Abstract:
System, method and antenna for an external power source for an implantable medical device having therapeutic componentry and a secondary coil operatively coupled to the therapeutic componentry. A housing has a first surface adapted to be placed closest to the secondary coil of the implantable medical device. A primary coil is operatively coupled to the external power and is capable of inductively energizing the secondary coil, the primary coil being wound forming generally concentric loops having an axis. The housing has a protrusion extending from the first surface.

Description:
FIELD  
       [0001]     The present invention relates generally to transcutaneous energy transfer for an implantable medical device and, more particularly, to an antenna for an external power source for such transcutaneous energy transfer.  
       BACKGROUND  
       [0002]     Implantable medical devices for producing a therapeutic result in a patient are well known. Examples of such implantable medical devices include implantable drug infusion pumps, implantable neurostimulators, implantable cardioverters, implantable cardiac pacemakers, implantable defibrillators and cochlear implants. Of course, it is recognized that other implantable medical devices are envisioned which utilize energy delivered or transferred from an external device.  
         [0003]     A common element in all of these implantable medical devices is the need for electrical power in the implanted medical device. The implanted medical device requires electrical power to perform its therapeutic function whether it is driving an electrical infusion pump, providing an electrical neurostimulation pulse or providing an electrical cardiac stimulation pulse. This electrical power is derived from a power source.  
         [0004]     Typically, a power source for an implantable medical device can take one of two forms. The first form utilizes an external power source that transcutaneously delivers energy via wires or radio frequency energy. Having electrical wires which perforate the skin is disadvantageous due, in part, to the risk of infection. Further, continuously coupling patients to an external power for therapy is, at least, a large inconvenience. The second form utilizes single cell batteries as the source of energy of the implantable medical device. This can be effective for low power applications, such as pacing devices. However, such single cell batteries usually do not supply the lasting power required to perform new therapies in newer implantable medical devices. In some cases, such as an implantable artificial heart, a single cell battery might last the patient only a few hours. In other, less extreme cases, a single cell unit might expel all or nearly all of its energy in less than a year. This is not desirable due to the need to explant and re-implant the implantable medical device or a portion of the device. One solution is for electrical power to be transcutaneously transferred through the use of inductive coupling. Such electrical power or energy can optionally be stored in a rechargeable battery. In this form, an internal power source, such as a battery, can be used for direct electrical power to the implanted medical device. When the battery has expended, or nearly expended, its capacity, the battery can be recharged transcutaneously, via inductive coupling from an external power source temporarily positioned on the surface of the skin.  
         [0005]     Several systems and methods have been used for transcutaneously inductively recharging a rechargeable used in an implantable medical device. Some examples of systems and method used for transcutaneously inductively charging or recharging an implantable medical device include U.S. Pat. No. 5,411,537, Munshi et al, Rechargeable Biomedical Battery Powered Devices With Recharging and Control System Therefor, (Intermedics, Inc.); U.S. Pat. No. 5,690,693, Wang et al, Transcutaneous Energy Transmission Circuit For Implantable Medical Device, (Sulzer Intermedics Inc.); U.S. Pat. No. 5,733,313, Barreras, Sr., FR Coupled Implantable Medical Device With Rechargeable Back-Up Power Source, (Exonix Corporation); U.S. Pat. No. 6,308,101, Faltys et al, Fully Implantable Cochlear Implant System, (Advanced Bionics Corporation); U.S. Pat. No. 6,324,430, Zarinetchi et al, Magnetic Shield For Primary Coil of Transcutaneous Energy Transfer Device, (Abiomed, Inc.); U.S. Pat. No. 6,516,227, Meadows et al, Rechargeable Spinal Cord Stimulator System, (Advanced Bionics Corporation); U.S. Pat. No. 6,505,077, Kast et al, Implantable Medical Device With External Recharging Coil Electrical Connection, (Medtronic, Inc.); European Patent Application 1,048,324, Schallhorn, Medical Li+ Rechargeable Powered Implantable Stimulator, (Medtronic, Inc.); PCT Patent Application No. WO 01/83029 A1, Torgerson et al, Battery Recharge Management For an Implantable Medical Device, (Medtronic, Inc.); and PCT Patent Application No. WO 01/97908 A2, Jimenez et al, An Implantable Medical Device With Recharging Coil Magnetic Shield, (Medtronic, Inc.).  
         [0006]     U.S. Patent Application Publication US 2005/0075700A1 (U.S. patent application Ser. No. 10/837,506, Schommer et al, External Power Source For An Implantable Medical Device Having An Adjustable Magnetic core and System and Method Related Therefore, filed Apr. 30, 2004), discloses an external power source, and system and method using such external power source, for an implantable medical device having therapeutic componentry and a secondary coil operatively coupled to the therapeutic componentry. A primary coil is capable of inductively energizing the secondary coil when externally placed in proximity of the secondary coil. A repositionable magnetic core associated with the primary coil is capable of being repositioned by a user of the external power source. An indicator is capable of providing the user with information relative to coupling between the primary coil and the secondary coil as a function of repositioning of the repositionable magnetic core.  
         [0007]     Transcutaneous energy transfer through the use of inductive coupling involves the placement of two coils positioned in close proximity to each other on opposite sides of the cutaneous boundary. The internal coil, or secondary coil, is part of or otherwise electrically associated with the implanted medical device. The external coil, or primary coil, is associated with the external power source or external charger, or recharger. The primary coil is driven with an alternating current. A current is induced in the secondary coil through inductive coupling. This current can then be used to power the implanted medical device or to charge, or recharge, an internal power source, or a combination of the two.  
         [0008]     For implanted medical devices, the efficiency at which energy is transcutaneously transferred is crucial. First, the inductive coupling, while inductively inducing a current in the secondary coil, also has a tendency to heat surrounding components and tissue. The amount of heating of surrounding tissue, if excessive, can be deleterious. Since heating of surrounding tissue is limited, so also is the amount of energy transfer which can be accomplished per unit time. The higher the efficiency of energy transfer, the more energy can be transferred while at the same time limiting the heating of surrounding components and tissue. Second, it is desirable to limit the amount of time required to achieve a desired charge, or recharge, of an internal power source. While charging, or recharging, is occurring the patient necessarily has an external encumbrance attached to their body. This attachment may impair the patient&#39;s mobility and limit the patient&#39;s comfort. The higher the efficiency of the energy transfer system, the faster the desired charging, or recharging, can be accomplished limiting the inconvenience to the patient. Third, amount of charging, or recharging, can be limited by the amount of time required for charging, or recharging. Since the patient is typically inconvenienced during such charging, or recharging, there is a practical limit on the amount of time during which charging, or recharging, should occur. Hence, the size of the internal power source can be effectively limited by the amount of energy which can be transferred within the amount of charging time. The higher the efficiency of the energy transfer system, the greater amount of energy which can be transferred and, hence, the greater the practical size of the internal power source. This allows the use of implantable medical devices having higher power use requirements and providing greater therapeutic advantage to the patient and/or extends the time between charging effectively increasing patient comfort.  
         [0009]     The efficiency of transcutaneous inductive energy transfer is directly related to the accuracy of positioning of the external, primary coil, to the internal, secondary coil. The two coils should be as close to each other as possible. Of course, since the position of the secondary coil is fixed following implantation, the closer that the primary coil can be positioned to the skin surface the better. The two coils should also be laterally aligned as close as possible. This alignment is typically accomplished by the patient by the attachment of the external power source/charger at the commencement of the charging process or when otherwise transferring power. It is often cumbersome and difficult for the patient, who typically is not a medical professional, to most accurately position the primary coil in the proper location. The lateral alignment is typically done tactilely by the patient. A typical implanted medical device is implanted close enough to the skin that the skin of the patient has a small protuberance at the site of implantation. This can be felt by the patient and can be used as a guide to position the external coil. However, this problem can be exacerbated because the lateral position of the secondary coil is not always laterally centered with the external protuberance providing the patient with tactile lateral location information.  
         [0010]     Even if the primary coil is properly placed at the initiation of energy transfer or of the charging process, energy transfer and/or charging can continue over a signification period of time. During this time, it is usually impracticable for the patient to remain absolutely immobile. Charging can typically occur over several, perhaps many, hours. It is desirable for the patient to be able to continue with as many normal activities as possible. For example, since charging often is accomplished at night, it is desirable that the primary coil not move during normal sleep activities of the patient. As the patient may move during energy transfer or during charging, motions and activities of the patient may cause the primary coil to move with respect to the secondary coil. If this should happen, the efficiency of energy transfer is not optimum which limits the rate at which energy can be transferred and resulting in an increase in charging time, if the system utilizes charging, or a decrease in the amount of energy available to the implanted medical device, if direct energy transfer is utilized.  
         [0011]     It also can be important to secure the primary coil in the proper location once the proper has been located by the patient.  
         [0012]     Prior art implantable medical devices, external power sources, systems and methods have not always provided the best possible benefit leading to efficiency of energy transfer and patient comfort.  
       SUMMARY  
       [0013]     A bump, or protrusion, on the surface of an external antenna closest to a secondary coil of an implanted medical device allows a greater efficiency of energy transfer as compared with external antennas having a flat, or relatively flat, surface closest to the implanted medical device.  
         [0014]     When the external antenna having such protrusion is positioned in charging position against the surface of the skin, the protrusion pushes skin away from the primary coil and allows the primary coil to get closer to the secondary coil in a direction orthogonal to the surface of the skin increasing energy transfer efficiency.  
         [0015]     Increased efficiency of energy transfer can result in a reduction in heat build-up during transcutaneous energy transfer and/or shorten the time required to transfer a given amount of energy, e.g., reducing the time required to charge or recharge an implanted medical device.  
         [0016]     In an embodiment, the present invention provides an antenna for an external power source for an implantable medical device having therapeutic componentry and a secondary coil operatively coupled to the therapeutic componentry. A housing has a first surface adapted to be placed closest to the secondary coil of the implantable medical device. A primary coil is operatively coupled to the external power and is capable of inductively energizing the secondary coil, the primary coil being wound forming generally concentric loops having an axis. The housing has a protrusion extending from the first surface.  
         [0017]     In an embodiment, the protrusion is a central protrusion centrally located with respect to the axis of the primary coil.  
         [0018]     In an embodiment, the central protrusion comprises a magnetic core.  
         [0019]     In an embodiment, a magnetic core is at least partially contained in the central protrusion.  
         [0020]     In an embodiment, the housing further has a peripheral protrusion extending from the first surface at least partially surrounding an outer periphery of the primary coil.  
         [0021]     In an embodiment, the peripheral protrusion comprises a plurality of bumps, spaced from each other, surrounding the outer periphery of the primary coil and extending from the first surface.  
         [0022]     In an embodiment, the peripheral protrusion extends from the first surface approximately one-half centimeter.  
         [0023]     In an embodiment, the peripheral protrusion extends from the first surface not greater than central protrusion extends from the first surface.  
         [0024]     In an embodiment, the peripheral protrusion extends from the first surface less than the central protrusion extends from the first surface.  
         [0025]     In an embodiment, the peripheral protrusion extends from the first surface approximately one-half of the distance the central protrusion extends from the first surface.  
         [0026]     In an embodiment, the protrusion is generally aligned with the axis of the primary coil.  
         [0027]     In an embodiment, the protrusion has a circular cross-section.  
         [0028]     In an embodiment, the protrusion has a round tip.  
         [0029]     In an embodiment, at least a portion of the protrusion is conical.  
         [0030]     In an embodiment, a diameter of the circular cross-section of the protrusion is not greater than a distance between an inner periphery of the primary coil.  
         [0031]     In an embodiment, the protrusion extends from the first surface approximately one-half centimeter.  
         [0032]     In an embodiment, an extent of extension of the protrusion from the first surface is adjustable.  
         [0033]     In an embodiment, the protrusion comprises a screw having threads adapted to mate with corresponding threads of the housing.  
         [0034]     In an embodiment, the present invention provides a system for transcutaneous energy transfer. An implantable medical device has therapeutic componentry and a primary coil capable of inductively energizing the secondary coil when externally placed in proximity of the secondary coil. An antenna has a housing having a first surface adapted to be placed closest to the secondary coil of the implantable medical device. A primary coil is operatively coupled to the external power and is capable of inductively energizing the secondary coil, the primary coil being wound forming generally concentric loops having an axis. The housing has a protrusion extending from the first surface.  
         [0035]     In an embodiment, the present invention provides a method of transcutaneous energy transfer to a medical device implanted in a patient having a secondary charging coil using an external power source having a housing containing a primary coil, the housing having a first surface nearest to the secondary coil, the housing having a protrusion extending from the first surface. The primary coil is positioned externally of the patient with respect to the secondary coil. The protrusion is pressed against skin of the patient. Energy is transcutaneously transferred from the primary coil to the secondary coil.  
         [0036]     In an embodiment, the protrusion is a central protrusion centrally located with respect to the axis of the primary coil.  
         [0037]     In an embodiment, the first surface is further pressed against the skin of the patient.  
         [0038]     In an embodiment, the distance of extension of the protrusion is adjusted.  
         [0039]     In an embodiment, a peripheral protrusion extending from the first surface at least partially surrounding an outer periphery of the primary coil is pressed against the skin of the patient.  
         [0040]     In an embodiment, the peripheral protrusion is a plurality of bumps, spaced from each other, surrounding the outer periphery of the primary coil and extending from the first surface.  
         [0041]     In an embodiment, a peripheral protrusion extending from the first surface at least partially surrounding an outer periphery of the primary coil is pressed against the skin of the patient.  
         [0042]     In an embodiment, the central protrusion comprises a magnetic core.  
         [0043]     In an embodiment, the housing further has a magnetic core carried at least partially in the central protrusion.  
         [0044]     In an embodiment, the protrusion, or bump, on the surface of the external antenna closest to the secondary coil can allow a greater efficiency of transcutaneous energy transfer as compared with an external antenna with a flat, or a relatively flat, surface. By adding a protrusion in the z-direction, i.e., the direction orthogonal to a plane defined by the cutaneous boundary, pushes skin away from the primary coil and allows the primary coil to be pressed closer to the secondary coil in the z-direction, thereby increasing transcutaneous energy transfer efficiency. Increased efficiency of energy transfer can result in a reduction in heat build-up during transcutaneous energy transfer and/or in a shortening of the time required to transfer a given amount of energy, e.g., reducing the time required to charge or recharge the implanted medical device.  
         [0045]     In an embodiment, a recess developed between peripheral protrusions in the surface of the external antenna closest to the secondary coil can assist with x-y positioning of the external antenna by providing a convenient location assist with the “bump” created by the implanted medical device.  
         [0046]     In an embodiment, a magnetic core at least partially located in a central protrusion on the surface of the external antenna closest to the secondary coil can help to additionally focus magnetic flux in the z-direction into the secondary coil. 
     
    
     DRAWINGS  
       [0047]      FIG. 1  is a block diagram of a charging system utilizing transcutaneous energy transfer;  
         [0048]      FIG. 2  is a cross-sectional illustration of a close-up view of a variation of a portion of a charging system;  
         [0049]      FIG. 3  is a cross-sectional illustration of an embodiment of an external antenna having a central protrusion contacting the cutaneous boundary of the patient;  
         [0050]      FIG. 4  is a cross-sectional illustration of an embodiment of an external antenna having a central protrusion being pressed against the cutaneous boundary of the patient;  
         [0051]      FIG. 5  is a cross-sectional illustration of an embodiment of the external antenna having peripheral protrusions and of an implantable medical device having an mating indent with a central protrusion of the external antenna;  
         [0052]      FIG. 6  is a bottom perspective view of the external antenna of  FIG. 5 ;  
         [0053]      FIG. 7  is a cross-sectional illustration of an embodiment of an external antenna having an adjustable central protrusion;  
         [0054]      FIG. 8  is a cross-sectional illustration of an embodiment of an external antenna having a rachetable central protrusion;  
         [0055]      FIG. 9  is a cross-sectional illustration of an embodiment of an external antenna having a magnetic core of non-uniform cross-section. 
     
    
     DETAILED DESCRIPTION  
       [0056]      FIG. 1  illustrates a system  10  into which an improved external antenna  32  may be utilized. System  10  consists of implantable medical device  12  and external power supply  14 .  
         [0057]     Implantable medical device  12  is situated under cutaneous boundary  16 . Implantable medical device  12  includes charging regulation module  14 , electronics module  16  and therapy module  18 . Charging regulation and therapy control is conventional. Implantable medical device  12  also has internal telemetry coil  20  configured in conventional manner to communicate through external telemetry coil  22  to an external programming device (not shown), charging unit  24  or other device in a conventional manner in order to both program and control implantable medical device and to externally obtain information from implantable medical device  12  once implantable medical device has been implanted. Internal telemetry coil  20 , rectangular in shape with dimensions of 1.85 inches (4.7 centimeters) by 1.89 inches (4.8 centimeters) constructed from 150 turns of 43 AWG wire, is sized to be larger than the diameter of secondary charging coil  26 .  
         [0058]     Internal antenna  25  contains secondary coil  26 , constructed with 182 turns of 30 AWG wire with an inside diameter of 0.72 inches (1.83 centimeters) and an outside diameter of 1.43 inches (3.63 centimeters) with a height of 0.075 inches (0.19 centimeters). Magnetic shield  28  is positioned between secondary charging coil  26  and housing  30  and sized to cover the footprint of secondary charging coil  26 .  
         [0059]     Internal telemetry coil  20 , having a larger diameter than secondary coil  26 , is not completely covered by magnetic shield  28  allowing implantable medical device  12  to communicate with the external programming device with internal telemetry coil  20  in spite of the presence of magnetic shield  28 .  
         [0060]     Rechargeable power source  24  can be charged while implantable medical device  12  is in place in a patient through the use of charging regulation module  14 . In a preferred embodiment, charging regulation module  14  consists of charging unit  24  and external antenna  32 . Charging unit  24  contains the electronics necessary to drive primary coil  34  with an oscillating current in order to induce current in secondary coil  26  when primary coil  34  is placed in the proximity of secondary coil  26 . Charging unit  24  is operatively coupled to primary coil  34  by cable  36 . In an alternative embodiment, charging unit  24  and antenna  32  may be combined into a single unit. Antenna  32  may also optionally contain external telemetry coil  22  which may be operatively coupled to charging unit  24  if it is desired to communicate to or from implantable medical device  12  with charging regulation module  14 . Alternatively, antenna  32  may optionally contain external telemetry coil  22  which can be operatively coupled to an external programming device, either individually or together with external charging unit  14 .  
         [0061]     As will be explained in more detail below, repositionable magnetic core  38  can help to focus electromagnetic energy from primary coil  34  to be more closely aligned with secondary coil  26 . Also as will be explained in more detail below, energy absorptive material  40  can help to absorb heat build-up in external antenna  32  which will also help allow for a lower temperature in implantable medical device  12  and/or help lower recharge times. Also as will be explained in more detail below, thermally conductive material  42  is positioned covering at least a portion of the surface of external antenna  32  which contacts cutaneous boundary  44  of the patient.  
         [0062]      FIG. 2  is a cross-sectional illustration of a close-up view of a variation of a portion of charging system  10 . Internal antenna  25  is shown having been implanted below cutaneous boundary  44 . Secondary coil  26  is positioned within internal antenna  25  above magnetic shield  28 .  
         [0063]     External antenna  38  contains primary coil  34  and is positioned in transcutaneous superposition with respect to internal antenna  25 . Primary coil  34  is aligned with secondary coil  26  in order to facilitate transcutaneous energy transfer using electromagnetic coupling. Magnetic core  38  helps to focus electromagnetic energy generated by primary coil  34  transcutaneously toward secondary coil  26 . In this embodiment, magnetic core  38  extends between windings of primary coil  34 . External antenna  32  has a generally planar surface  48  intended to contact cutaneous boundary  44 . An edge of magnetic core  38  is coplanar with surface  48  to help promote electromagnetic fields to extend from primary coil  34  and be captured more readily by secondary coil  26 . Insulation  46  between magnetic core  38  and primary coil  34 , particularly on the side of external antenna  32  facing surface  48 , protects magnetic core  38  from collecting heat produced by primary coil  34  and increasing the surface of cutaneous boundary  44 .  
         [0064]      FIG. 3  illustrates an alternative embodiment of external antenna  32  used in charging system  10 . Again, external antenna  38  contains primary coil  34  and is positioned in transcutaneous superposition with respect to internal antenna  25 . Primary coil  34  is aligned with secondary coil  26  in order to facilitate transcutaneous energy transfer using electromagnetic coupling. Magnetic core  38  helps to focus electromagnetic energy generated by primary coil  34  transcutaneously toward secondary coil  26 . Central protrusion may be aligned with the axis of primary coil  34 .  
         [0065]     However, external antenna  32  illustrated in  FIG. 3  extends magnetic core  38  further toward cutaneous boundary  44  and past surface  48  creating protrusion  50 . Central protrusion  50  extends beyond surface  48  creating a noticeable bump on surface  48  contacting cutaneous boundary  44 . In an embodiment, central protrusion  50  is circular in cross-section and has a conically shaped end intended to contact cutaneous boundary  44 . In an embodiment, central protrusion  50  is approximately 0.5 centimeters in diameter and extends approximately 0.5 centimeters beyond surface  48 . The outer diameter of central protrusion  50  may be not greater than, and perhaps less than, the inner diameter of primary coil  34 .  
         [0066]     External antenna  32  may be pressed by the user against cutaneous boundary  44  as illustrated in  FIG. 4 . Central protrusion  50  pushes a portion of cutaneous boundary  44  away from its point of impact allowing external antenna  32 , in general, and magnetic core  38 , in particular, to come closer to secondary coil  26 .  
         [0067]     Central protrusion  50  allows primary coil  34  to more efficiently electromagnetically couple with secondary coil  26  by allowing magnetic core  38  to be closer to internal antenna  25  and secondary coil  26 . Commonly, an external antenna  32  having a planar surface  48  may be able to come within 1 centimeter of secondary coil  26  of internal antenna  25 . However, central protrusion  50  is able to indent cutaneous boundary  44  and reduce the distance, commonly referred as the “air gap distance” between primary coil  34  and secondary coil  26 .  
         [0068]     An alternative embodiment of external antenna  32  of charging system  10  can be seen by referring to  FIG. 5  and  FIG. 6 .  FIG. 5  is a cross-sectional view of external antenna  32  placed in the proximity of implantable medical device  12  and secondary coil  26 .  FIG. 6  is an underside perspective view of external antenna  32  unencumbered by implantable medical device  12 .  
         [0069]     As in  FIG. 3  and  FIG. 4 , external antenna  32  of  FIG. 5  and  FIG. 6  has central protrusion  50  enhancing electromagnetic coupling between primary coil  34  and secondary coil  26  as discussed above. In addition, external antenna  32  contains a plurality of peripheral protrusions  52  extending beyond surface  48  in a similar fashion to central protrusion  50 . Peripheral protrusions  52  may be sized and positioned to “fit” around the periphery of internal antenna  25  of implantable medical device  12  to further reduce the gap between primary coil  34  and secondary coil  26 . One or more peripheral protrusions  52  may be used. Peripheral protrusions may be circular in cross-section, conically shaped, square, rectangular or arcuate. Typically, peripheral protrusions  52  extend a similar distance beyond surface  48  as central protrusion  50 , however, peripheral protrusions  52  may extend farther or less far from surface  48  than central protrusion  50 . Peripheral protrusions  52  may extend approximately one-half of the distance that central protrusion  50  extends from surface  48 .  
         [0070]     Peripheral protrusions  52  may be spaced from one another as illustrated in  FIG. 6  or may be more or less continuous around a periphery of external antenna  32  forming, to a large extent or entirely, a peripheral ring around external antenna  32  extending below surface  48 .  
         [0071]     In an embodiment illustrated in  FIG. 5 , implantable medical device  12 , and, in particular, internal antenna  25 , has an indent  54  on the surface facing cutaneous boundary  44  aligned with central protrusion  50 . So configured, indent  54  of internal antenna  25  provides a locating feature allowing the user to tactilely determine the optimum positioning of external antenna  32  and will help hold external antenna  32  in proper position for electromagnetic energy transfer and will help ensure efficient energy transfer. Further, indent  54  may allow magnetic core  38  in protrusion  50  to get even closer to secondary coil  26  making energy transfer even more efficient.  
         [0072]     In an embodiment illustrated in  FIG. 5  and  FIG. 6 , peripheral protrusions  52  may be sized and positioned to have a pattern, perhaps a circular pattern, slightly larger in diameter than internal antenna  25  allowing peripheral protrusions  52  to “fit” over the edge of internal antenna  25  enabling ease of tactile positioning of external antenna  32  with respect to internal antenna  25 . Further, peripheral protrusions  52  positioned in this manner may tend to push away skin of cutaneous boundary  44  and stretch cutaneous boundary  44  to be more thin over internal antenna  25  allowing external antenna  32  and, hence, primary coil  34 , to be closer to secondary coil  26  and increasing the efficiency of energy transfer.  
         [0073]      FIG. 7  illustrates an embodiment of external antenna  32  having screw  56  facilitating implementation of central protrusion  50 . Screw  56  may be turned clockwise or counter-clockwise to either increase the amount of protrusion or decrease the amount of protrusion of central protrusion  50  from surface  48 . Screw  56  may be turned by hand or by using a tool such as a screwdriver in a slot of the top surface of screw  56 . Screw  56  may extend through external antenna  32  as shown, facilitating tool manipulation, or may extend only partly through external antenna  32  allowing manipulation, for example, by hand turning central protrusion  50 . Screw  56  may be adjusted to create a greater or lesser extension of central protrusion  50  to account for patient comfort, varying implant locations and implant depths and type of skin or amount of fat tissue surrounding implantable medical device  12 .  
         [0074]      FIG. 8  illustrates an embodiment of external antenna  32  similar to the embodiment illustrated in  FIG. 7 . However, in the embodiment illustrated in  FIG. 8 , adjustable central protrusion  50  is designed to an adjustable plunger that ratchets within the body of external antenna  32 . The ratchet mechanism can allow central protrusion  50  to be adjusted relative to surface  48 .  
         [0075]      FIG. 9  illustrates an embodiment of external antenna  32  in which magnetic core  38  has a non-uniform cross-sectional area. Screw  56  is constructed of magnetic core  38  having a larger cross-section nearer the tip of central protrusion  50  and a smaller cross-section farther away from the tip of central protrusion  50 . The greater amount of magnetic core  38  nearer the tip of central protrusion  50  increases the focusing effect of magnetic core  38  and increases the efficiency of energy transfer by keeping electromagnetic flux within magnetic core  38  farther toward secondary coil  26 . The remainder of screw  56  may be comprised of a non-magnetic protective material  60  such as an injection molded thermoplastic such as nylon  12 , nylon PPA, polycarbonate or ABS. While the embodiment of  FIG. 8  is illustrated with magnetic core  38  contained within screw  56 , it is to be recognized and understood that magnetic core  38  could also be contained within external antenna  32  and within central protrusion  50  without an adjustable screw  56 . That is, central protrusion  50  could be fixed and still contain magnetic core  38  on non-uniform cross-section.  
         [0076]     While various embodiments of central protrusion  50  have been described, it is to be recognized and understood such embodiments and techniques could be used for one or more of peripheral protrusions  52 , either in addition to be used with central protrusion  50  or alternative to being used with central protrusion  50 .  
         [0077]     While peripheral protrusions  52  have been illustrated and described as being used with central protrusion  50 , it is to be recognized and understood that peripheral protrusions  52  could be used to benefit in external antenna  32  without central protrusion  50 .  
         [0078]     In an embodiment, a portion of insulating material  46  facing surface  48  could be formed of a low permeable material, such as bismuth graphite, to assist in forcing the electromagnetic field generated by primary coil  34  toward secondary coil  26 .  
         [0079]     Thus, embodiments of the invention are disclosed. One skilled in the art will appreciate that the present invention can be practiced with embodiments other than those disclosed. The disclosed embodiments are presented for purposes of illustration and not limitation, and the present invention is limited only by the claims that follow.