Abstract:
Conductors within an implantable medical lead that carry stimulation signal signals are at least partially embedded within a lead body of the medical lead over at least a portion of the length of the conductors while being surrounded by a radio frequency (RF) shield. A space between the shield and the conductors is filled by the presence of the lead body material such that body fluids that infiltrate the lead over time cannot pool in the space between the shield and the conductors. The dielectric properties of the lead body are retained and the capacitive coupling between the shield and the conductors continues to be inhibited such that current induced on the shield is inhibited from being channeled onto the conductors. Heating at the electrodes of the medical lead is prevented from becoming excessive.

Description:
TECHNICAL FIELD 
       [0001]    Embodiments are related to implantable medical leads having shields for blocking electromagnetic energy from coupling onto conductors. More specifically, embodiments are related to reducing the coupling of the shield to the conductor(s) within the implantable medical device. 
       BACKGROUND 
       [0002]    Implantable medical leads are used to provide electrical stimulation from a pulse generator to a target site within a body of a patient. The lead includes electrical conductors that extend from a proximal end that is coupled to the pulse generator to a distal end. The conductors carry stimulation signals to electrodes on the distal end that are positioned at the target site and deliver the stimulation signals to the tissue. 
         [0003]    The presence of the lead presents a risk if the patient undergoes a magnetic resonance imaging (MRI) scan. Radio frequency (RF) energy that is present during the MRI scan may couple to the conductor(s) within the lead which results in electrical current on the conductor that can cause potentially dangerous heating of tissue nearby the electrode. This is especially problematic for neurostimulation leads where the electrode is placed in very sensitive neurological tissue such as within the brain or spine. 
         [0004]    Various techniques have been devised to try to lessen the current being induced onto the conductor by the RF energy to thereby lessen the amount of heating at the electrode. One technique is to include a conductive RF shield that surrounds the conductor. The RF energy is largely blocked from reaching the conductor and the induced current and tissue heating are reduced. 
         [0005]    The conductor is typically located in a lumen of the lead while the shield may be present outside of the lumen, typically in a polymer jacket. Over time, body fluids infiltrate the polymer jacket of the lead and reach the lumen which fills with the fluid. Thus, a significant amount of body fluid could be present between the shield and the conductor being shielded. Because the body fluid presents a high dielectric constant, capacitive coupling may occur to some degree between the shield and the conductor which could result in some of the RF energy being transferred to the conductor. 
       SUMMARY 
       [0006]    Embodiments address issues such as these and others by providing a lead where at least a portion of the diameter of the conductor is embedded within a lead body that contains the shield such that a space between a shield and the conductor is entirely filled with the lead body material. This eliminates body fluid from being pooled between the shield and the conductor and thereby lessens the capacitive coupling that occurs to thereby limit increases in heating over time. 
         [0007]    Embodiments provide a method of providing a medical lead that includes providing a conductor having a diameter and providing a radio frequency (RF) shield that surrounds the conductor such that a space exists between the shield and the conductor. The method further involves providing a lead body with a lumen where the lead body encapsulates the shield and surrounds the conductor with a portion of the conductor diameter being embedded within the lead body and the lead body filling the space. The method further involves providing an electrode attached to the lead body and electrically coupled to the conductor. 
         [0008]    Embodiments provide a method of providing a medical lead. The method involves forming an inner lead body layer of a lead body about a conductor to embed a portion of a diameter of the conductor within the inner lead body layer and positioning a radio frequency (RF) shield about the lead body inner layer. The method further involves forming an outer lead body layer of the lead body about the shield and the inner lead body layer to encapsulate the shield and to bond with the inner lead body layer and providing an electrode attached to the lead body and electrically coupled to the conductor. 
         [0009]    Embodiments provide an implantable medical lead that includes a conductor having a diameter and a radio frequency (RF) shield that surrounds the conductor such that a space exists between the shield and the conductor. The lead includes a lead body with a lumen, the lead body encapsulating the shield and surrounding the conductor with a portion of the conductor diameter being embedded within the lead body and the lead body filling the space. The lead further includes an electrode attached to the lead body and electrically coupled to the conductor. 
         [0010]    Embodiments provide an implantable medical system that includes a pulse generator and a medical lead. The medical lead includes a conductor having a diameter, the conductor being electrically coupled to the pulse generator. The medical lead includes a radio frequency (RF) shield that surrounds the conductor such that a space exists between the shield and the conductor. The lead includes a lead body with a lumen and the lead body encapsulates the shield and surrounds the conductor with a portion of the conductor diameter being embedded within the lead body and with the lead body filling the space. The lead further includes an electrode attached to the lead body and electrically coupled to the conductor. 
         [0011]    Embodiments provide an implantable medical lead that includes a conductor having a diameter and a radio frequency (RF) shield that surrounds the conductor such that a space exists between the shield and the conductor. The lead includes a lead body with a lumen, the lead body encapsulating the shield and surrounding the conductor with a first longitudinal section of the conductor diameter being at least partially embedded within the lead body and with a second longitudinal section of the conductor diameter that is distal of the first section and that is less embedded by the lead body than the first section, and the lead body filling the space between the first longitudinal section of the conductor and the shield. The lead also includes an electrode attached to the lead body and electrically coupled to the conductor. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  shows an example of an implantable medical system that may include embodiments of the lead to reduce the coupling of the shield to the lead conductor(s). 
           [0013]      FIG. 2A  shows a first example of a longitudinal cross-section of an implantable medical lead that includes a full-length partially embedded conductor to reduce the coupling of the shield to the lead conductor(s). 
           [0014]      FIG. 2B  shows a second example of a longitudinal cross-section of an implantable medical lead that includes a partial-length partially embedded conductor to reduce the coupling of the shield to the lead conductor(s). 
           [0015]      FIG. 2C  shows a third example of a longitudinal cross-section of an implantable medical lead that includes a partial-length fully embedded conductor to reduce the coupling of the shield to the lead conductor(s). 
           [0016]      FIG. 2D  shows a fourth example of a longitudinal cross-section of an implantable medical lead that includes a full-length fully embedded conductor to reduce the coupling of the shield to the lead conductor(s). 
           [0017]      FIG. 2E  shows a fifth example of a longitudinal cross-section of an implantable medical lead that includes a partial-length embedded conductor to reduce the coupling of the shield to the lead conductor(s) while providing increased conductor compliance near the electrode. 
           [0018]      FIG. 3  shows an example of a set of operations to construct an implantable medical lead with a conductor that is at least partially embedded according to the embodiments of  FIGS. 2A-2D . 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    Embodiments provide methods, medical leads, and systems where the medical leads have one or more conductors that are at least partially embedded for at least a portion of the length of the lead and where a shield is present within the lead and surrounds the conductors. Where the conductor is at least partially embedded, the lead body fills the space between the conductor and the shield so that fluids that infiltrate the lead body and reach a lumen of the lead body over time cannot pool between the conductor and the shield where the conductor is at least partially embedded. 
         [0020]      FIG. 1  shows an example of an implantable medical system  100  that may be used to provide electrical stimulation therapy and that may reduce coupling between a shield and a conductor of a lead  104 . The implantable medical system  100  includes a pulse generator  102  that includes a housing  106  that contains a stimulation engine  110  that produces the electrical stimulation signals. The pulse generator  102  may include a header  108  that includes a bore that receives a proximal end of the lead  104 . The header  108  includes electrical connectors  114  that physically contact conductive contacts  116  of the lead  104 . A feedthrough  114  transfers electrical signals from the sealed housing  106  to the connectors  114  of the header  104 . 
         [0021]    The lead  104  carries the electrical signals from the contacts  114  to the electrodes  120  that are coupled to the distal end of the lead body and are located at the target site within the body.  FIG. 2A  shows a longitudinal cross-section of a first example of the lead  104 A. In this example, the lead  104 A includes a collection of coiled conductors  208  that are electrically coupled to the contacts  114  of  FIG. 1  and to the electrodes  120  via a radially extending portion  212 . The lead  104 A also includes a radio frequency (RF) shield  206  that in this example is a braid of conductive wires where the braid surrounds the conductors  208 . In this example, the shield  20  is encapsulated within the lead body  118  where the lead body  118  is constructed within an insulative inner layer  204 A and an insulative outer layer  202  overmolded onto the shield  206  and the inner layer  204 A. Each of these layers  202 ,  204 A may be various biocompatible and mechanically compliant materials such as polyurethane or silicone rubber. These layers  202 ,  204 A and may have varying degrees of hardness ranging, for instance according to some embodiments the hardness may range from Shore 45A to Shore 80D. 
         [0022]    As can be seen in this cross-section, the conductor  208  has a diameter  218  and the conductor  208  is partially embedded with a portion of the diameter  218  residing within the inner layer  204 A and a portion residing within a lumen  210 . In this example, one half of the diameter is embedded but it will be appreciated that the amount of the diameter  218  that is embedded may vary from one application to another. The conductor  208  in this example is partially embedded over the entire length of the conductor  208  from the proximal end at the contact  114  to the distal end at the electrode  114 , which provides a high degree of isolation of the conductors  208  from the shield  206 . A space  214  exists between the shield  206  and the conductors  208 , and the inner layer  204 A entirely fills the space  214  such that body fluids cannot pool between the conductors  208  and the shield  206 . The coupling of the shield  206  to the conductor  208  is inhibited to avoid unwanted currents being channeled from the shield  206  to the conductors  208 . 
         [0023]      FIG. 2B  shows another example of a lead  104 B. In this example, a first longitudinal section of the conductor  208  is partially embedded into the inner layer  204 B of the lead body  118 , such as one half of the diameter  218  being embedded as shown. However, the conductors  208  are not embedded to this degree over the full length and become less embedded, including being completely unembedded as shown, at a second longitudinal section in an area between a termination of the shield  206  and the proximal edge of the distal electrode  120 . The conductor  208  may be less embedded by the inside diameter of the inner layer  204  increasing in size as shown, or by the outer diameter of the coil of the conductor  208  shrinking in size. This configuration continues to isolate the conductors  208  from the shield  206  to a high degree as the conductor  208  remains embedded in layer  204 B for some distance beyond the shield termination but provides greater mechanical compliance of the conductors  208  near the electrodes  120  which may be beneficial in some situations. 
         [0024]      FIG. 2C  shows another example of a lead  104 C. In this example, a first longitudinal section of the conductor  208  is fully embedded into the inner layer  204 C of the lead body  118  with the full diameter  218  being present within the inner layer  204 C. However, the conductors  208  are not embedded to this degree over the full length and become less embedded, including being completely unembedded as shown, at a second longitudinal section in an area between a termination of the shield  206  and the proximal edge of the distal electrode  120 . This configuration continues to isolate the conductors  208  from the shield  206  to a high degree while providing increased stiffness relative to a partially embedded state as in  FIG. 2B . However, like the example in  FIG. 2B , the lesser embedded portion provides greater mechanical compliance of the conductors  208  near the electrodes  120  which may be beneficial in some situations. 
         [0025]      FIG. 2D  shows another example of a lead  104 D. In this example, the conductor  208  is fully embedded into the inner layer  204 C of the lead body  118  with the full diameter  218  being present within the inner layer  204 C. In this case, the conductors  208  are fully embedded over the full length from the contact  114  to the electrode  120 . This configuration continues to isolate the conductors  208  from the shield  206  to a high degree while providing increased stiffness relative to a partially embedded state as in  FIG. 2C  and additionally stiffness at the electrodes  214  as well, which may be beneficial in some situations. 
         [0026]      FIG. 2E  shows another example of a lead  104 E. In this example, a first longitudinal section of the conductor  208  is partially embedded into the inner layer  204 E of the lead body  118 , such as one half of the diameter  218  being embedded as shown or could also be fully embedded. However, the conductors  208  are not embedded to this degree over the full length and become less embedded, including being completely unembedded as shown, at a second longitudinal section prior to a termination of the shield  206 . This configuration continues to isolate the conductors  208  from the shield  206  over a significant length of the conductor  208  but provides significantly greater mechanical compliance of the conductors  208  near the electrodes  120  where this larger degree of mechanical compliance near the electrodes  120  may be beneficial in some situations. 
         [0027]      FIG. 3  shows one example of a set of operations used to construct the embodiments of the lead  104 . In this example, the conductors  208  are coiled and therefore straight wires are coiled around a mandrel at a coiling operation  302  to form the coiled conductors  208 . The conductors may be coiled at the desired pitch and spacing as is typical for coiled conductors in medical leads. An inner layer  204  of the insulative lead body  118  is then overmolded onto the coiled conductors  208  at a molding operation  304 . The overmolding may occur while the coiled conductors  208  remain on the coiling mandrel or the coiled conductors  208  may first be removed from the coiling mandrel and placed on a molding mandrel such as a stainless steel pin or wire that is coated with a polytetrafluoroethylene (PTFE) such as the Teflon® polymer from the DuPont Corporation. This overmolding operation  304  dictates the degree to which the diameter  214  of the conductors  208  is embedded into the inner layer  204 . This overmolding operation  304  also dictates the length of the conductors  208  that become embedded. The overmolding operation  304  may be performed by using a heat shrink tubing as at least a portion of the inner layer  204  that contacts the conductors  208 . The depth to which the diameter of the conductors  208  is present within the heat shrink tubing is controlled by the amount of shrink resulting from the chosen time and temperature of the heat shrink process. In that case, the longitudinal length of the conductors  208  that are at least partially embedded into the layer  204  is controlled by the length of the heat shrink tubing being applied to the conductors  208 . 
         [0028]    At this point, the lead assembly is ready for application of the shield  206 , which may be created by braiding wires onto the inner layer  204  of the lead body  118  at a shielding operation  306 . As an alternative, a conductive foil may be wrapped around the inner layer  204  at the shielding operation  306  to provide the shielding. The outer layer  202  of the lead body  118  is then overmolded atop the shielding  206  at a molding operation  308  in order to encapsulate the shield within the lead body  118 . The construction of the lead  104  is completed at a conductor operation  310  by radially extending the conductor portion  212  to the position for the contact  116  on the proximal end and to the position for the electrode  120  on the distal end. The contact  116  and the electrode  120  are installed onto their respective positions on the lead body  118  with a weld or other conductive bond of the conductors  208  to the corresponding contacts  116  and electrodes  120 . Other methods of manufacture may also be done, such as extruding the polymer layer over the coil while present on the mandrel, although the starting and stopping points along the length of the coil where the coil is being embedded may be less precise than where a heat shrink with a specified length is being used to achieve the embedding. 
         [0029]    As discussed above, cabled conductors may be used in place of coiled conductors and in such a case, the cabled conductors may be positioned at their designated circumferential positions on a molding pin. Then the cabled conductors are overmolded with the inner layer  204  at the molding operation  304  and the process of  FIG. 3  continues. 
         [0030]    While embodiments have been particularly shown and described, it will be understood by those skilled in the art that various other changes in the form and details may be made therein without departing from the spirit and scope of the invention.