Abstract:
An improved telemetry head that employs two coils is disclosed. A first, larger diameter outer coil is generally employed to initiate telemetric communication between the programmer or monitor and the implanted device. In the event that successful communication is not accomplished using the outer coil, the programmer or monitor attempts communication using the inner coil. Any resultant reduced signal strength that may be associated with use of the smaller diameter inner coil is acceptable based on the assumption that the antenna of the implanted device is located in the null associated with the outer antenna coil and thus is located in close proximity to the inner coil. To prevent loss of signal strength, addition turns may be employed to implement the inner coil. In one embodiment, at least one of the first and second coils is of a dual-coil configuration having two coils in series opposition to improve the signal-to-noise ratio.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates generally to programmers or monitors intended for use with implantable medical devices and more specifically to antennas adapted for communication between implanted medical devices and external programmers or monitors. 
     In the context of implantable medical devices, it has become common to provide a communication link between the implanted device and an external programmer or monitor in order to allow for transmission of commands from the external device to the implanted device and to allow for transmission of stored information and/or sensed physiological parameters from the implanted device to the external programmer. Conventionally, communication between an implanted device and an external programmer or monitor has been accomplished by means of a telemetry system which includes a transceiver located within the implanted medical device and an external programmer or monitor, each having a radio transmitter/receiver and an associated antenna. 
     The implanted device typically includes an antenna located either within the hermetic device housing containing the circuitry, as disclosed in U.S. Pat. No. 4,542,532 issued to McQuilkin, in a plastic header or connector block used to interconnect the device to electrical leads as disclosed in U.S. Pat. No. 5,697,958 issued to Patrick et al. or mounted to the device housing as in U.S. Pat. No. 5,861,019 issued to Sun et al. and U.S. Pat. No. 5,720,770 issued to Nappholz et al., all incorporated herein in their entireties. The programmer or monitor typically includes or consists of a telemetry head containing an antenna, intended to be placed on the patient&#39;s body in close proximity to the implanted device. The telemetry head may be coupled to the external programmer or monitor by means of a cord, as disclosed in U.S. Pat. No. 5,766,232 issued to Grevious et al. In alternative systems, as described in U.S. Pat. No. 5,404,877 issued to Nolan and U.S. Pat. No. 5,113,869 issued to Nappholz, the programmer or monitor may be provided with an antenna located some feet away from the implanted device. 
     One common telemetry head antenna configuration is a coil antenna mounted parallel to a major surface of the telemetry head&#39;s outer enclosure, which surface is located on or closely adjacent the patient&#39;s body during telemetric communication. Such telemetry heads are disclosed in U.S. Pat. Nos. 5,527,348 and 5,562,714, both incorporated herein by reference in their entireties. Coil antenna telemetry systems such as these have the disadvantage that they display a signal strength minimum or “null” in the region directly beneath and close to the coil. In the context of telemetry systems employing transmission antennas located at substantial distances from their associated receiving antennas, one solution to eliminating null spots has been to provide diversity antenna arrays employing antennas which are either oriented to display different polarizations or are separated spatially from one another. Examples of such antenna arrays are disclosed in U.S. Pat. No. 6,009,878 issued to Wiejand. In the context of programmers for implantable medical devices, the problem of nulls is generally addressed by the physician moving the telemetry head relative to the implanted device until an appropriate spatial relationship is attained. 
     SUMMARY OF THE INVENTION 
     The present invention is directed toward an improved telemetry head which employs two coils that may alternatively be employed to communicate with the implanted device. A first, larger diameter outer coil is generally employed in conjunction with initiation of telemetric communication between the programmer or monitor and the implanted device. The telemetry head is first placed in a location that the physician believes to be located generally over the implanted device, and the larger diameter outer coil is first employed in an attempt to initiate telemetric communication with the implanted device. In the event that successful communication is not accomplished using the outer coil, programmer or monitor attempts communication using the inner coil, under the assumption that the implanted device is located directly under some portion of the outer coil at a relatively shallow depth, falling into the null. Any resultant reduced signal strength that may be associated with use of the smaller diameter inner coil is acceptable based on the assumption that the antenna of the implanted device is located in the null associated with the outer antenna coil and thus is located in close proximity to the inner coil. It may be noted that additional coil turns may be used to implement the smaller inner coil to maintain signal strength. Only if successful communication between the programmer and the implanted device cannot be accomplished using either of the two coils, will the physician be required to move the telemetry head to an alternative location. Therefore, in the particular circumstances associated with telemetry heads used in close proximity to implanted medical devices, the multi-coil antenna array provides a simple and elegant mechanism for simplifying the process of locating the telemetry head relative to the implanted device. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a programmer or monitor provided with a telemetry head according to the present invention. 
     FIG. 2 is a schematic view of a telemetry head according to the present invention. 
     FIG. 3 is a functional diagram illustrating the interconnection of the antenna array, RF circuitry and other functional components of the programmer or monitor illustrated in FIG.  1 . 
     FIG. 4 is a schematic diagram of an alternative dual-coil embodiment of an antenna that may be used in the system of the present invention. 
     FIG. 5 is a diagram illustrating the manner in which two dual-coil antennas may be positioned with respect to each other. 
     FIG. 6A is a diagram illustrating an inner and outer coil configuration wherein the inner and outer coils are not concentric. 
     FIG. 6B is a diagram illustrating an inner and outer coil antenna wherein the inner and outer coils are not concentric, and wherein both the inner and outer coils include two coils is series opposition to improve the signal-to-noise ratio. 
     FIG. 7 is a diagram illustrating an inner and outer coil antenna wherein the inner and outer coils are not concentric or co-planar. 
     FIG. 8 is a diagram illustrating an inner and outer coil antenna wherein the inner and outer coils have arbitrary geometries with respect to each other. 
     FIG. 9 is a diagram illustrating an inner and outer coil antenna wherein only the inner coil is of a dual-coil configuration. 
     FIG. 10 is a diagram illustrating an inner and outer coil antenna wherein one of the inner coils is co-planar with respect to an associated outer coil, and wherein the other inner coil is not co-planar with respect to the associated outer coil. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is an illustration of a programmer or monitor and associated telemetry head incorporating the present invention. The programmer or monitor  10  is provided with a display  16  which may be employed to display information received from or to be sent to an implanted device. Keyboard  18  is employed control the operation of the programmer or monitor, to select parameters for the implanted device, and to determine the sorts of information desired to be received from the implanted device. A touch sensitive display may of course be substituted for separate keyboards and display, for example as disclosed in U.S. Pat. No. 5,345,362, issued to Winkler, incorporated herein by reference in its entirety. 
     The telemetry head  12  is illustrated as coupled to the programmer or monitor  10  by means of a coiled multi-conductor cable  13 , and is provided with one or more indicators and/or switches  14  which may be employed as switches and indicators have typically been employed in the context of programmers for implantable devices, including as indicators of a valid telemetry link and as controls for initiating telemetry transmissions. For example, such indicators and control buttons are included in U.S. Pat. No. 5,324,315 issued to Grevious and incorporated herein by reference in its entirety. The lower major surface  12   a  of the telemetry head is intended to be placed on or adjacent the skin  17  of the patient. Located within telemetry head  12  are two concentric, generally co-planar coil antennas located parallel to the lower major surface  12   a  of the telemetry head, each coupled to the programmer or monitor  10  by means of cable  13 . Programmer or monitor  10  is provided with an R-F transceiver located therein, which may be selectively coupled to either of the two coil antennas in the telemetry head  12  by means of an R-F switching circuit, located either internal to programmer or monitor  10  or internal to telemetry head  12 . In additional alternative embodiments, the R-F transceiver circuitry may also be located internal to telemetry head  12 , or the entire programmer or monitor may be incorporated into a unitary, hand held device, for example as disclosed in U.S. Pat. No. 5,311,449 issued to Adams, and U.S. Pat. No. 5,565,005 issued to Erickson. 
     Illustrated at  15  is an implanted pacemaker, located under the skin of the patient, indicated by broken line  17 . In use, the telemetry head  12  is placed on the patient&#39;s skin  17 , over the implanted pacemaker  15 , and an attempt to establish a telemetry link is first made using the outermost of the two coils located within telemetry head  12 . In response to a failure to establish a communication link using the outermost of the two coils, the innermost of the two coils is then employed in an attempt to establish a telemetry link with the implanted device  15 , on the assumption that it is located in the null directly under a portion of the outermost coil. In the event that neither coil may be successfully employed to establish a telemetry link, the physician will move the telemetry head slightly, in an effort to find a better location, in a manner analogous to that employed by prior art single-coil or dual-coil telemetry heads. 
     FIG. 2 is a schematic illustration of the structure of the telemetry head  12 , with the enclosure  30  of the telemetry head  12  indicated by broken line. Located within the enclosure  30  are two multi-turn coil antennas  20  and  22 , each coupled to a pair of mutually insulated conductors,  28  and  26 . Conductor pairs  26  and  28  may extend to the programmer  10  (FIG. 1) within the cable  13 , or alternatively, may be coupled to switching circuitry located within the telemetry head enclosure  30 . Optionally located within the inner coil  22  is a permanent magnet  24  to be employed in activating the reed switch on those implanted devices in which reed switches are employed in conjunction with telemetry operations. The physical structure of the telemetry head  12  may, for example, correspond to that illustrated in U.S. Pat. No. 5,527,348 issued to Winkler et al. and incorporated herein by reference in its entirety, with the exception that two concentric coils are located in the telemetry head rather than a single coil as illustrated in the Winkler et al. patent. 
     FIG. 3 is a diagram illustrating the interconnection of the antenna array of the programmer or monitor  10  of FIG. 1 with the circuitry and other functional components included therein. The controller circuitry  46  selects which of the two antennas  20  and  22  is employed by means of control line  34  which controls antenna switch  32 . RF signals are carried between the antenna switch  32  and transceiver  38  via RF line  36 . 
     The circuitry within the programmer or monitor includes a microprocessor  50  which controls the operation of the device as a function of programming stored in RAM/ROM  54  and/or hard disk drive  52 , both of which are coupled to the microprocessor via data/address bus  48 . Commands from the physician are provided to the microprocessor via keyboard  18  and/or any additional control buttons on the telemetry head  12  (FIG. 1) and/or, if the display  16  is touch sensitive, from the display as well. Information regarding the operation of the programmer or monitor and information received from the associated implanted device are displayed on display  16 , under control of graphics circuitry  56 . The graphics circuitry, microprocessor, hard disk drive, RAM/ROM circuitry, keyboard and display may all correspond to corresponding components of personal computers and/or prior art programmers and monitors such as those described in the patents incorporated by reference above. 
     Operation of the telemetry system is controlled by controller circuit  46  which operates under control of microprocessor  50 . Transceiver  38  may be any of any appropriate type, including those described in the patents referred to above. In the particular embodiment illustrated, transceiver  38  may be configurable to operate at multiple frequencies or using multiple communication protocols, under control of controller  46  via control bus  43 . Controller  46  also configures the transceiver  38  to either transmit RF signals to the antennas or receive RF signals from the antennas by means of transmit/receive line  47 . Controller  46  provides the data to be telemetered to the implanted device to data processing circuitry  44  and receives decoded received data from the implanted device from data processing circuit  44 , via data bus  45 . Data provided by controller  46  to data processing circuitry  44  is converted therein from parallel to serial format and provided serially to transceiver  38  on TX data line  42 . Correspondingly, data received by transceiver  38  is provided in serial format on DEMOD line  40  to data processing circuitry  44 , and is converted therein to parallel format and provided to the microprocessor  50  via controller circuitry  46 . Controller  46  is also capable of monitoring the integrity of the telemetry link to allow for selection between the two antennas as described above. 
     In operation, during initialization of telemetry or monitoring functions requiring a telemetry link between the programmer/monitor  10  (FIG. 1) and an implanted device, the controller operates to first employ the larger diameter, outer coil antenna  20 , selected by means of switch  32  under control of control line  34 . In response to receipt of a valid telemetry transmission, the implanted device will in turn provide a return transmission so indicating. In the event that the received return telemetry transmission from the implanted device is acceptable, the outer coil antenna  20  is thereafter employed for purposes of maintaining the telemetry link. In the event that either no return telemetry transmission is detected using coil  20 , or in the event that the return transmission is of substandard amplitude and/or has a higher error rate than acceptable, the controller may thereafter switch to inner antenna  22  for use in attempting to establish a telemetry link with the implanted device. In the event that the received return telemetry transmission from the implanted device is acceptable, the inner coil antenna  22  may thereafter be employed for purposes of maintaining the telemetry link. In the event that neither of the two coil antennas provides an acceptable telemetry link, the device may be provided with an indicator, for example in the form of an LED located on the housing of the telemetry head, or an indicator on the display  16  in the programmer or monitor  10 . In such case, the physician would have to move the telemetry head  12  to reestablish a telemetry link. Similarly, in the event that after selection of the inner or outer coil antenna ( 22 ,  20 ), the telemetry link is thereafter lost, the controller  46  may thereafter attempt to employ one of the two antennas not previously in use to reestablish the telemetry link, and, if unsuccessful, the device may provide an indication of an inability to establish an accurate telemetry link, similarly requiring the physician to reposition the telemetry head. 
     The particular mechanisms employed by the controller to determine the integrity of the telemetry link may correspond to those generally known in the prior art, and may include, for example, monitoring the signal strength of the return transmission from the implanted device, as described in U.S. Pat. No. 5,683,432 issued to Goedeke et al., incorporated herein by reference in its entirety. Alternatively, the mechanism by which the programmer or monitor  10  determines the integrity of the transmission may involve a measurement of the number of errors per received transmission, as described in U.S. Pat. No. 4,531,523 issued to Anderson, also incorporated herein by reference in its entirety. However, other mechanisms for monitoring the integrity of the telemetry length may be substituted, within the confines of the present invention. 
     In conjunction with the physical structure of a programmer or monitor embodying the present invention, it is expected that in most cases, the outer and inner coils  20  and  22 , along with mutually insulated conductors  26  and  28  will be located in the cable  13  (FIG. 1) that connects the telemetry head to the programmer or monitor  10 . However, in alternative embodiments, the switching circuitry  32  illustrated in FIG. 3 might also be located in the telemetry head (FIG.  1 ), with control and R-F lines  34  and  36 , respectively, located within the cable  13  (FIG.  1 ). In other embodiments, the entire programmer/monitor structure illustrated in FIG. 3 might be incorporated into a small, hand held device, for use directly applied to the patient&#39;s body. The specific physical compartmentalization of the components of the monitor or programmer  10  (FIG. 1) is not critical to successful implementation of the present invention. 
     FIG. 4 is a schematic diagram of an alternative embodiment of a coil antenna that may be used in the system of the present invention. This antenna is the type shown and described in U.S. Pat. No. 4,542,532 to McQuilkin, incorporated herein by reference in its entirety. This antenna utilizes two substantially identical coils connected in series opposition to cancel noise signals common to both coils and to receive desired signals present at only one coil. 
     The embodiment of the dual coil antenna shown in FIG. 4 includes transmitter  110  and receiver  152 , both of which would be included with the accompanying circuit components in transceiver  38  of FIG.  3 . Transmitter  110  has its output connected to a parallel combination of a fixed resonant capacitor  116  and an adjustable resonant capacitor  114 , which is used for resonating the antenna at the transmitter frequency. The series-resonant capacitors are connected between the transmitter circuitry and the junction point  120 . Junction point  120  lies between the two series-connected antenna coils shown as  20   a  and  20   b , which in the present embodiment, may be substituted for the single coil  20  of FIG.  3 . 
     The two small black dots adjacent the junction points  126  and  128  of FIG. 4 indicate the polarity of winding of the coils  20   a  and  20   b . The winding polarity is such that the coils are in series-opposition. Capacitors  130  and  132  are used to form a parallel resonant circuit with series-opposed coils  20   a  and  20   b . Capacitor  130  provides fine-tuning of this resonant frequency. 
     Four diodes are employed in the transceiver circuit, two of which are electrically coupled to the electrical junction point  126  and the other two of which are coupled to the junction point  128 . Diode  134  has its cathode coupled to the junction point  126  and its anode coupled to ground while diode  136  has its anode coupled to the junction point  126  and its cathode coupled to ground. Similarly, diode  138  has its cathode connected to the junction point  128  and its anode connected to ground, while diode  140  has its anode connected to junction point  128  and its cathode connected to ground. Thus, any signals that are developed across the diode pair  134 ,  136  or the diode pair  138 ,  140  will be limited in magnitude to the voltage drops that may be developed across these diodes. 
     The cathode of the diode  134  and the anode of the diode  136  are connected by the line  142  to one end of an antenna load resistor  146 . The diodes  134  and  136  may thus be said to be “oppositely-poled.” The other end of the antenna load resistor  146  is connected by the line  144  to the cathode of the diode  138  and the anode of the diode  140 . The voltage developed across the load resistor  146  is coupled to the receiver  152  by the coupling capacitors  148  and  150 . The field  109  from the implanted medical device  15  will have a stronger effect on coil  20   a  than on coil  20   b . When the device is in the receive mode the signal seen at the receiver input is the difference between the voltages generated across each individual coil. The receiver  152  desirably employs a differential amplifier input stage to avoid any noise introduction due to voltage differences between the antenna and system ground. 
     In the receiving mode, the antenna coil  20   a , which is closest to the implanted device, picks up a substantially larger signal than the more remotely located coil  20   b . For example, the implanted cardiac pacemaker  15  creates a field  109  when it is transmitting telemetry information which links the coil  20   a  with a substantially stronger signal than the coil  20   b . On the other hand, a noise source  106 , positioned at a more remote location provides a field  107  which tends to link both coils,  20   b  and  20   a , with the same field. Since the coils are wound in series opposition, however, the noise field component from the noise source  106  will be cancelled, leaving primarily the signal component at the input of the receiver  152 . 
     During telemetry reception all of the diodes are essentially non-conductive due to the low antenna signal voltages that are typically applied across the diodes. These are generally less than 5 millivolts peak-to-peak and, therefore, diodes  134 ,  136  and  138 ,  140  effectively act as very high resistances to ground. The interference field which links the antenna coils  20   b  and  20   a  will generate equal but opposite antenna voltages which will cancel when summed at the receiver input and the common mode rejection of the receiver differential signal will also attenuate undesirable electric-field pick-up. The telemetry field thus generates antenna voltages unequally on the coils  20   b  and  20   a . Thus the telemetry signals appear at the input of the receiver  152  through the coupling capacitors  148  and  150 . During the receive mode of operation, the transmitter circuit  110  will be effectively connected to a null point between the oppositely wound coils  20   b ,  20   a  and will, thus, have no appreciable effect on the receiver circuit. 
     According to one embodiment of the current invention, each of the coil antennas  20  and  22  of FIG. 2 may be dual-coil antenna of the type shown in FIG.  4 . 
     FIG. 5 is a diagram illustrating the manner in which two dual-coil antennas may be positioned with respect to each other. Outer coil  20   a  is positioned adjacent to outer coil antenna  20   b  to provide improved signal-to-noise ratio as compared to the single-coil antenna  20  of FIG.  3 . Similarly, inner coil  22   a  is positioned within outer coil  20   a  and adjacent to inner coil antenna  22   b  to provide an improved signal-to-noise ratio as compared to single-coil antenna  22 . Dashed line  30  represents the enclosure housing telemetry head  12 . 
     The embodiments of FIGS. 2 and 5 show inner and outer coil pairs that are substantially concentric, co-planar, and generally circular. It may be noted that none of these aspects are necessary for proper operation of the system. The inner coil may be of any arbitrary shape that is the same or different as compared to the arbitrary geometry of the outer coil. The inner and outer coils need not be concentric or co-planar. Furthermore, one or both of the antennas may be of the dual-coil configuration shown in FIG.  5 . 
     FIG. 6A is a diagram illustrating an inner and outer coil configuration wherein the inner and outer coils are not concentric. The inner coil may be positioned anywhere within the outer coil. It may be noted that if both the inner and outer coil are dual-coil antennas of the type shown in FIG. 5, both of the inner coils must be positioned in substantially the same location with respect to the respective outer coil, and must be of substantially the same shape as discussed further below. 
     FIG. 6B is a diagram illustrating an inner and outer coil antenna wherein the inner and outer coils are not concentric, and wherein both the inner and outer coils are of the dual-coil configuration. 
     FIG. 7 is a diagram illustrating an inner and outer coil antenna wherein the inner and outer coils are not concentric or co-planar. If desired, one of the inner coils may be co-planar with respect to the associated outer coil, whereas the other inner coil may be positioned such that it is not co-planar with the respective outer coil. It may be noted that only one pair of inner and outer coils as shown in FIG. 7 is necessary to practice the current invention. 
     FIG. 8 is a diagram illustrating an inner and outer coil antenna wherein the inner and outer coils have arbitrary geometries with respect to each other. As discussed above in reference to FIG. 7, although the geometry of an inner coil need not be the same as that of the outer coil, the geometry and spatial relationship of the two inner coils with respect to each other must be substantially similar. The same is true of the geometry and spatial relationship of the two outer coils with respect to each other. As discussed above in reference to FIG. 7, only one pair of inner and outer coils is necessary to practice the current invention. Additionally, if desired, each of the inner coils need not be in the same plane as the respective outer coil. 
     FIG. 9 is a diagram illustrating an inner and outer coil antenna wherein only the inner coil is of a dual-coil configuration. The second inner coil is not necessary to practicing the current invention. If desired, a second outer coil may be added. 
     FIG. 10 is a diagram illustrating an inner and outer coil antenna wherein one of the inner coils is co-planar with respect to an associated outer coil, and wherein the other inner coil is not co-planar with respect to the associated outer coil. 
     The embodiments of FIGS. 6A through 10 illustrate that the spatial relationship, and the geometry, of the inner coil with respect to an associated outer coil, may vary. However, if a two-coil inner antenna is used, these two inner coils must be of substantially the same size and geometry, and include the same number of turns with respect to each other. These inner coils must further be positioned in substantially the same location with respect to the associated outer coil. That is, the inner coils are vertically “stacked” with respect to each other. Similarly, when a two-coil outer antenna is used, the two outer coils must be of substantially the same size and geometry, and include the same number of turns. These two outer coils are also vertically stacked with respect to each other.