formed according to yet another embodiment of the present invention. 
           [0040]      FIG. 5   a  is a perspective view of the wire layer used in  FIG. 5  according to some embodiments of the present invention. 
           [0041]      FIG. 5   b  is a perspective view of the set of wires used to form the layer  27  of the multi stranded cable in  FIG. 5  according to an embodiment of the present invention. 
           [0042]      FIG. 6   a  is a perspective view of a coil formed by a multi filar wire set utilizing at least two different winding pitches over one or more sections of the coil according to some embodiments of the present invention. 
           [0043]      FIG. 6   b  is a perspective view of a coil formed by a multi filar wire set with the insulation removed over one or more sections of the coil according to some embodiments of the present invention. 
           [0044]      FIG. 6   c  is a perspective view of a coil formed with the multi filar wire set shown in  FIG. 6   d  according to some embodiments of the present invention. 
           [0045]      FIG. 6   d  is a perspective view of the multi filar wire set used to form the coil in  FIG. 6   c . The wire set utilizes wires including alternating insulated and bare wire sections created on a mechanically continuous wire according to some embodiments. 
           [0046]      FIG. 6   e  is a perspective view of a coil formed with the multi filar wire set shown in  FIG. 6   f  according to some embodiments of the present invention. 
           [0047]      FIG. 6   f  is a perspective view of the multi filar wire set used to form the coil in  FIG. 6   e . The wire set utilizes wires including alternating insulated and conductive coating sections. 
       
    
    
     DETAILED DESCRIPTION 
       [0048]    Before any embodiments of the invention are explained in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of components set forth in the following description or illustrated in the following drawings. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. Also, it is to be understood that the phraseology and terminology used herein is for the purpose of description and should not be regarded as limiting. The use of “including,” “comprising,” or “having” and variations thereof herein is meant to encompass the items listed thereafter and equivalents thereof as well as additional items. 
         [0049]    Unless specified or limited otherwise, the terms “mounted,” “connected,” “supported,” and “coupled” and variations thereof are used broadly and encompass both direct and indirect mountings, connections, supports, and couplings. Further, “connected” and “coupled” are not restricted to physical or mechanical connections or couplings. 
         [0050]    Also, it is to be understood that phraseology and terminology used herein with reference to device or element orientation (such as, for example, terms like “central,” “upper,” “lower,” “front,” “rear,” “distal,” “proximal,” and the like) are only used to simplify description of the present invention, and do not alone indicate or imply that the device or element referred to must have a particular orientation. In addition, terms such as “first” and “second” are used herein for purposes of description and are not intended to indicate or imply relative importance or significance. 
         [0051]    With reference to the Figures, a thin tube or rod, from hereon referred to as a shaft, according to the present invention is shown in  FIG. 1 . It will be understood by those of skill in the art that the shaft  1  could be part of any of a number of medical devices, including but not limited to guide wires, guide cannulars, EP mapping and ablation catheters, transseptal needles, etc. 
         [0052]    In the embodiment of  FIG. 1 , a thin continuous dielectric layer is plated, extruded, “heat shrunk”, glued or in some other form deposited on a mechanically continuous shaft that could be made, for example, from Nitinol, stainless steel, brass or a carbon nano tube. The dielectric layer completely covers the shaft with the exception of a small area at the tip  2 , allowing the transfer of low frequency signals through this area. A second, electrically conductive layer is plated or in some form deposited on the dielectric layer, for example as secondary tubing or tubing sections slipped and glued over the core/dielectric layer assembly; or for example as part of a dielectric/polymer material that has been “doped” in sections to be conductive. The purpose of the conductive layer is to force AC signal propagation on the top conductive surface and to fully or partially shield the core material. In contrast to the dielectric layer, the conductive layer contains patterns  7 ,  8  and  9 , of varying length, thickness and/or conductivities, leaving sections  3 ,  4 ,  5  and  6  of the dielectric layer exposed. 
         [0053]    The ability to form standing waves and the propagation efficiency of AC signals along the shaft surface at specific frequencies or frequency bands is significantly affected by the alternating conductive—dielectric material pattern. In some embodiments, a third, thin dielectric layer covers the outer surface to reduce the interaction with the surrounding material, either for electromagnetic reasons and/or to maintain biocompatibility. Optimized patterns for specific frequencies or frequency bands are determined by equivalent circuit analysis combined with computer simulations to determine circuit parameters such as capacitive coupling to the core material and if a third layer is present, capacitive coupling to the surrounding material. In some embodiments, the highest possible shaft impedance for AC signals is desired at specific frequencies or frequency bands, whereas in other embodiments a shaft impedance matching its surrounding material is more preferred. 
         [0054]    As an example, the equivalent circuit for a standard shaft consisting only of the core material (“the core shaft”) is compared to a shaft constructed according to the embodiment shown in  FIG. 1  with sections  7 ,  8  and  9  of equal length, thickness and conductivity (“the modified shaft”). Furthermore, for this example individual top layer conductive sections of the modified shaft are assumed to be short compared to the wavelength of interest. The voltage across each section can then be assumed to have a constant amplitude and phase, even though both amplitude and phase may vary between sections. As can be seen from  FIG. 1   a , capacitive coupling from the top conductive layers, here sections  8  and  9 , to the core layer  2  is controlled by the thickness of the dielectric layer  5  and the length of the sections  8  and  9 . Similarly, capacitive coupling to the external layer is controlled by the thickness of the third, top dielectric layer (not shown in the Figures). 
         [0055]    The resulting equivalent circuit for the modified shaft representing area  10  of  FIG. 1 , including a third dielectric layer, is shown in  FIG. 1   c . Each top conductive section is represented by a series of inductors, resistors and voltage sources L G , R G , V G1  and V G3 . The gap  5  is represented by the resistance R D ; however, capacitive coupling to the core layer via the capacitors C B  and to the external layer via C X  create parallel conduction pathways, reducing the maximum achievable impedance. Taking into account phase shifts between the different conductive sections and the voltages induced in the external as well as the core layer, the AC propagation characteristic may be optimized further. The impedance of the core shaft over a section equivalent to the gap  5  is very small and can be approximated by the resistance R B1 . The modified shaft has substantially different AC propagation characteristics compared to the core shaft without degrading the mechanical characteristics of the continuous core material. 
         [0056]    In a second embodiment, shown in  FIG. 2 , the top conductive layer has sections  12 ,  13 ,  14  and  15 . In contrast to the embodiment shown in  FIG. 1 , the conductive sections are now connected via conductive patterns  16 ,  17  and  18 , resembling solenoid inductors. An equivalent circuit for the shaft modified according to this embodiment, with sections  12 ,  13 ,  14  and  15  of equal length, thickness and conductivity, as well as sections  16 ,  17  and  18  of equal length, thickness, conductivity and turn density for the solenoids, is shown in  FIG. 2   a . The resistance R D  of  FIG. 1   c  is now replaced by the inductor L G2 . This inductor and the capacitors C X  and C B  can be made to form a parallel resonant circuit, effectively suppressing AC current propagation along the shaft; or alternatively the AC propagation characteristics can be matched to the external material by appropriately selecting the capacitor ratios. 
         [0057]    In some embodiments according to  FIG. 2 , the tip section of the shaft remains partially exposed, allowing the conduction of low frequency AC signals through the core as well as the top conductive layer. In other embodiments, the tip section can be covered by the dielectric material  5 , preventing any low frequency propagation through the core of the shaft. 
         [0058]    It will be apparent to those of skill in the art that other surface patterns can be created, such as for example the one shown  FIG. 3 , that allow the inductive, resistive and capacitive characteristics of the shaft to be manipulated to result in the desired AC impedance and propagation performance. 
         [0059]    It will be apparent to those of skill in the art that fewer or additional layers can be used in the creation of the shaft  1 . It will also be apparent to those of skill in the art that patterns can be created on more than one layer and that these overlaying patterns result in additional degrees of freedom to adjust the AC response of the resulting shaft. 
         [0060]    Furthermore, it will be apparent to those skilled in the art that the order of material properties, such as conductivity and dielectric constant, can be reversed or arranged to result in more beneficial AC responses in different frequency bands. 
         [0061]    With reference to the Figures, a multi stranded cable, modified according to the present invention is shown in  FIG. 4 . It will be understood by those of skill in the art that the cable  33  could be incorporated in any of a number of medical devices, including EP mapping catheters, imaging catheters, RF ablation catheters, neurostimulator leads, ICD and pacemaker leads. The cable  33  consists of three conductor layers  25 ,  26  and  34  separated by insulating layers  28  and  29 . The cable layer  34  in  FIGS. 4 and 4   b  electrically presents a string of one or more inductors  35  connected via electrical short or low resistance section  36 . The mechanically continuous cable layer  34  is formed by braiding (twisting) the wire set  37  of  FIG. 4   c  around the longitudinal axis of the cable. The wire set  37  consists of single continuous wires  40  that, as shown in  FIG. 4   d , include insulated sections  38  and conductive sections  39 . The conductive sections  39  either represent sections of bare wire and/or sections in which a conductive coating has been applied in some form over the sections of the wire. The latter approach allows the diameter of the conductive section to be manipulated to either be less than, equal to, or greater than the diameter of the insulated section. Because the wire  40  is mechanically continuous, the transition points between the insulated and non-insulated sections  35  and  36  of the cable layer  34  are mechanically continuous and do not require any means of joining such as soldering, welding, etc. It will be understood by those of skill in the art that the cable layer  34  of  FIG. 4   b  could be comprised of more sections  35  and  36  or that the wire set  37  of  FIG. 4   c  could include more or fewer wires  40 , or that the wire set could include bare wires, or insulated wires or non-conductive wires or any combination thereof. It will also be understood by those of skill in the art that the cable  33  of  FIG. 4  could have more or fewer layers and that one or more cable layers  34  could be used in the cable structure. 
         [0062]    It will also be understood by those of skill in the art that the insulating layer  28  and  29  could be single insulating structures or could be double sided such that one side is conductive and the other is non-conductive or that one side contains patterns, such as for example described in the embodiments shown of  FIGS. 1 ,  2  and  3 . 
         [0063]    In the embodiment shown in  FIG. 4 , the layer  26  consists of bare wire and is separated from the layer  34  via an insulating layer  28 . The resulting equivalent circuit for this configuration is shown in  FIG. 4   a . The third layer is represented by a string of inductors, resistors and voltage source L T , R T  and V T1  and V T3 , respectively, separated by a resistive section containing the voltage source V T2 . The sections are considered short such that the voltage source has constant amplitude and phase over the section at the wavelength of interests; however, amplitude and phase may vary from section to section. The bare wire section will primarily be responsible for the capacitive coupling C T  to the second layer. The second layer is to first order approximated by a string of resistive elements because the outer/third layer acts as a shield. If the shielding is insufficient, the insulating layer  28  can be modified to contain one conductive surface, in contact with layer  26 , and one non-conductive surface, in contact with layer  34 . The resulting equivalent circuit is shown in  FIG. 4   a  and consists of series inductors joined across shunt capacitors; a typical low pass filter. The circuit can be transformed into a series of resonant LC circuits at specific frequencies or frequency band by appropriate choice of inductor and capacitor values, i.e., section length, dielectric constant and thickness of layer  28 . 
         [0064]    In some embodiments similar to that shown in  FIG. 4 , the insulation layer of the wire can be made very thin, for example, between 0.1 and 0.25 mil. This increases the turn-to-turn parasitic capacitance and effectively replaces the inductor L T  in  FIG. 4   a  with a parallel LC circuit where the capacitance is distributed over the “inductor windings”. Choosing an appropriate pitch and section length, a resonant “tank” circuit is created, suppressing AC currents of the layer. Varying the pitch and length along the cable results in an AC current suppression at multiple frequencies or frequency bands. 
         [0065]    In some embodiments, the alternating insulated and non-insulated sections  38  and  39  of the wire structure  40  are created by a removal process that removes partial sections from a fully insulated wire by chemical, mechanical, optical, or thermal means (e.g., chemical etching, mechanical grinding, laser burning, etc.). In other embodiments, the alternating insulated and non-insulated sections  38  and  39  of the wire structure  34  are created by a covering process that covers sections of a bare (non-insulated) wire with insulation material my means of partial extrusions, chemical deposition, etc. In yet other embodiments, the alternating insulated and non-insulated sections  38  and  39  of the wire structure  34  are created by a coating or extrusion process utilizing alternating or multiple types of coating/extrusion materials. These materials may include PTFE, PEEK, conductive polymers, etc. 
         [0066]    In some embodiments, alternating insulated and non-insulated sections  35  and  36  of the structure  34  are formed by initially creating the structure using fully insulated wire and subsequently removing partial sections from the fully insulated section by chemical, mechanical, optical or thermal means. In other embodiments, the alternating insulated and non-insulated sections  35  and  36  of the structure  34  are formed initially from bare wire and sections are subsequently covered with insulation material by means of “dipping” or chemical deposition. 
         [0067]    In yet another embodiment of the invention, shown in  FIG. 5 , the multi stranded cable  24  utilizes a third layer  27 . The cable layer  27  in  FIGS. 5 and 5   a  electrically presents a string of one or more inductors  30  connected via electrical short or low resistance section  31 . The mechanically continuous cable layer  27  is formed by braiding (twisting) the wire set  32  of  FIG. 5   b  around the longitudinal axis of the cable. The wire set  32  consists of mechanically continuous bare and insulated wires  41  and  42 , respectively. Because the wires  41  and  42  are mechanically continuous, the transition points between the insulated and non-insulated sections  30  and  31  of the cable layer  27  are mechanically continuous and do not require any means of joining such as soldering, welding, etc. It will be understood by those of skill in the art that the cable layer  27  of  FIG. 5   a  could be comprised of more sections  30  and  31  or that the wire set  32  of  FIG. 5   b  could include more or fewer wires  41  or  42 , or that the wire set could include non-conductive wires or wires of differing conductivities or any combination thereof. It will also be understood by those of skill in the art that the cable  24  of  FIG. 5  could have more or fewer layers and that one or more cable layers  27  could be used or that other cable layers, such as  34  could be used in combination with layer  27  in the cable structure. At the lower end of the frequency spectrum (10 MHz to 3 GHz), it is advantageous to utilize thin wire insulation to increase the parasitic capacitance between the insulated windings and thereby increase the impedance of the insulated sections. 
         [0068]    In yet another embodiment of the invention, shown in  FIG. 6   a , the coil(s) of pacemaker or ICD leads or other medical devices incorporating coiled wire to transfer diagnostic and therapeutic energy from the system end to the patient end are modified to form high impedance sections  46  by closely winding the insulated wire  45  coaxially along the lead body. The sections will behave as lumped elements as long as the coiled length is small compared to the wavelength at the frequency of interest. This is achieved by introducing a variable pitch, resulting in a gap  47 . The impedance of section  46  can be increased compared to the impedance of an ideal inductor by adjusting the parasitic turn-to-turn capacitance by appropriate choice of the insulation thickness. Since the inductor section  46  forms a parallel LC circuit with the parasitic capacitance, it is possible to significantly increase the impedance; however, when the section becomes too long, the impedance will start to decrease and become capacitive. The precise behavior is controlled by varying the pitch over small sections. This approach essentially results in a string of high impedances joined by small inductive impedances. 
         [0069]    In the embodiment shown in  FIG. 6   b , a constant pitch is maintained and the high impedance sections  46  are now joined by bare wire sections of the same pitch. The bare sections can be created, for example, by sand blasting a wire section and thereby removing the insulation locally. 
         [0070]    In the embodiment of  FIG. 6   c , wire(s) including alternating insulated and bare wire sections (e.g., see  FIG. 6   d ) are coiled along the lead body. The pitch is adjusted to result in a tightly wound coil consisting of insulated (inductor) and bare (short circuit) sections. The high impedance sections are now joined by node like sections. For large insulation thickness, a noticeable step down in diameter is observed as well as a change in pitch. 
         [0071]    In the embodiment of  FIG. 6   e , wire(s) including alternating insulated and conductive sections (e.g., see  FIG. 6   f ) are coiled along the lead body. The alternating sections are, for example, created via a coating or extrusion process in which the material is switched during the process. The resulting structure can be a string of high impedances joined by short circuit sections. In contrast to  FIG. 6   c , there now is full control over the coil diameter. The conductive sections now can be made to have a smaller, equal or larger diameter than the insulated sections. In some cases, it is useful to use hydrophilic material for the conductive sections since this can result in a swelling of these sections, forcing electrical turn-to-turn contact. 
         [0072]    The embodiments described above and illustrated in the figures are presented by way of example only and are not intended as a limitation upon the concepts and principles of the present invention. As such, it will be appreciated by one having ordinary skill in the art that various changes in the elements and their configuration and arrangement are possible without departing from the spirit and scope of the present invention as set forth in the appended claims.