Abstract:
A system and method wirelessly transfers information electromagnetically using a detachable helical antenna. In an example, the detachable helical antenna can include a first threaded portion. In an example, the detachable helical antenna can be configured to mechanically threadably engage an implantable medical device.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This patent application claims the benefit of priority, under 35 U.S.C. Section 119(e), to Greg Carpenter et al., U.S. Provisional Patent Application Ser. No. 61/033,535, entitled “ANTENNA FOR IMPLANTABLE MEDICAL DEVICE,” filed on Mar. 4, 2008 (Attorney Docket No. 00279.G29PRV), incorporated herein by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    Medical devices can be implanted in a body to perform tasks including monitoring, detecting, or sensing physiological information in the body, diagnosing a physiological condition or disease, treating a physiological condition or disease, or restoring or otherwise altering the function of an organ or a tissue. Examples of an implantable medical device can include a cardiac rhythm management device, such as a pacemaker, a cardiac resynchronization therapy device, a cardioverter or defibrillator, a neurological stimulator, a neuromuscular stimulator, or a drug delivery system. Implantable medical devices can include a telemetry circuit configured to provide wireless communication between the implantable medical device and an external device, e.g., to send information (such as physiological information) from the implantable medical device to the external device, or to receive information (e.g., such as programming instructions) at the implantable medical device from the external device. 
       Overview  
       [0003]    This document discusses, among other things, systems and methods for wirelessly transferring information electromagnetically using a detachable helical antenna. In an example, the detachable helical antenna can include a first threaded portion. In an example, the detachable helical antenna can be configured to mechanically attach to an implantable medical device at least in part using the first threaded portion. 
         [0004]    The present inventor has recognized, among other things, that it can be advantageous to provide a telemetry circuit or antenna for an implantable medical device that is one or more of adaptable, compact in size, efficient, that increases a communication range, suitable for providing communication over various media (e.g., tissue, air, etc.), or otherwise increases the abilities or options for communication between an implantable device and an external device or between multiple implantable devices. 
         [0005]    This overview is intended to provide an overview of subject matter of the present patent application. It is not intended to provide an exclusive or exhaustive explanation of the invention. The detailed description is included to provide further information about the present patent application. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document. 
           [0007]      FIG. 1  illustrates generally an example of a system including an implantable medical device (IMD) having a telemetry circuit. 
           [0008]      FIG. 2  illustrates generally an example of a system including an IMD and an external device. 
           [0009]      FIGS. 3-5  illustrate generally an example of an IMD including a housing and a header. 
           [0010]      FIG. 6  illustrates generally an example of an antenna. 
           [0011]      FIGS. 7 ,  7   a,    8 , and  8   a  illustrate generally an example of a helically coiled antenna. 
           [0012]      FIG. 9  illustrates generally an example of an IMD having a housing and a header. 
           [0013]      FIGS. 10   a - 10   c  illustrate generally examples of loaded wire antennas. 
           [0014]      FIG. 11  illustrates generally an example of a multiband loaded antenna. 
           [0015]      FIG. 12  illustrates generally an example of a capacitive loaded antenna. 
           [0016]      FIGS. 13   a - 13   f  illustrate generally an example of a loaded antenna configuration or location with respect to an IMD. 
           [0017]      FIGS. 14-15  illustrate generally an example of an IMD including a housing, a header, and an antenna. 
           [0018]      FIGS. 16   a - 16   c  illustrate generally an example of a multi-length antenna. 
           [0019]      FIG. 17  illustrates generally an example of a Smith Chart illustrating the impedance of a multi-length antenna. 
           [0020]      FIG. 18  illustrates generally a multi-band antenna having more than one resonant length 
           [0021]      FIGS. 19   a - 19   b  illustrate generally a multi-band/multi-length antenna. 
           [0022]      FIG. 20  illustrates generally an example of a multi-length antenna oriented along more than one axis. 
           [0023]      FIGS. 21-22  illustrate generally examples of a holder for an antenna. 
           [0024]      FIGS. 23   a - 23   b  illustrate generally examples of multi-length antennas. 
           [0025]      FIG. 24  illustrates generally an example of a multi-length antenna. 
           [0026]      FIG. 25  illustrates generally an example of a capacitively loaded antenna having a first capacitor and a second capacitor. 
           [0027]      FIGS. 26   a - 26   c  illustrates generally a capacitively loaded antenna section. 
           [0028]      FIG. 27  illustrates generally an example of an IMD including a housing, a header, and an antenna located inside of the header. 
           [0029]      FIGS. 28   a - 28   b  illustrates generally an example of an IMD including a housing, a header, and an antenna on the surface of the housing. 
           [0030]      FIGS. 29   a - 29   b  illustrate generally an example of a telemetry circuit coupled to a first antenna or a second antenna. 
           [0031]      FIG. 30  illustrates generally an example of a telemetry circuit having a transceiver, a matching network, and an antenna. 
           [0032]      FIGS. 30   a - 30   f  illustrate alternate embodiments regarding the components of the matching network. 
           [0033]      FIG. 31  illustrates generally an example of an IMD having a housing, a header, and an antenna. 
           [0034]      FIG. 31   a  illustrates generally an example of a conductor over a dielectric on the surface of the housing or the header. 
           [0035]      FIG. 31   b  illustrates generally an example of a conductor located in the middle of a dielectric on the surface of the housing or the header. 
       
    
    
     DETAILED DESCRIPTION 
       [0036]      FIG. 1  illustrates generally an example of a system  100  including an implantable medical device (IMD)  105  having a telemetry circuit  110 . In an example, the IMD  105  can include a medical device configured to be implanted in a body. In certain examples, the IMD  105  can be implanted in the body, or the IMD  105  can be capable of being implanted, but can reside outside the body (e.g., the IMD  105  can operate in air outside of the body, such as before being implanted in the body or after being explanted from the body). 
         [0037]    In an example, the telemetry circuit  110  can be configured to provide far-field telemetry with another medical device, such as an external medical device. In certain examples, the telemetry circuit  110  can include at least one of an antenna, a receiver, a transmitter, or an energy source. In certain examples, at least a portion of the telemetry circuit  110  (e.g., the antenna, for example) can come into contact with biological material (e.g., skin, tissue, body fluid, etc.). Thus, at least a portion of the telemetry circuit  110  (e.g., the portion that can come into contact with biological material) can be made or manufactured using one or more biocompatible materials, or materials that can be safely implanted in the body. 
         [0038]    In an example, the receiver can include an amplifier, a demodulator, or other receiver circuit. In an example, the transmitter can include an amplifier, a modulator, a radio frequency (RF) carrier generator, or other transmitter circuit. In certain examples, the telemetry circuit  110  can include a transceiver, including both a transmitter and a receiver. 
         [0039]      FIG. 2  illustrates generally an example of a system  200  including an IMD  105  and another device, such as an external device  115 . In certain examples, the IMD  105  can include a telemetry circuit  110 , or the external device  115  can include a telemetry circuit  120 . In an example, the IMD  105  can be communicatively coupled, such as by using wireless telemetry, to the external device  115 . 
         [0040]    In the example of  FIG. 2 , the IMD  105  can be implanted in a subject  101 , and can be communicatively coupled to an external device  115  located outside the body. In certain examples, the IMD  105  can be configured to be implanted in the body, but can be located outside of the body, such as before implant or after explant, for example. In that example, the IMD  105  can be an externally-located component that can be communicatively coupled to the external device  115 . 
         [0041]      FIG. 3  illustrates generally an example of an IMD  105  including a housing  106  and a header  107 . In an example, the housing  106  can include within an energy delivery circuit, a physiological information detector circuit, a controller, or other implantable medical device circuit. In an example, the exterior of the housing  106  (also referred to as a “case” or a “can”) can include a conductive material, such as titanium or other conductive biocompatible material. In certain examples, the exterior of the housing  106  can include other non-conductive biocompatible material, such as ceramic. In an example, the header  107  can include a receptacle, e.g., to receive an intravascular or other lead, an electrode, or other component. In an example, the header  107  can be formed at least in part by using an insulative or non-conductive material, such as molded plastic. In certain examples, the header  107  can be substantially transparent, at least in part, such as to include a line-of-sight to at least a portion of the receptacle (e.g., the portion of the receptacle where the lead or the electrode is received) so that a user can visually verify a correct placement of the lead or electrode in the receptacle. 
         [0042]    In an example, at least a portion of the telemetry circuit  110  (e.g., the antenna), can be located at least in part inside or on the housing  106 , inside or on the header  107 , outside the housing  106  or the header  107 , or any combination or permutation of inside or on the housing  106 , inside or on the header  107 , or outside the housing  106  or the header  107 . For example, an energy source, a receiver, or a transmitter can be located inside of the housing  106 , while the antenna can be located entirely within the header  107 . In certain examples, at least a portion of the antenna (e.g., at least a portion of the radiating part of the antenna, such as to avoid shielding in an example in which the housing  106  is conductive) can be located in or on the header  107 , while the remainder of the antenna can be located outside of the housing  106  or the header  107  (e.g., at least a portion of the antenna can attach to the header  107  through a receptacle, for example, similar to that used to attach a lead), or while the remainder of the antenna can be located in or on the housing  106 . In certain examples, the energy source, the receiver, or the transmitter can be located outside of the header or the housing. 
       Antenna Threaded Into IMD 
       [0043]      FIGS. 4 and 5  illustrate generally an example of an IMD  105  including a housing  106  and a header  107 . In an example, the IMD  105  can include an antenna  405 . In certain examples, the antenna  405  can include a stub helical antenna configured to be integrated into the IMD  105  (e.g., the antenna  405  can be configured to attach to the header  107 ). 
         [0044]    The present inventor has recognized, among other things, that it can be advantageous to provide a convenient (e.g., manually, without requiring any special tool) attachable or detachable antenna for an implantable medical device. This can allow for separate production of the antenna and the implantable medical device. This can also allow for the attachment of different antennas for different regulatory regions or frequencies (e.g., one antenna for the MICS band and one antenna for the ISM band). In certain examples, the antenna  405  can be configured to wirelessly transfer information in a specified one of one or more of the following operating frequency ranges: 
         [0045]    (1) a Short Range Device (SRD) band range (e.g., 862-870 MHz); 
         [0046]    (2) a first Industrial-Scientific-Medical (ISM) band range (e.g., 902-928 MHz); 
         [0047]    (3) a second Industrial-Scientific-Medical (ISM) band range (e.g., 2.4-2.5 GHz); 
         [0048]    (4) a Medical Implant Communications Service (MICS) band range (e.g., 402-405 MHz); or 
         [0049]    (5) one or more other frequency band ranges configured for communication between an IMD and one or more other implantable or external devices. 
         [0050]    The present inventor has also recognized that integration of the antenna  405  (e.g., into the header the device) can reduce the overall size of the device. The present inventor has also recognized that using a stub helical antenna allows for using a smaller package without compromising performance, further allowing the overall size of the IMD  105  to be reduced. 
         [0051]    In the example of  FIGS. 4 and 5 , the antenna  405  can be configured to be integrated into an opening in the header. In certain examples, the opening in the header can be at an opposite end from a face providing one or more intravascular or other lead receptacles. This curved portion of the header can accommodate a tapered helix of decreasing diameter (in an outward direction from the housing  106 ), and such portion of the lead volume may be readily available for this use, in that it need not be used to provide lead receptacle bores, electrical connections, or the like. 
         [0052]    In an example, the antenna  405  can be connected to the IMD  105  by threading or otherwise attaching the antenna  405  into a conductive connector block attached to the IMD  105  (e.g., a conductive connector block in the header  107  or the housing  106 ). In certain examples, the antenna  405  can be connected to the IMD  105  by directly fixing the antenna  405  to the IMD  105  (e.g., by threading the antenna  405  into the IMD  105 , including into the header  107  or the housing  106 ), or by snapping the antenna  405  into the IMD  105  (e.g., by snapping into an undercut feature of the housing  106  or the header  107 ). Further, the electrical connection between the antenna  405  and the IMD  105  can use a direct electrical contact (e.g., a compression spring electrical contact) or an indirect electrical coupling (e.g., a capacitive coupling connection). 
         [0053]    In an example, using the capacitive coupling connection can include electrically coupling the antenna  405  to an offset plane or surface in the IMD  105 . The offset can be adjusted (e.g., by adjusted a distance or an angle) such as to help obtain a desired tuning of the telemetry circuit  110 . In an example, the offset plane or surface can include a biocompatible conductive material insert, an integrated plated header, a plated insert, or other conductive surface. The offset plane or surface can be electrically coupled to an amplifier, a receiver, a transmitter, or other telemetry component. In an example, if the offset plane or surface is located in the header  107  and at least a portion of the telemetry circuit  110  (e.g., a receiver, a transmitter, or other telemetry component) is located in the housing  106 , the offset plane or surface can be electrically coupled to the remainder of the telemetry circuit  110  using a biocompatible feed-through electrical connection from the header  107  to the housing  106 . 
         [0054]    In an example, the space in the IMD  105  where the antenna  405  attaches (e.g., the space in the header  107 ) can be filled, e.g., once the antenna  405  is inserted, such as for aesthetics, tuning (e.g., with a material of a desired permittivity) or the like. In certain examples, the space can be filled with a biocompatible material, such as a medical adhesive or some other biocompatible material that can be chosen, such as based on the permittivity of the material. 
         [0055]      FIG. 6  illustrates generally an example of an antenna  405 . The antenna  405  can include a helically coiled antenna  406 , which can be mounted in, encapsulated in, or surrounded by an antenna overmold material  407 . In certain examples, the antenna  405  can be created using a plated overmold. In an example, the helically coiled wire  406  can include one or more of a first end coupled to a first contact  408  and a second end coupled to a second contact  410 . In an example, the first contact  408  can be separated from the second contact  410  such as by using an insulator  409 . In certain examples, the feedwire (e.g., the conductor that couples the antenna to another portion of the telemetry circuit  110 ) for the first contact  408  and the second contact  410  can include a single coaxial feedwire. 
         [0056]    In certain examples, the helically coiled antenna  406  can be loaded (e.g., at least partially filled inside of, or covered on top of or around, or any combination of at least partially filled or covered) with a material having a dielectric constant or permittivity greater than air. In an example, loading an antenna with a material having a high dielectric constant (e.g., ceramic) can modify the resonance characteristics of the radiating element. In certain examples, the loading dielectric material can increase the effective length of the antenna. In certain examples, the antenna can be loaded with a material that matches or is close to the dielectric constant of the antenna&#39;s surrounding medium (e.g., 20≦ε R ≦50 for tissue medium or 50≦ε R ≦70 for fluid medium). 
         [0057]    In an example, the helically coiled antenna  406  (or other antenna capable of being similarly loaded with a high dielectric constant material, such as a spiral antenna, a cylindrical antenna, a half or quarter split cylindrical antenna, etc.) can be loaded using a polymer material having a high dielectric constant (e.g., 20-100). In an example, the polymer material can be fabricated through injection molding or other techniques. 
         [0058]    In certain examples, another dielectric antennas configuration can be used, such, as for example, a dielectric resonator antenna, a high dielectric antenna, etc. 
         [0059]      FIG. 7  illustrates generally an example of a helically coiled antenna  406 . The present inventor has recognized that using a balanced structure, such as the helical antenna in particular, can provide an antenna that radiates relatively consistently regardless of lead configuration, lead positioning, or header geometry, and can therefore be relatively consistent across entire product lines. 
         [0060]    In the example of  FIG. 7 , the helically coiled antenna  406  can include a tapered helical configuration. In an example, the helical antenna can be tapered in order to obtain a desired shape of the overall package of the antenna  405  (e.g., to conveniently fit in the space in the curved portion of header shown in  FIGS. 4  and  5 ), or to tune the telemetry circuit  110 . In certain examples, the helically coiled antenna  406  can include more than one coil, such as a first tapered or non-tapered outer coil and one or more than tapered or non-tapered one inner coil. 
         [0061]      FIG. 7   a  illustrates generally an example of a cross sectional top view of the example of the helically coiled antenna  406  of  FIG. 7 . 
         [0062]      FIG. 8  illustrates generally an example of a helically coiled antenna  406  having a uniform or substantially non-tapered helix. In an example, the helical form can be uniform, such as to shape the overall package of the antenna  405  into a desired shape (e.g., to fit in the space in the header shown in  FIGS. 4 and 5 ), or to tune the telemetry circuit  110  as desired. In certain examples, the helically coiled antenna  406  can include more than one coil, such as a first outer coil and one or more than one inner coil. 
         [0063]      FIG. 8   a  illustrates generally a cross sectional top view of the helically coiled antenna  406  of  FIG. 8 . 
         [0064]    In certain examples, the antenna  406  can include a printed circuit (PC) board type antenna in which an antenna can be printed or otherwise formed on or mounted to the PC board and then mounted or inserted on or into the IMD  105  (e.g., on or into the housing  106  or the header  107 ). In an example, an antenna layer can be printed on each side of a PC board. In certain examples, the PC board can include multiple layers, with one or more of the layers including an antenna. Either or both of the area and layer dimensions of the PC board can be used to obtain a desired two-dimensional or three-dimensional antenna structure. In certain examples, the PC board can have a first layer having a first antenna configuration configured to communicate at a first frequency and a second layer having a second antenna configuration configured to communicate at a second frequency. In certain examples, such first and second different-frequency antennas can be formed on the same layer of the PC board. 
       HOUSING ANTENNA EXAMPLES 
       [0065]      FIG. 9  illustrates generally an example of an IMD  105  including a housing  106  and a header  107 . In certain examples, the outer or inner surface of the housing  106  can include or be formed of a conductive material. In an example, at least a portion of the conductive surface of the housing  106  can be insulated and an antenna  905  can be etched, stamped, deposited or otherwise formed on the insulated layer. In certain examples, the inner or outer surface of the housing  106  can include an insulator (e.g., ceramic, plastic, etc.). If the inner or outer surface of the housing  106  includes an insulator, then the antenna  905  can be etched, stamped, deposited, or otherwise formed on the insulator. 
         [0066]    In an example, at least a portion of the antenna  905  can be configured to be located (e.g., etched, deposited, or otherwise formed) underneath the header  107 . In certain examples, at least a portion of the antenna  905  can be configured to be located on at least one of the outer surface or the inner surface of the housing  107 . If the housing  107  is conductive, then it may be desirable to locate the portion of the antenna  905  on the outer surface of the housing  107 , such as to avoid or reduce shielding, or to use the housing as a ground plane, if desired. In an example, the antenna  905  can include one or more of a fractal antenna, a spiral antenna, a serpentine antenna, a loop antenna, a straight wire antenna, a patch antenna, or other antenna configuration. 
         [0067]    Further, the antenna  905  can be configured in a position or an orientation such that at least a portion of the antenna  905  can be useful as a visual or other identifier (e.g., a brand, model, bar code, etc.) for the IMD  105 . In an example, a fractal antenna can be positioned to appear as an identifier (e.g., a name, number, or other signifier that can identify an individual unit, a model, a brand, a user, a patient, etc.). In certain examples, other antenna configurations can be positioned to appear as an identifier. In certain examples, the identifier can be separate from the antenna, but formed together with the antenna  905 . For example, when the antenna is etched, deposited, or otherwise formed, the identifier can be concurrently etched, deposited, or otherwise formed using all or part of the process used for forming the antenna  905 . 
       LOADED ANTENNA EXAMPLES  
       [0068]    Generally, the length of an antenna determines the frequency the antenna is configured to transmit or receive. For example for a straight wire antenna, the antenna length should be approximately one-quarter of the desired wavelength. A loaded antenna includes an antenna (e.g., a wire antenna) that can be reactively loaded, such as by forming a coil or by placing an inductor in the length of the antenna. Placing the coil or the inductor in the length of the antenna can change the appearance or behavior of the antenna (e.g., change the impedance of the antenna, make the antenna appear electromagnetically longer, thereby allowing the antenna to be physically shorter to receive a desired frequency, etc.). The loaded antenna can be small in size, omni-directional, and it can be tuned for different frequencies or environments such as by simply adjusting or changing the inductance or location of the coil or inductor. This is in contrast to placing a coil or an inductor within a device (rather than within the antenna) to transfer or match the impedance of an antenna to the impedance of other telemetry circuit  110  components (e.g., the receiver, the transmitter, the transceiver, etc.). However, there can be a tradeoff between the physical size of the antenna and efficiency. Typically, as the length of the coil increases, the overall size of the antenna decreases, but the efficiency of the overall antenna decreases as well. 
         [0069]      FIG. 10   a  illustrates generally an example of a base loaded wire antenna  1005 . The base loaded wire antenna  1005  includes a coil  1007  at or near the base of the antenna (e.g., the base of the antenna being located at the end of the antenna that is proximal to a local transceiver electrically driving the antenna) coupled to a straight wire portion  1006 . In certain examples, other types of antennas (e.g., helical, spiral, serpentine, etc.) can be base loaded using a coil or an inductor. 
         [0070]      FIG. 10   b  illustrates generally an example of a center loaded wire antenna  1010 . The center loaded wire antenna  1010  includes a straight wire length  1011  having a coil at or near the center of the antenna. In certain examples, other types of antennas (e.g., helical, spiral, serpentine, etc.) can be center loaded using a coil or an inductor. 
         [0071]      FIG. 10   c  illustrates generally an example of a top loaded wire antenna  1015 . The top loaded wire antenna  1015  includes a straight wire length  1016  having a coil at or near the top of the antenna (e.g., the top of the antenna being located at the end of the antenna that is distal to the local transceiver electrically driving the antenna). In certain examples, other types of antennas (e.g., helical, spiral, serpentine, etc.) can be top loaded using a coil or an inductor. 
         [0072]    The location of the coil or the inductor in the loaded antenna can affect the antenna&#39;s power profile. Generally, as the coil or the inductor is moved away from the base and toward the top of the antenna length, the power profile becomes better. By configuring the location of the coil or the inductor to be adjustable, for example, the antenna can be tuned, such as to match the RF output of the telemetry circuit  120  of the external device  115 , without opening the housing  106  or adding additional components. Also, the same main assembly can be used for different frequencies because tuning can be accomplished external to the device (e.g., such as if the location of the coil or inductor is outside of the housing  106 ). (See generally sketches of current distribution along an antenna length across from  FIGS. 10   a - 10   c. ) 
         [0073]      FIG. 11  illustrates generally an example of a multiband loaded antenna  1105 . In certain examples, the multiband loaded antenna  1105  includes a straight wire length  1106  having one or more than one coil or inductor (e.g., more than one coil or inductor, such as the first inductor  1107  and the second inductor  1108 ) in its length. In certain examples, the multiband loaded antenna  1105  can include one or more other types of antennas (e.g., helical, spiral, serpentine, etc.) having one or more than one inductor. 
         [0074]    The one or more than one coil or inductor can effectively be used to create a multiband antenna such as by blocking high frequencies along at least a portion of the length of the antenna. The frequency being blocked depends on the inductance or the location of the one or more than one coil or inductor. Generally, at high frequencies the multiband loaded antenna  1105  appears to have a first length (e.g., high band  1110 ). At low frequencies, the multiband loaded antenna  1105  appears to have a second length (e.g., low band  1111 ). Thus, the multiband loaded antenna  1105  can be tuned to operate in more than one frequency (e.g., by altering the location or value of the one or more than one inductor). In certain examples, the multiband loaded antenna  1105  can be configured with its tuning coils or inductors located so as to obtain operation in each of the MICS and ISM bands. 
         [0075]      FIG. 12  illustrates generally an example of a capacitive loaded antenna  1205 . An antenna can be tuned by adding capacitance or inductance. In an example, the capacitive loaded antenna  1205  can include a straight wire length  1206  and a capacitor  1215 . In an example, the capacitor  1215  can include a conductive disk or other object capable of storing a charge. In certain examples, the capacitive loaded antenna  1205  can include one or more than one inductor  1207  (e.g., configured to further tune or reduce the effective length of the capacitive loaded antenna  1205 ). 
         [0076]      FIGS. 13   a - 13   f  illustrate generally an example of a loaded antenna configuration or location with respect to an IMD  105 . 
         [0077]      FIG. 13   a  illustrates generally an example of an IMD  105  including a base loaded antenna  125  having a coil in the housing  106  and the remainder of the base loaded antenna  125  located in the header  107 . In an example, the coil (or inductor) can be shielded in the housing  106  because the loaded antenna has a majority of its radiation coming from the non-coiled portion of the loaded antenna. The non-coiled portion of the loaded antenna can be located outside of the housing  106 , such as if the housing  106  is conductive so as to act as a shield. The coil can be used to tune the antenna to a desired frequency, and need not be relied upon to radiate. 
         [0078]      FIG. 13   b  illustrates generally an example of an IMD  105  including a base loaded antenna  126  having a coil and the remainder of the base loaded antenna  126  located in the header  107 . 
         [0079]      FIG. 13   c  illustrates generally an example of an IMD  105  including a base loaded antenna  127  having a coil located in the header  107  and the remainder of the base loaded antenna located in a combination of the header  107  and outside of the header  107  and the housing  106 . In this example, the remainder of the base loaded antenna  127  can be configured to be a certain distance from the outside of the housing  106 . It can be advantageous for at least a portion of the antenna to remain equidistant from the housing  106  or other conductive surface (e.g., a ground plane). In certain examples, the remainder of the base loaded antenna  127  can be configured to move away from the IMD  105  or along the header  107 . 
         [0080]      FIG. 13   d  illustrates generally an example of an IMD  105  including a base loaded antenna  128  having a coil and the remainder of the base loaded antenna  128  located outside of the IMD  105 . In this example, the coil and the remainder of the base loaded antenna  128  can be configured to be a certain distance from the outside of the housing  106 . In certain examples, the coil or the remainder of the base loaded antenna  128  can be configured to move away from the IMD  105  or along the header  107 . 
         [0081]      FIG. 13   e  illustrates generally an example of an IMD  105  including a center loaded antenna  129  having a coil located between a first and second portion of the remainder of the center loaded antenna  129 . In this example, the first portion of the center loaded antenna  129  can be located in the header and the coil and the second portion of the center loaded antenna  129  can be located outside of the IMD  105  along the housing  106 . In an example, the center loaded antenna  129  (including the coil and the first and second portions) can be configured to be a certain distance from the outside of the housing  106 . In certain examples, the coil or the second portion of the center loaded antenna  129  can be configured to move away from the IMD  105  or along the header  107 . 
         [0082]      FIG. 13   f  illustrates generally an example of an IMD  105  including a top loaded antenna  130  having a coil and the remainder of the top loaded antenna  130  located outside of the IMD  105 . In this example, the coil and the remainder of the top loaded antenna  130  can be configured to be a certain distance from the outside of the housing  106 . In certain examples, the coil or the remainder of the top loaded antenna  130  can be configured to move away from the IMD  105  or along the header  107 . 
         [0083]    In the examples of  FIGS. 13   a - 13   f,  at least a portion of the antennas can be separated from the outside of the housing  106 , if the housing is conductive, using an insulator (e.g., molded plastic, ceramic, etc.). Further, any materials used for the antenna outside of the IMD  105  can be biocompatible or capable of being safely implanted and safely reside inside a body. 
       Multi-Length Antenna 
       [0084]    Generally, it is desirable not only that an antenna used for an IMD have good performance when implanted, but also that the antenna have good performance in air before the IMD is implanted. For example, it can be beneficial to establish a communication link before the IMD is implanted to test the device before implantation, to program, preprogram, or reprogram the device prior to implantation, or to otherwise communicate with the IMD prior to implantation. However, electromagnetic waves travel differently in air (having a dielectric constant of ˜1) than they do in tissue (having a dielectric constant of ˜50-70). One way to accommodate for this difference is to detune the antenna to have acceptable performance in both air and tissue. However, this decreases the antenna performance after implantation. In contrast to detuning the antenna, an antenna can be tuned to receive a desired frequency in more than one medium (e.g., tissue and air) having different physical characteristics (e.g., different dielectric constants). 
         [0085]      FIG. 14  illustrates generally an example of an IMD  105  including a housing  106 , a header  107 , and an antenna  1405 . In this example, the antenna can be configured to have or maintain a set distance from either the housing  106  or the outer surface of the header  107 . In certain examples, a ground plane can be placed between the header  107  and the housing  106 . 
         [0086]    Generally, if the antenna  1405  is placed too close to body fluid or tissue, the variations in the medium properties (e.g., the dielectric constant) or the conductivity of the tissue or body fluid can affect the performance of the antenna. Further, the radiation power of the antenna  1405  is proportionate to the area between the housing  106  and the antenna  1405 . So, if the antenna  1405  is placed too close to the housing  106  (if the housing  106  includes a conductor), the radiation field will collapse. Thus, by adjusting the distance from the housing  106  and the body fluid or tissue, a point of best performance can be found. 
         [0087]      FIG. 15  illustrates generally an example of an IMD  105  having a housing  106 , a header  107 , a first antenna  1505 , and a second antenna  1510 . Generally, the desired antenna length is roughly inversely proportionate to the square root of the dielectric constant of the antenna&#39;s medium. Thus, while an antenna having a first length is optimal in a medium having a high dielectric constant (tissue˜50), the optimal length in a medium having a low dielectric constant (air˜1) is much longer. It can be desirable to couple a removable external antenna to the MD for in-air communication prior to or following implantation in a body. 
         [0088]    In this example, the second antenna  1510  can include an external antenna coupled to the first antenna  1505  of the IMD  105 . In an example, the second antenna  1510  can be capacitively coupled to the first antenna  1505  (or directly coupled), increasing the overall length of the combined antenna in the IMD for in-air communication. In certain examples, the second antenna  1510  can be attached to the outer surface of the IMD  105  (e.g., using an adhesive or other attachment method). The second antenna  1510  can be removed prior to implantation of the IMD  105  into a body. In certain examples, the second antenna  1510  can be a different color than the rest of the IMD  105  to draw attention, or the second antenna  1510  can include a tag with a warning label or other notification to a physician so it is not implanted with the IMD  105 . 
         [0089]      FIGS. 16   a - 16   c  illustrates generally an example of a multi-length antenna  1605 . An antenna structure in a first medium having a first dielectric constant can appear electrically different than the same antenna structure in a second medium having a second dielectric constant. In an example, the multi-length antenna  1605  is configured in an antenna structure that can appear to be a first shape having a first length in a first medium, and that can appear to have a second shape having a second length in a second medium. 
         [0090]      FIG. 16   b  illustrates generally an example of a first equivalent multi-length antenna  1606 , or the electrical equivalent of the multi-length antenna  1605  in a medium having a low dielectric constant (e.g., air, which has a dielectric constant of about 1). In contrast,  FIG. 16C  illustrates generally an example of a second equivalent multi-length antenna  1606 , or the electrical equivalent of the multi-length antenna  1605  in a medium having a high dielectric constant (e.g., tissue, which has a dielectric constant of about 20-50, or body fluid, which has a dielectric constant of about 50-70). In certain examples, the electrical equivalent antenna varies depending on the distance between the sections of the antenna. Thus, in a medium having a higher dielectric constant, the distance between the sections of the antenna can be farther apart and still couple, creating the shorter electrical equivalent antenna. However, in a medium having a lower dielectric constant, the distance between the sections of the antenna need to be closer together to couple. In order to have an antenna that appears to be one length in one medium and another length in another medium, the distance between the sections of antenna must be such that the coupling occurs in one medium and does not occur in the other. 
         [0091]    Generally, there is lower capacitive coupling between two conductors in a medium having a low dielectric constant (e.g., air). Therefore, in the example of  FIG. 16   b,  the first equivalent multi-length antenna  1606  remains similar to the original multi-length antenna  1605 . In contrast, there is higher capacitive coupling in a medium having a high dielectric constant (e.g., body fluid). Thus, in the example of  FIG. 16   c,  the second equivalent multi-length antenna  1607  is significantly different than the original multi-length antenna  1605 . 
         [0092]    In an example, the multi-length antenna  1405  can appear as a first antenna in a first medium (e.g.,  FIG. 16   b ) and a second antenna in a second medium (e.g.,  FIG. 16   c ). Therefore, one antenna can be tuned to receive a desired frequency in multiple mediums having different dielectric constants. 
         [0093]      FIG. 17  illustrates generally an example of a Smith Chart  1700  illustrating the impedance of a multi-length antenna (similar to that shown in  FIG. 16   a ). The two points on the chart illustrate that it can be possible to match the impedance of the antenna in the first and second mediums even though their dielectric constants are different. Matching the impedance in the first and second medium can allow for an increase in overall performance of the telemetry circuit  110 . 
         [0094]      FIG. 18  illustrates generally a multi-band antenna  1805  having more than one resonant length (e.g., L 1  and L 2 ). In this example, the first length (L 1 )  1806  will resonate at a first frequency, and the antenna&#39;s second length will resonate at a second frequency. 
         [0095]      FIGS. 19   a - 19   b  illustrate generally a multi-band/multi-length antenna  1905 . In the example of  FIG. 19   a,  the multi-band/multi-length antenna  1905  appears as a first length in air and is configured to receive both a first desired frequency and a second desired frequency, depending upon the set spacing between antenna elements. 
         [0096]    In the example of  FIG. 19   b,  the multi-band/multi-length antenna  1905  appears as a second length in tissue or fluid and is configured to receive both a first desired frequency and a second desired frequency, depending upon the set spacing between antenna elements. 
         [0097]      FIG. 20  illustrates generally an example of a multi-length antenna  2005  oriented along more than one axis. In certain examples, the orientation of a multi-length or multi-band antenna can be changed in order to save or accommodate spatial requirements or to better direct radiation in desired directions. The example in  FIG. 20  illustrates a multi-length antenna  2005  with a 90 degree change along a first axis. In other examples, other orientations, such as an angles or bends other than 90 degrees or angles or bends along other axis besides that shown in  FIG. 20 . Further, in an example, the multi-length antenna  2005  can twist along one or more axis to further increase the direction of radiation. 
         [0098]      FIGS. 21-22  illustrate generally examples of a holder  2110  for an antenna  2105 . In an example, the holder  2110  can be composed of a non-conductive material (e.g., plastic, etc.). Generally, the holder  2110  includes a gap running along the length of the holder  2110  in which the antenna  2105  can reside.  FIG. 21  illustrates an example of the holder  2110  on the header of the IMD.  FIG. 22  illustrates an example of the holder  2110  on the housing of the IMD. In certain examples, the antenna  2105  can be placed in the gap along the length of the holder  2110 . In an example, the antenna  2105  can be flush along the sides of the holder  2110 . In other examples, there can be space between the holder  2110  and the antenna  2105  allowing tissue or fluid to encompass, contact, or surround at least a portion of the antenna  2110 . In certain examples, the dielectric value of the tissue or body fluid can assist the performance of the antenna. Generally, the closer the impedance of the antenna and the medium the antenna communicates through, the better the performance of the communication. 
         [0099]      FIGS. 23   a - 23   b  illustrate generally examples of multi-length antennas. It can be advantageous to keep a first point of an antenna and a second point on the antenna as perpendicular as possible to increase radiation. 
         [0100]      FIG. 23   a  illustrates generally an example of a multi-length antenna  2305  having a substantial portion of the antenna perpendicular to a first point on the antenna. Thus, the distance between the beginning of the antenna and each point along each perpendicular line is roughly equivalent. 
         [0101]      FIG. 23   b  illustrates generally an example of a multi-length antenna  2306  not having a substantial portion of the antenna perpendicular to a first point on the antenna. In contrast to the antenna shown in  FIG. 23   a,  the distance between the beginning of the antenna and each point along each perpendicular line is not roughly equivalent. Rather, the distance is greater along the ends of each perpendicular line. 
         [0102]      FIG. 24  illustrates generally an example of a multi-length antenna  2405 . In this example, the multi-length antenna  2405  shares a common horizontal line amidst the remaining structure. 
         [0103]      FIG. 25  illustrates generally an example of a capacitively loaded antenna  2505  having a first capacitor  2506  and a second capacitor  2507 . Although capacitive loading does not change the effective length of the antenna, in certain examples, capacitance can be added to an antenna in order to tune the antenna or provide a desired impedance along its surface or length. 
         [0104]      FIGS. 26   a - 26   c  illustrates generally a capacitively loaded antenna section  2605 .  FIG. 26   a  illustrates that a wire antenna (or other antenna) can be pinched in order to change the geometry or the electrical appearance of the antenna. In an example, changing the geometry or the electrical appearance of the antenna can help tune the antenna to better receive a desired frequency (e.g., increase or decrease the bandwidth or efficiency). In certain examples, pinching the antenna can result in disk-like features along the length similar to adding capacitors.  FIG. 26   b  illustrates a side view of  FIG. 26   a.    
         [0105]      FIG. 26   c  illustrates an alternative to pinching the antenna section. In an example, the wire antenna (or other antenna) can be smashed (or otherwise stamped), such as smashed into a crevice or other apparatus to create a sharp bend or other deformity in the antenna that changes the electrical appearance of the antenna section. 
       OTHER EXAMPLES  
       [0106]      FIG. 27  illustrates generally an example of an IMD  105  including a housing  106 , a header  107 , and an antenna  2710  located inside of the header  107 . In an example, the antenna  2710  can include a patch antenna. Generally, a patch antenna includes a piece of metal over a ground plane. In certain examples, the patch antenna or the ground plane can be folded or curved to increase the directional coverage of the antenna. 
         [0107]    In other examples, other types of antennas can be included in the header, such as an overmold antenna, or the header  107  itself can have a conductor placed on its outer or inner surface to use as an antenna. Generally, the overmold antenna can include a molded support having metal placed over the surface of the mold, where at least one of the support or the metal has been shaped to transmit or detect at a desired frequency. 
         [0108]      FIGS. 28   a - 28   b  illustrate generally an example of an IMD  105  including a housing  106 , a header  107 , and an antenna  2805  on the surface of the housing  106 . In an example, the antenna  2805  (e.g., a patch antenna or other relatively flat antenna) can be placed on the exterior surface of the housing  106 . If the housing  106  is conductive, then an insulator must first be put down between the antenna  2805  and the housing  106 . As shown in  FIG. 28   a,  two half-circle shaped antennas (as well as many other configurations, e.g.,  FIG. 28   b ) can be used as the antenna  2805 . If the housing  106  is not conductive, then the antenna  2805  can be put directly on (or in) the surface of the housing  106 . In other examples, the antenna  2805  can be placed on the inside surface of the housing  106 . 
         [0109]      FIG. 29   a  illustrates generally an example of a telemetry circuit  110  coupled to a first antenna  2905  or a second antenna  2906  through a switch  2907 . In an example, the first antenna  2905  or the second antenna  2906  can include a patch antenna or other type of antenna. In certain examples, the first antenna  2905  can be tuned to a first desired frequency and the second antenna  2906  can be tuned to a second desired frequency. In other examples, the first antenna  2905  can be tuned to a desired frequency in a first medium (e.g., air) and the second antenna  2906  can be tuned to the same (or another) desired frequency in a second medium (e.g., tissue). The switch  2907  then operates to select which antenna to present to the remainder of the telemetry circuit  110 . In an example, the operation of the switch  2907  can be controlled dependent upon the information received from the first antenna  2905  or the second antenna  2906 . In other examples, the state of the switch is changed following or during implantation of the IMD  105 . In an example, the switch  2907  continuously or periodically changes states until information is received from one antenna and not the other, or until the information received using one antenna is determined to be incorrect, noisy, or too weak to receive. 
         [0110]      FIG. 29   b  illustrates generally an example of a telemetry circuit  110  coupled to a first antenna  2908  and a second antenna  2909 . In this example, the telemetry circuit  110  can drive both the first antenna  2908  and the second antenna  2909 . However, because the first antenna  2908  and the second antenna  2909  were either tuned for separate frequencies or for separate mediums (having different dielectric constants), one antenna resonates. In an example, the other antenna can reflect the non-resonating information. Whereas in  FIG. 29   a  the switching occurred at the antenna, here, the determination of antenna comes from the control of the driving signal. In certain examples, the same methods regarding switching for  FIG. 29   a  can be used to determine which frequency to drive the antennas at. 
       Impedance Match Tuning Networks 
       [0111]      FIG. 30  illustrates generally an example of a telemetry circuit  110  having a transceiver  3005 , a matching network  3010 , and an antenna  3015 . A system and method can be used for tuning the telemetry circuit  110  impedance before or after implantation of the telemetry circuit  110  in a body. First, an impedance must be sensed or estimated. In an example, the IMD can monitor the transmitter current (a real current) to estimate the tuning of the telemetry circuit  110 . In other examples, actual measurements of impedance can be made to estimate the tuning of the telemetry circuitry. 
         [0112]    In an example, once the indication of tuning is received, the matching network  3010  can alter the impedance of the tuning circuit.  FIGS. 30   a - 30   f  illustrate alternate embodiments regarding the components of the matching network. 
         [0113]      FIGS. 30   a  and  30   c  illustrate generally a matching network  3010  having switched capacitors in parallel with the transceiver  3005 .  FIG. 30   b  illustrates generally the matching network  3010  having switched capacitors in series with the transceiver  3005 .  FIGS. 30   d  and  30   f  illustrates generally the matching network  3010  having switched inductors in parallel with the transceiver  3005 .  FIG. 30   e  illustrates generally the matching network  3010  having switched inductors in series with the transceiver. 
       Transmission-Line Antenna 
       [0114]      FIG. 31  illustrates generally an example of an IMD  105  having a housing  106 , a header  107 , and an antenna  3105 . In this example, the antenna  3105  can include a transmission-line style antenna (e.g., a microstrip over a ground plane, a coaxial wire, a twisted pair, etc.). However, to have an effective transmission-line antenna, the distance between the conductor and the ground plane must remain constant. If the distance varies, the electrical characteristics of the telemetry circuitry can change, which can introduce loss or poor radiation. 
         [0115]    In the example of  FIG. 31 , the conductor includes a microstrip. The microstrip can be fabricated using printed circuit board (PCB) technology. In an example, the ground plane can include the housing  106 . If the housing  106  is not a grounded conductor, then a ground plane can be deposited or otherwise added below or around the microstrip. In an example, the ground plane can be added using the same PCB as the microstrip conductor (e.g., using a dual sided board, using a multilayer board, etc.). 
         [0116]      FIG. 31   a  illustrates generally an example of a conductor  3105  over a dielectric  3110  on the surface of the housing  106  or the header  107 . In certain examples, the dielectric  3110  and the conductor  3105  can be located over the header  107  and not the housing  106 , the housing  106  and not the header  107 , or a combination of the header  107  and the housing  106 . In certain examples, the conductor  3105  over the dielectric  3110  can include a piece of metal placed over the dielectric  3110 , a layer of metal deposited on the dielectric  3110 , etc. 
         [0117]      FIG. 31   b  illustrates generally an example of a conductor  3105  located in the middle of a dielectric  3110  on the surface of the housing  106  or the header  107 . In an example, the thickness of the dielectric can be controlled during the production of the In an example, the distance from the housing  107  (or other ground plane) and the medium surrounding the IMD  105  (e.g., tissue or fluid) can be optimized or tuned to give a desired power at a desired bandwidth. 
         [0118]    In an example, a lossy transmission line having a controlled and predictable impedance (the impedance changes in relation to the distance) can be preferred. In other examples, a distributed transmission line can be preferred because of the continuous structure. 
         [0119]    In certain examples, the telemetry circuit  110  can be tuned to increase the efficiency at the cost of reducing bandwidth. By reducing the bandwidth of the telemetry circuit  110 , the amount of unwanted noise (e.g., MRI noise, 60 Hz noise, or any unwanted communication or electromagnetic field) entering the IMD  105  through the telemetry circuit  110  can decrease. In other examples, other techniques can be used to not allow noise through the telemetry circuit (including the feed-through into the housing  106 ), such as shorting received information (e.g., transmission or noise) to the housing when the IMD  105  is not expecting to receive information. In an example, the time for receipt of information from the telemetry can be cycled so as to regularly check to receive information, but also filtering out unwanted noise for a majority of the cycle. In an example, transmission can be allowed while reception is being shorted to the housing. 
       Other Notes 
       [0120]    The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” All publications, patents, and patent documents referred to in this document are incorporated by reference herein in their entirety, as though individually incorporated by reference. In the event of inconsistent usages between this document and those documents so incorporated by reference, the usage in the incorporated reference(s) should be considered supplementary to that of this document; for irreconcilable inconsistencies, the usage in this document controls. 
         [0121]    In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. 
         [0122]    The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to comply with 37 C.F.R. §1.72(b), to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separate embodiment. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.