Abstract:
The problem of accessing an injection port transcutaneously is resolved using wireless position transducers in an inflation port assembly and in an injection syringe. The measurements provided by the transducers indicate to the practitioner the position and orientation of syringe relative to the injection port. A console provides a visual indication of the relative position and orientation so as to guide the practitioner to insert the syringe at the proper site and in the proper direction and to penetrate the port cleanly and correctly.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to determining the positions of objects inside a living body. More particularly, this invention relates to determining the position and alignment of an injector relative to an injection port located inside a living body. 
         [0003]    2. Description of the Related Art 
         [0004]    Gastric bands are used to restrict food intake in cases of morbid obesity. An inflatable gastric band is inserted surgically so as to encircle a portion of a patient&#39;s stomach. The band forms a small proximal pouch with a constricted stoma that allows food to slowly pass therethrough. The band may be inflated or deflated by a medical practitioner in order to adjust the size of the stoma and thus control the patient&#39;s food intake. 
         [0005]    In typical gastric band systems, the band is connected by a tube to an inflation port near the body surface. To inflate or deflate the band, the practitioner inserts a syringe into the port and either injects or withdraws fluid through the port. Finding the port is often difficult, particularly in very obese patients, and may require a substantial amount of trial and error. This is inconvenient to the patient, and often produces substantial discomfort. 
         [0006]    U.S. Pat. No. 6,450,946, issued to Forsell, proposes to restrict food intake using a restriction device implanted in a patient and engaging the stomach or the esophagus to form an upper pouch of the stomach and a restricted stoma or passage in the stomach or esophagus. An energy transmission device for wireless transmission of energy of a first form from outside the patient&#39;s body is provided. An implanted energy transfer device transfers the energy of the first form transmitted by the energy transmission device into energy of a second form, different from the first form. The energy of the second form is used to control the operation of the restriction device to vary the size of the restricted passage. 
         [0007]    U.S. Pat. No. 6,305,381, issued to Weijand, et al., describes a system and method for locating an implantable medical device. The system consists of a flat “pancake” antenna coil positioned concentric with the implantable medical device target, e.g., a drug reservoir septum. The system further features an antenna array, which is separate from the implantable device and external to the patient. The antenna array features three or more separate antennas, which are used to sense the energy emitted from the implanted antenna coil. The system further features a processor to process the energy ducted by the antenna array. The system senses the proximity to the implant coil and, thus, the implant device by determining when an equal amount of energy is present in each of the antennas of the antenna array and if each such ducted energy is greater than a predetermined minimum. When such a condition is met, the antenna array is aligned with the implant coil. 
         [0008]    U.S. Pat. Nos. 5,391,199 and 5,443,489, issued to Ben-Haim, whose disclosures are incorporated herein by reference, describe systems wherein the coordinates of an intrabody probe are determined using one or more field sensors, such as a Hall effect device, coils, or other antennae carried on the probe. Such systems are used for generating three-dimensional location information regarding a medical probe or catheter. Preferably, a sensor coil is placed in the catheter and generates signals in response to externally applied magnetic fields. The magnetic fields are generated by three radiator coils, fixed to an external reference frame in known, mutually spaced locations. The amplitudes of the signals generated in response to each of the radiator coil fields are detected and used to compute the location of the sensor coil. Each radiator coil is preferably driven by driver circuitry to generate a field at a known frequency, distinct from that of other radiator coils, so that the signals generated by the sensor coil may be separated by frequency into components corresponding to the different radiator coils. 
         [0009]    U.S. Pat. No. 6,198,963, issued to Ben-Haim et al., whose disclosure is incorporated herein by reference, describes simplified apparatus for confirmation of intrabody tube location that can be operated by nonprofessionals. The initial location of the object is determined as a reference point, and subsequent measurements are made to determine whether the object has remained in its initial position. Measurements are based upon one or more signals transmitted to and/or from a sensor fixed to the body of the object whose location is being determined. The signal could be ultrasound waves, ultraviolet waves, radio frequency (RF) waves, or static or rotating electromagnetic fields. 
       SUMMARY OF THE INVENTION 
       [0010]    According to disclosed embodiments of the invention, the problem of transcutaneously accessing the injection port of an inflatable restriction device is solved by using wireless position transponders in the inflation port assembly and in an injection device that is used to inflate and deflate the port. The signals provided by the transponders indicate to the practitioner the position and orientation of the injection device relative to the injection port. In some embodiments, a console provides a visual indication of the relative positions and alignment of the injection device and the port. The visual indication guides the practitioner in maneuvering the injection device so that it penetrates the port cleanly and correctly. 
         [0011]    An embodiment of the invention provides a method for adjusting an inflatable gastric restriction device within a body of a living subject, which is carried out by disposing a wireless transponder on the gastric restriction device. The wireless transponder generates a location signal relative to a receiver, which has a known relation to an injection device that is adapted to a port of the gastric restriction device. The method is further carried out by irradiating the wireless transponder with a driving field, the wireless transponder being powered at least in part by the driving field. The method is further carried out by wirelessly transmitting an output signal by the wireless transponder responsively to the driving field, receiving and processing the output signal to determine respective locations and orientations of the injection device and the port, and responsively to the respective locations and orientations, navigating the injection device within the body to introduce the injection device into the port, and changing a fluid content of the gastric restriction device using the injection device. 
         [0012]    An aspect of the method includes disposing on the injection device a second wireless transponder that generates a second output signal, and generating a plurality of electromagnetic fields at respective frequencies in a vicinity of the wireless transponder and in a vicinity of the second wireless transponder, wherein the output signal and the second output signal include information indicative of respective strengths of the electromagnetic fields at the wireless transponder and the second wireless transponder. 
         [0013]    One aspect of the method includes storing first electrical energy and second electrical energy derived from the driving field in the wireless transponder and the second wireless transponder, respectively, and transmitting the output signal and the second output signal using the first electrical energy and the second electrical energy, respectively. 
         [0014]    According to one aspect of the method, the wireless transponder and the second wireless transponder are powered exclusively by the driving field. 
         [0015]    An additional aspect of the method includes transmitting telemetry signals from the gastric restriction device, the telemetry signals containing information of a state of the gastric restriction device. 
         [0016]    An embodiment of the invention provides a location system for adjusting an inflatable gastric restriction device within a living subject, The system includes an injection device that is receivable by a port of the gastric restriction device. The injection device has a second wireless transponder. The first wireless transponder and the second wireless transponder each comprise a position sensor, a transmitter for irradiating the first wireless transponder and the second wireless transponder with a driving field. The first wireless transponder and the second wireless transponder are each powered at least in part by the driving field to energize the position sensor thereof. The first wireless transponder and the second wireless transponder are operative responsively to the driving field for wirelessly transmitting a first output signal and a second output signal, respectively. The system further includes electrical circuitry for receiving and processing the first output signal and the second output signal to determine respective locations and orientations of the port and the injection device, and a console that is operative for displaying visual indications of the respective locations and orientations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    For a better understanding of the present invention, reference is made to the detailed description of the invention, by way of example, which is to be read in conjunction with the following drawings, wherein like elements are given like reference numerals, and wherein: 
           [0018]      FIG. 1  schematically illustrates a system for sensing a position and orientation of an injection or aspiration device relative to a port in accordance with a disclosed embodiment of the invention; 
           [0019]      FIG. 2  schematically illustrates details of a wireless position transponder for use in the system shown in  FIG. 1 , in accordance with a disclosed embodiment of the invention; 
           [0020]      FIG. 3  schematically shows details of driving and processing circuits in a processor in the system shown in  FIG. 1 , in accordance with a disclosed embodiment of the invention; 
           [0021]      FIG. 4  is a block diagram showing details of an embodiment of the front end of a receiver in the circuitry shown in  FIG. 3 , which is adapted to receive signals from a plurality of transponders concurrently, in accordance with a disclosed embodiment of the invention; 
           [0022]      FIG. 5  is a schematic diagram of a system for sensing a position and orientation of an injection or aspiration device relative to an injection port that is located within a body of a living subject, in accordance with an alternate embodiment of the invention; 
           [0023]      FIG. 6  schematically illustrates a system for sensing a position and orientation of an injection or aspiration device relative to a port located in a living subject in accordance with an alternate embodiment of the invention; 
           [0024]      FIG. 7  schematically illustrates details of a wireless position transponder, in accordance with an alternate embodiment of the invention; 
           [0025]      FIG. 8  schematically shows details of a wireless transponder in accordance with an alternate embodiment of the invention; 
           [0026]      FIG. 9  is a block diagram of driving and processing circuitry, which are cooperative with the transponder shown in  FIG. 8 , in accordance with a disclosed embodiment of the present invention; 
           [0027]      FIG. 10  is a flow chart of a method for transmitting a digital signal, using the transponder and circuitry shown in  FIG. 8  and  FIG. 9 , in accordance with a disclosed embodiment of the invention; 
           [0028]      FIG. 11  is a block diagram of driving and processing circuitry, which are cooperative with the transponder shown in  FIG. 8 , in accordance with an alternate embodiment of the present invention; and 
           [0029]      FIG. 12  is a block diagram of a wireless position transponder in accordance with an alternate embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0030]    In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent to one skilled in the art, however, that the present invention may be practiced without these specific details. In other instances, well-known circuits, and control logic have not been shown in detail in order not to obscure the present invention unnecessarily. 
       Embodiment 1 
       [0031]    Turning now to the drawings, reference is initially made to  FIG. 1 , which schematically illustrates a system  10  for sensing a position and orientation of an injection or aspiration device relative to a port of injection and aspiration of fluid that is located within a body of a living subject, in accordance with a disclosed embodiment of the invention. A body surface  12  is represented by a vertical line. A generic inflatable gastric restriction device  14  is emplaced on a stomach  16  near its esophagogastric junction  18 . The restriction device  14  constricts the gastric lumen, segmenting the stomach  16  into proximal portion  20  and a distal portion  22 . The restriction device  14  creates a relatively narrow stoma or passage, which retards the movement of food from the proximal portion  20  to the distal portion  22 . 
         [0032]    In the restriction device  14 , a band  24  engages and at least partially wraps around the stomach  16 . An inflation port  26 , usually disposed near the body surface  12 , is adapted to receive an injection device, which is typically a syringe  28 . Typically, a tube  30  connects the inflation port  26  with the band  24 . To inflate or deflate the band  24 , and thereby respectively enlarge or constrict the passage, the practitioner inserts the syringe  28  into the inflation port  26 , and injects or withdraws fluid, as the case may be. Finding the inflation port  26  is often difficult, particularly in very obese patients, and may require a substantial amount of trial and error. The band  24  and the inflation port  26  may include sensors  32  that measure such parameters as intraluminal pressure. 
         [0033]    In order to position the syringe  28  in alignment with the inflation port  26 , at least one transmitter is implanted on the restriction device  14 , at the inflation port  26  or at least in a known relationship to the inflation port  26 . A second transmitter may additionally be emplaced at the syringe  28 , shown in  FIG. 1  as wireless position transponders  34 ,  36 , which in this example are affixed to the inflation port  26  and the syringe  28 , respectively. The disposition of the transponders is not critical. They can be externally or internally located, so long as offsets between the transponders and points of interest on the inflation port  26  and the syringe  28  are known. Measurements derived from signals provided by the transponders indicate to the practitioner the position and orientation of the syringe  28  relative to the inflation port  26 . Signals originating from the transponders  34 ,  36  are transmitted to a field receiving unit  38 , which processes the transponder signals in order to determine the locations of the transponders  34 ,  36 , and hence the locations of the inflation port  26  and the syringe  28 . Typically, in the receiving unit  38 , a processor  40  receives wireless signals  42 ,  44  from the trans-ponders  34 ,  36 , and after suitable signal processing, a console  46  displays a visual indication of the relative position and orientation of the syringe  28  and the inflation port  26 . The display guides the practitioner to navigate the syringe  28  so as to penetrate the body surface  12  and then reach the inflation port  26  correctly. 
         [0034]    Reference is now made to  FIG. 2 , which schematically illustrates details of a wireless position transponder  48 , which can be used as the transponders  34 ,  36  ( FIG. 1 ), in accordance with a disclosed embodiment of the invention. The transponder  48  comprises a power coil  50  and a sensing coil  52 , coupled to control circuitry, typically embodied as a control chip  54 . The control chip  54  comprises a voltage-to-frequency converter  56 , which generates a RF signal whose frequency is proportional to the voltage produced by the current through the sensing coil  52  flowing across a load, which can be measured as a voltage drop across the sensing coil  52 . Additional modulation can be imposed on the RF signals transmitted by the transponder  48 , using a modulator  58 . This allows incorporation of information taken from the sensors  32  ( FIG. 1 ) into the transmitted signals. Any suitable modulation scheme may be employed by the modulator  58 . 
         [0035]    The power coil  50  is preferably optimized to receive and transmit high-frequency signals, in the range above 1 MHz. The sensing coil  52 , on the other hand, is preferably designed for operation in the range of 1-3 kHz. As will be explained below, the sensing coil  52  is operationally disposed within an electromagnetic field having a frequency in the range of 1-3 kHz. Alternatively, other frequency ranges may be used, as dictated by application requirements. The entire transponder  48  is typically 2-5 mm in length and 2-3 mm in outer diameter, enabling it to be housed conveniently in the syringe  28  and the inflation port  26  ( FIG. 1 ). 
         [0036]    Reference is now made to  FIG. 3 , which schematically shows details of driving and processing circuits in the processor  40  of the receiving unit  38  ( FIG. 1 ), in accordance with a disclosed embodiment of the invention. The processor  40  comprises a RF power driver  60 , which drives an antenna  62  to emit a power signal, preferably in the 2-40 MHz range. Values in the Industrial, Scientific and Medical (ISM) bands of 13, 27, and 40 MHz have all been found to be suitable. A plurality of field generator coils  64 , driven by driver circuitry  66 , produce electromagnetic fields at different frequencies that energize the transponder  48  ( FIG. 2 ), as explained below. 
         [0037]    Referring again to  FIG. 2 , the power signal produced by the antenna  62  ( FIG. 3 ) causes a current to flow in the power coil  50 , which is rectified by the control chip  54  and used to power its internal circuits. The control chip  54  in the transponder  48  ( FIG. 2 ) uses the RF signal received by the power coil  50  not only as its sole power source, but also as a frequency reference. 
         [0038]    Meanwhile, electromagnetic fields produced by the field generator coils  64  ( FIG. 3 ) cause a current to flow in the sensing coil  52 . This current has frequency components at the same frequencies as the driving currents flowing through the field generator coils  64 . The frequency components are proportional in amplitude to the strengths of the components of the respective magnetic fields produced by the field generator coils  64  in a direction parallel to the axis of the sensing coil  52 . Thus, the amplitudes of the currents indicate the position and orientation of the sensing coil  52  relative to the field generator coils  64 . 
         [0039]    The control chip  54  measures the currents flowing in the sensing coil  52  at the different field frequencies. It encodes this measurement in a high-frequency signal, which it then transmits back via the power coil  50  to the antenna  62  ( FIG. 3 ). Preferably, the RF signal produced by the control chip  54  has a carrier frequency in the range 50 MHz-2.5 GHz. ISM frequencies of 433,915 MHz and 2.5 GHz have been found to be suitable. The RF signal produced in this manner is modulated with three different frequency modulation (FM) components that vary over time at the respective frequencies of the fields generated by the field generator coils  64 . The magnitude of the modulation is proportional to the current components at the three frequencies. An advantage of using frequency modulation, rather than amplitude modulation, to convey the sensor coil amplitude measurements from the transponder  48  to the antenna  62  is that the information in the signal (i.e., the frequency) is unaffected by the variable attenuation of the body tissues through which the signal must pass. 
         [0040]    Referring again to  FIG. 3 , The signal transmitted by the power coil  50  ( FIG. 2 ) is picked up by the antenna  62  and input to a receiver  68 . The receiver  68  demodulates the signal to generate a suitable input for signal processing circuitry  70 . Typically, the receiver  68  amplifies, filters and digitizes the signals from the transponder  48  ( FIG. 2 ). The digitized signals are received and used by the signal processing circuitry  70  to compute the position and orientation of the transponder  48 . Using pre-established offsets, the position and orientation of a structure connected to the transponder  48  can then be derived. The signal processing circuitry  70  may be realized as dedicated circuitry, or as a general-purpose computer, which is programmed and equipped with appropriate input circuitry for processing the signals from the receiver  68 . 
         [0041]    The processor  40  includes a clock synchronization circuit  72 , which is used to synchronize the driver circuitry  66  and the power driver  60 . Using the frequency reference provided by the power driver  60 , both the control chip  54  in the transponder  48  ( FIG. 2 ) and the receiver  68  are able to apply phase-sensitive processing as known in the art to the current signals generated by the sensing coil  52  ( FIG. 2 ), in order to detect the current of the sensing coil  52  in phase with the driving fields generated by the field generator coils  64 . In the case of the receiver  68 , input is also taken from the clock synchronization circuit  72 . Such phase-sensitive detection methods enable the transponder  48  to achieve an enhanced signal/noise ratio, despite the low amplitude of the current signals in the sensing coil  52 . 
         [0042]    A point of possible ambiguity in determining the orientation coordinates of the trans-ponder  48  ( FIG. 2 ) is that the magnitude of the currents flowing in the sensing coil  52  is invariant under reversal of the direction of the axis of the coil. In other words, flipping the trans-ponder  48  by 180 degrees through a plane perpendicular to the axis of the sensing coil  52  has no effect on the current amplitude. Under some circumstances, this symmetrical response could cause an error of 180 degrees in determining the position and orientation coordinates of the transponder  48 . This ambiguity is usually not relevant in practice, as the orientation is known from the operating environment. 
         [0043]    While the magnitude of the current in the sensing coil  52  is unaffected by flipping the coil axis, the 180 degree reversal reverses the phase of the current relative to the phase of the electromagnetic fields generated by the field generator coils  64 . The clock synchronization circuit  72  can be used to detect this phase reversal and thus overcome the ambiguity of orientation when 180 degree rotation occurs. Synchronizing the modulation of the RF signal returned by the control chip  54  ( FIG. 2 ) to the receiver  68  with the driving currents of the field generator coils  64  enables the receiver  68  to determine the phase of the currents in the sensing coil  52  relative to the driving currents. By checking whether the sensor currents are in phase with the driving currents, or are 180 degrees out of phase, the signal processing circuitry  70  is able to decide in which direction the transponder  48  is pointing. 
         [0044]    Reference is now made to  FIG. 4 , which is a block diagram showing details of an embodiment of the front end of the receiver  68  ( FIG. 3 ), which is adapted to receive signals from both of the transponders  34 ,  36  ( FIG. 1 ) concurrently, in accordance with a disclosed embodiment of the invention. Embodiments of the transponders  34 ,  36  may or may not transmit at different frequencies, or otherwise use different signatures. In any case, it is necessary for the receiver  68  (and the signal processing circuitry  70 ) to differentiate among the transponders. In the embodiment of  FIG. 4 , it is assumed that the frequencies emitted by the transponders  34 ,  36  are different. The antenna  62  is coupled to a plurality of tuning circuits  74 , each tuned to a respective frequency emitted by one of the transponders  34 ,  36 . A switch  76  time multiplexes the outputs of the tuning circuits  74 , and directs them to further signal processing circuitry, as is known in the receiver art. Other multiplexing techniques known in the art may also be employed to allow a single receiver to process signals from a plurality of transponders. 
         [0045]    Alternatively, it is possible to switch the signals of the transponders  34 ,  36  using many other switching circuits known in the art. Alternatively, components of the receiver  68  and the signal processing circuitry  70  could be duplicated and dedicated to transponders  34 ,  36 , respectively. However, this alternative would generally be more expensive and hence, less satisfactory. 
         [0046]    Further details of the transponder  48  ( FIG. 2 ) and the processor  40  ( FIG. 3 ) are described in PCT Publication WO 96/05768, the above-noted U.S. Pat. No. 6,690,963, and in U.S. Patent Application Publication Nos. 2003/0120150 and 2005/0099290, the disclosures of which are herein incorporated by reference. 
       Operation 
       [0047]    Referring again to  FIG. 1 , to operate the system  10 , a subject is placed in a magnetic field generated by the field generator coils  64  ( FIG. 3 ). For example, the field generator coils  64  may be disposed in a pad disposed beneath the subject (not shown). A reference electromagnetic sensor (not shown) is preferably fixed relative to the patient, for example, taped to the patient&#39;s back, and the syringe  28  is advanced into the patient toward the inflation port  26 . The processor  40  constantly updates the relative positions and orientations of the inflation port  26  and the syringe  28  and displays a visual indication on the console  46 . Thus an operator is able at all times during the procedure to determine the precise location of the tip of the syringe  28  relative to the inflation port  26 . When the inflation port  26  is suitably engaged by the syringe  28 , fluid is injected or aspirated from the band  24  as required. Subsequently, the syringe  28  is withdrawn to terminate the operation. 
       Embodiment 2 
       [0048]    Referring again to  FIG. 1 , both of the transponders  34 ,  36  may be configured as transmitters, and their positions may be determined relative to a separate receiving location pad on the patient&#39;s body or fixed outside the body. 
         [0049]    Alternatively, various position and orientation configurations may be used in the system  10 . For example, one of the transponders  34 ,  36  may be configured as a magnetic field transmitter, while the other is configured as a receiver. 
       Embodiment 3 
       [0050]    Continuing to refer to  FIG. 1 , various sensors may be associated with the gastric band, such as a pressure sensor or temperature sensor. The magnetic field transducer associated with the inflation port  26  may then also be used as a data transmitter for purposes of telemetry, in order to transmit measurement values relating to the state of the gastric band to the console, e.g., the fluid pressure in the band  24 . The telemetry signals may be received by a suitable receiver in the syringe  28  (not shown) or at a telemetry antenna  78  of a receiver unit  80 , which can be a separate unit as shown in  FIG. 1 , or can be integrated in the processor  40 . 
       Embodiment 4 
       [0051]    Other types of position sensing may be used, such as ultrasonic position sensing. Reference is now made to  FIG. 5 , which is a schematic diagram of a system  82  for sensing a position and orientation of an injection or aspiration device relative to an injection port that is located within a body of a living subject, in accordance with an alternate embodiment of the invention. 
         [0052]    In this embodiment, a wireless transponder  84 , attached to an injection port  86  located within the body of a patient, receives its operating power not from an electromagnetic field, but from acoustic energy generated by an ultrasound transmitter  88 . A device of this sort is shown, for example, in U.S. Patent Application Publication No. 2003/0018246, the disclosure of which is herein incorporated by reference. The acoustic energy generated by the ultrasound transmitter  88  excites a miniature transducer, such as a piezoelectric crystal  90 , in the wireless transponder  84 , to generate electrical energy that powers the transponder. The electrical energy causes a current to flow in one or more coils in the wireless transponder  84 , such as the power coil  50  ( FIG. 2 ) described above. The currents in the coils in the wireless transponder  84  generate electromagnetic fields outside the patient&#39;s body, which are in this case received by field receivers  92 . The amplitudes of the currents flowing in coils at the frequency of the applied acoustic energy are measured to determine the position of the wireless transponder  84  in relationship with an injection device or syringe  94 , which contains a trans-ponder as described above, which may be wireless or powered by a cable  96 . 
         [0053]    A display  98  preferably comprises a distance guide  100  and an orientation target  102 . A mark  104  on the distance guide  100  indicates how far the tip of the syringe  94  is from the location of the port  86 . A cursor  106  on the orientation target  102  indicates the orientation of tool  76  relative to the axis required to reach the port  86 . When the cursor  106  is centered on the orientation target  102 , it means that the syringe  94  is pointing directly toward the port  86 . The console  46  ( FIG. 1 ) preferably works on a similar principle. 
       Embodiment 5 
       [0054]    Reference is now made to  FIG. 6 , which schematically illustrates a system  108  for sensing a position and orientation of an injection or aspiration device relative to a port located in a living subject in accordance with an alternate embodiment of the invention. In this embodiment, the processor  40  is retrofitted to an existing tracking system, such as the CartoBiosense® Navigation System, available from Biosense Webster Inc., 3333 Diamond Canyon Road, Diamond Bar CA 91765. The processor  40  is designed to receive and process signals received over a cable  110  from one or more sensor coils in a transponder  112 , using the signal processing circuitry  70  ( FIG. 3 ) to determine the position and orientation of the transponder. The wire  110  may also conduct power signals to the transponder  112 , which is constructed similar to the transponder  48  ( FIG. 2 ), except that the power coil  50  can be omitted. The transponder  34  can be wireless, as shown in  FIG. 6 . Alternatively, it is also possible, but less convenient, for wires (not shown) leading from the transponder  34  to be brought out to the body surface  12  and connected to the processor  40 , in which case a copy of the transponder  112  can be substituted for the transponder  34 . In any case, the receiver  68  demodulates the signals generated by either or both of the transducers  34 ,  36  so as to reconstruct the variable current signals generated by respective instances of the sensing coil  52 . The existing processing circuits use this information to determine the position and orientation of the transponders, just as if the sensor coil currents had been received by a wireless connection. 
       Embodiment 6 
       [0055]    Reference is now made to  FIG. 7 , which schematically illustrates details of a wireless position transponder  114 , which can be used as the transponders  34 ,  36  ( FIG. 1 ), in accordance with an alternate embodiment of the invention. The transponder  114  is similar to the transponder  48  ( FIG. 2 ), except that a control chip  116  includes a sampling circuit  118  and an analog-to-digital converter  120  (A/D), which digitizes the amplitude of the current flowing in the sensing coil  52 . In this case, the control chip  116  generates a digitally modulated signal, and RF-modulates the signal for transmission by the power coil  50 . Any suitable method of digital encoding and modulation may be used for this purpose. Other methods of signal processing and modulation will be apparent to those skilled in the art. 
       Embodiment 7 
       [0056]    Reference is now made to  FIG. 8 , which schematically shows details of a wireless transponder  122  in accordance with an alternate embodiment of the invention. The trans-ponder  122  is similar to the transponder  48  ( FIG. 2 ), except that a control chip  124  comprises an arithmetical logic unit  126  (ALU) and a power storage device, such as a capacitor  128 , typically having a capacitance of about 1 microfarad. Alternatively, the power storage device comprises a battery or other power storage means known in the art. The entire transponder  122  is typically 2-5 mm in length and 2-3 mm in outer diameter. 
         [0057]    The control chip  124  measures the voltage drop across the sensing coil  52  at different field frequencies, as explained hereinabove. Employing the arithmetical logic unit  126 , the control chip  124  digitally encodes the phase and amplitude values of the voltage drop. For some applications, the measured phase and amplitude for each frequency are encoded into a 32-bit value, e.g., with 16 bits representing phase and 16 bits representing amplitude. Inclusion of phase information in the digital signal allows the resolution of the above-noted ambiguity that would otherwise occur in the signals when a 180 degree reversal of the sensing coil axis occurs. The encoded digital values of phase and amplitude are typically stored in a memory  130  in the control chip  124  using power supplied by the capacitor  128 . The stored digital values are subsequently transmitted by the transponder  122  using a digital RF signal, as described hereinbelow. For some applications, the control chip  124  digitally encodes and transmits only amplitude values of the voltage drop across the sensing coil  52 , and not phase values. 
         [0058]    Reference is now made to  FIG. 9 , which schematically show details of the driving and processing circuitry  132 , which are cooperative with the transponder  122  ( FIG. 8 ), in accordance with a disclosed embodiment of the invention. The circuitry  132  comprises a RF power driver  134 , which drives the antenna  62  to emit a power signal, typically in the megahertz range, e.g., about 13 MHz. An optional switch  136 , embodied in hardware or software, couples the RF power driver  134  to the antenna  62  for the duration of the emission of the power signal. The power signal causes a current to flow in the power coil  50  of the trans-ponder  122 , which current is rectified by the control chip  124  and used to charge the capacitor  128 . Typically, but not necessarily, the circuitry  132  includes a clock synchronization circuit  138 , which is used to synchronize the RF power driver  134  and the driver circuitry  66 . As mentioned hereinabove, the driver circuitry  66  drive the field generator coils  64  to generate electromagnetic fields. The electromagnetic fields cause a time-varying voltage drop across the sensing coil  52  of the transponder  122  ( FIG. 8 ). 
         [0059]    The digitally modulated RF signals transmitted by the transponder  122  ( FIG. 8 ) is picked up by a receiver  140 , which is coupled to the antenna  62  via the switch  136 . The switch  136 , shown connecting the receiver  140  to the antenna  62 , can be decoupled from the receiver  140  to connect the antenna  62  with the RF power driver  134 . The receiver  140  demodulates the signal to generate a suitable input to signal processing circuitry  142 . The digital signals are received and used by the signal processing circuitry  142  to compute the position and orientation the transponder  122  ( FIG. 8 ) as described above. 
         [0060]    Reference is now made to  FIG. 10 , which is a flow chart that schematically illustrates a method for transmitting a digital signal, using the transponder  122  ( FIG. 8 ) and the circuitry  132  ( FIG. 9 ), in accordance with a disclosed embodiment of the invention. It is emphasized that the particular sequence shown in  FIG. 10  is by way of illustration and not limitation, and the scope of the present invention includes other protocols that would be obvious to a person of ordinary skill in the art. 
         [0061]    The method begins at initial step  144  in which the power driver  60  ( FIG. 3 ) generates a first RF power signal, typically for about 5 milliseconds, which causes a current to flow in the power coil  50 , thereby charging the capacitor  128  ( FIG. 8 ). Subsequently, in step  146 , the driver circuitry  66  drives the field generator coils  64  ( FIG. 9 ) to produce electromagnetic fields, typically for about 20 milliseconds and thereby generate position signals. 
         [0062]    At step  148  the fields generated in step  146  induce a voltage drop across the sensing coil  52  of the transponder  122 , which is measured by the control chip  124 . 
         [0063]    Next, at step  150 , using the power stored in the capacitor  128  ( FIG. 8 ), the arithmetical logic unit  126  converts the amplitude and phase of the sensed voltage into digital values, and stores these values in the memory  130 . 
         [0064]    If the capacitor  128  is constructed such that at this stage it has largely been discharged, then at step  152 , the power driver  60  generates a second RF power signal, typically for about 5 milliseconds, to recharge the capacitor  128 . In applications in which the capacitor  128  retains sufficient charge to power the operations described below, step  152  can be omitted. 
         [0065]    Next, at step  154  Using the stored energy, the control chip  124  generates a digitally modulated signal based on the stored digital values, and RF-modulates the signal for transmission by the power coil  50 . Alternatively, the signal is transmitted using the sensing coil  52 , for example, if a lower frequency is used. This transmission typically requires no more than about 3 milliseconds. Any suitable method of digital encoding and modulation may be used for this purpose, and will be apparent to those skilled in the art. 
         [0066]    Next, at step  156 , the receiver  140  receives and demodulates the digitally modulated signal. 
         [0067]    Next, at step  158 , the signal processing circuitry  142  uses the demodulated signal to compute the position and orientation of the transponder  122 . 
         [0068]    Control now proceeds to decision step  160 , where it is determined whether another operation cycle the transponder  122  is to be performed. If the determination at decision step  160  is affirmative, then control returns to initial step  144 . Typically, step  144  through step  158  are repeated continuously during use of the transponder  122  to allow position and orientation coordinates to be determined in real time. 
         [0069]    If the determination at decision step  160  is negative, then control proceeds to final step  162 , and the procedure terminates. 
         [0070]    The process steps are shown in a linear sequence in  FIG. 10  for clarity of presentation. Typically, the RF driving field is received and electrical energy stored in the transponder during a first time period, and the digital output signal is transmitted by the transponder during a second time period. However, it will be evident that these steps could be performed concurrently or in many different orders. In embodiments in which the method of  FIG. 10  is performed using a plurality of transponders concurrently, the process steps may be interleaved among the different transponders in many different combinations. 
       Embodiment 8 
       [0071]    Reference is now made to  FIG. 11 , which schematically show details of driving and processing circuitry  164 , which are cooperative with the transponder  122  ( FIG. 8 ), in accordance with an alternate embodiment of the invention. 
         [0072]    The circuitry  164  is similar to the circuitry  132  ( FIG. 9 ), except that the switch  136  has been replaced by two band pass filters  166 ,  168 . The band pass filter  166  couples the RF power driver  134  to the antenna  62 , and, for example, may allow energy in a narrow band surrounding 13 MHz to pass to the antenna. The band pass filter  168  couples the receiver  140  to the antenna  62 , and, for example, may allow energy in a narrow band surrounding 433 MHz to pass from the antenna to the receiver. Thus, RF power generated by the RF power driver  134  is passed essentially in its entirety to the antenna  62 , and substantially does not enter circuitry of the receiver  140 . 
         [0073]    Further details of the embodiments shown in  FIGS. 8 ,  9 , and  11  are disclosed in the above-noted U.S. Patent Application Publication No. 2005/0099290. 
       Embodiment 9 
       [0074]    Referring again to  FIG. 3 , in some applications, quantitative measurement of the position and orientation of the transponder to a reference frame is necessary. This requires at least two non-overlapping field generator coils  64  that generate at least two distinguishable AC magnetic fields, the respective positions and orientations of the field generator coils  64  relative to the reference frame being known. The number of radiators times the number of sensing coils is equal to or greater than the number of degrees of freedom of the desired quantitative measurement of the position and orientation of the sensors relative to the reference frame. 
         [0075]    In the embodiment of  FIG. 2 , the single sensing coil  52  is generally sufficient, in conjunction with field generator coils  64 , to enable the signal processing circuitry  70  to generate three dimensions of position and two dimensions of orientation information. The third dimension of orientation (typically rotation about the longitudinal axis) can be inferred if needed from mechanical information or, from a comparison of the respective coordinates of two transponders. However, in some applications, a larger number of degrees of freedom of the quantitative measurements is required. 
         [0076]    Reference is now made to  FIG. 12 , which schematically illustrates details of a wireless position transponder  170 , which can be used as the transponders  34 ,  36  ( FIG. 1 ), in accordance with an alternate embodiment of the invention. The transponder  170  has a plurality of sensing coils  172 ,  174 ,  176 , which are preferably mutually orthogonal, and are connected to a control chip  178 . One of the axes of the sensing coils  172 ,  174 ,  176  may be conveniently aligned with the long axis of the device with which the transponder  170  is associated. The transponder  170  operates similarly to the transponder  48  ( FIG. 2 ). However, the signal processing circuitry  70  ( FIG. 3 ) can now determine all six position and orientation coordinates of the transponder  170  unambiguously. 
         [0077]    The sensing coils  172 ,  174 ,  176  (and the sensing coil  52  ( FIG. 2 )) are preferably wound on air cores. The sensing coils  172 ,  174 ,  176  are closely spaced to reduce the size of the transponder  170 , so that the transponder  170  is suitable for incorporation in a small device. The sensing coils can have an inner diameter of 0.5 mm and have 800 turns of 16 micrometer diameter to give an overall coil diameter of 1-1.2 mm. The effective capture area of each coil is preferably about 400 mm 2 . It will be understood that these dimensions may vary over a considerable range and are only representative of a preferred range of dimensions. In particular, the size of the coils could be as small as 0.3 mm (with some loss of sensitivity) and as large as 2 or more mm. The wire size can range from 10-31 micrometers and the number of turns between 300 and 2600, depending on the maximum allowable size and the wire diameter. The effective capture area should be made as large as feasible, consistent with the overall size requirements. While the preferred sensor coil shape is cylindrical, other shapes can also be used. For example a barrel shaped coil can have more turns than a cylindrical shaped coil for the same diameter of implant. Also, square or other shaped coils may be useful depending on the geometry of the catheter. 
         [0078]    A plurality of sensing coils may optionally be incorporated, mutatis mutandis, in the transponder  114  ( FIG. 7 ) and the transponder  122  ( FIG. 8 ). 
         [0079]    It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather, the scope of the present invention includes both combinations and sub-combinations of the various features described hereinabove, as well as variations and modifications thereof that are not in the prior art, which would occur to persons skilled in the art upon reading the foregoing description.