Abstract:
A surgical tracking system includes a wireless position sensor, which is adapted to be implanted in a bone of a subject, and responsively to externally-applied magnetic fields within a working volume of the tracking system, to generate and transmit sensor signals indicative of coordinates of the position sensor within the bone. A plurality of field generator coils are adapted to generate the magnetic fields so as to define the working volume. The field generators are fixed in predetermined locations to a reference structure, which is movable relative to the subject in order to position the working volume so as to intercept the bone. A system controller is coupled to receive and process the sensor signals so as to determine the coordinates of the position sensor within the bone.

Description:
[0001]     This claims the benefit of U.S. Provisional Application No. 60/550,924 filed on Mar. 5, 2004. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to intrabody tracking systems, and specifically to wireless methods and devices for tracking the position and orientation of an object in the body.  
       BACKGROUND OF THE INVENTION  
       [0003]     Various methods and systems are known in the art for tracking the position of a medical probe or implant inside the body of a subject.  
         [0004]     For example, U.S. Pat. Nos. 5,391,199 and 5,443,489 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.  
         [0005]     PCT Patent Publication WO 96/05768, U.S. Pat. No. 6,690,963 and the corresponding U.S. patent application Ser. No. 09/414,875, to Ben-Haim et al. (published as U.S. Patent Application Publication U.S. 2002/0065455 A1), whose disclosures are incorporated herein by reference, describe a system that generates six-dimensional position and orientation information regarding the tip of a catheter. This system uses a plurality of sensor coils adjacent to a locatable site in the catheter, for example near its distal end, and a plurality of radiator coils fixed in an external reference frame. These coils generate signals in response to magnetic fields generated by the radiator coils, which signals allow for the computation of six location and orientation coordinates.  
         [0006]     U.S. Pat. No. 6,239,724 to Doron et al., whose disclosure is incorporated herein by reference, describes a telemetry system for providing spatial positioning information from within a patient&#39;s body. The system includes an implantable telemetry unit having (a) a first transducer, for converting a power signal received from outside the body into electrical power for powering the telemetry unit; (b) a second transducer, for receiving a positioning field signal that is received from outside the body; and (c) a third transducer, for transmitting a locating signal to a site outside the body, in response to the positioning field signal.  
         [0007]     The above-mentioned U.S. patent application Ser. No. 10/029,473 to Govari, published as U.S. Patent Application Publication 2003/0120150, describes apparatus for tracking an object. The apparatus includes a plurality of field generators, which generate electromagnetic fields at different, respective frequencies in a vicinity of the object, and a radio frequency (RF) driver, which radiates a RF driving field toward the object. A wireless transponder is fixed to the object. The transponder includes at least one sensor coil, in which a signal current flows responsive to the electromagnetic fields, and a power coil, which receives the RF driving field and conveys electrical energy from the driving field to power the transponder. The power coil also transmits an output signal responsive to the signal current to a signal receiver, which processes the signal to determine coordinates of the object.  
       SUMMARY OF THE INVENTION  
       [0008]     Embodiments of the present invention provide magnetic tracking systems for use in tracking the positions of objects within the body of a patient. In some embodiments, these systems are used in orthopedic procedures, such as implantation of implants such as screws, nails, rods or prosthetic joints or any other orthopedic device or tool. For this purpose, wireless magnetic position sensors may be inserted into the patient&#39;s bone, into implants such as screws, nails, rods or other orthopedic devices that can be inserted into the patient&#39;s bone or into prosthetic implants and into tools used during surgery. The track system determines the coordinates of the sensors, and thus enables the surgeon to visualize the locations and orientations of these elements while reducing or eliminating the need for intraoperative X-ray imaging. Implanted position sensors may also be used in post-operative follow-up.  
         [0009]     The wireless magnetic position sensors generate position signals in response to magnetic fields that are produced by multiple field generators that are deployed outside the body. The intersection region of the magnetic fields that are produced by the various field generators defines a working volume, in which the tracking system is capable of determining the coordinates of the position sensors with accuracy sufficient for the medical procedure in question. The field generators are typically mounted on a reference structure that holds the field generators in fixed relative positions, but which is capable of moving relative to the patient. In this manner, the working volume of the tracking system can be conveniently positioned to cover the area of a surgical procedure, without interfering with the surgeon&#39;s ability to work in the area.  
         [0010]     There is therefore provided, in accordance with an embodiment of the present invention, a surgical tracking system, including: 
        a wireless position sensor, which is adapted to be implanted in a bone of a subject, and responsively to externally-applied magnetic fields within a working volume of the tracking system, to generate and transmit sensor signals indicative of coordinates of the position sensor within the bone;     a plurality of field generator coils, which are adapted to generate the magnetic fields so as to define the working volume;     a reference structure, to which the field generator coils are fixed in predetermined locations, and which is movable relative to the subject in order to position the working volume so as to intercept the bone; and     a system controller, which is coupled to receive and process the sensor signals so as to determine the coordinates of the position sensor within the bone.        
 
         [0015]     In some embodiments, the reference structure includes multiple arms, each holding a respective one of the field generator coils, and a base, which is coupled to support the arms. The base is typically adapted to adjust at least one of a height, a rotation and a tilt of the arms, while maintaining the arms in a fixed mutual relation.  
         [0016]     In other embodiments, the reference structure includes multiple arms, each holding a respective one of the field generator coils, and a boom, which is coupled to support the arms above the subject. Typically, the boom is adapted to adjust at least one of a height, a rotation and a tilt of the arms, while maintaining the arms in a fixed mutual relation. In one embodiment, the system includes a lamp, which is held over the subject by the boom so as to illuminate an area of the working volume.  
         [0017]     Typically, the bone has an axis, and the locations at which the field generators are fixed to the reference structure define a plane, and the reference structure is adapted to position the field generators so that the plane is approximately parallel to the axis. Alternatively or additionally, the reference structure is adapted to position the field generators so that the plane is approximately perpendicular to the axis. In one such embodiment, the reference structure includes a semicircular holder, to which the field generators are fixed, and which partly surrounds the axis. The reference structure may also include a base, to which the semicircular holder is movably attached, so as to permit at least one of a height, a rotation and a tilt of the semicircular holder to be adjusted, while maintaining the field generators in a fixed mutual relation.  
         [0018]     In another embodiment, the system includes an operating table having an underside and including a base, which contains the system controller, wherein the reference structure includes multiple arms, each holding a respective one of the field generator coils, and an articulated mount to which the arms are fixed and which is fixed to the underside of the table in order to support the arms. Typically, the articulated mount is adapted to adjust at least one of a height, a rotation and a tilt of the arms, while maintaining the arms in a fixed mutual relation.  
         [0019]     In yet another embodiment, the reference structure is configured to be inserted into an opening in an operating table.  
         [0020]     In disclosed embodiments, the position sensor includes one or more sensor coils, which are adapted to sense the magnetic fields so as to generate the sensor signals. In some embodiments, the system includes a driving antenna, which is adapted to radiate a radio frequency (RF) electromagnetic field toward the sensor, and the position sensor includes a power coil, which is coupled to receive the RF electromagnetic field so as to provide electrical power to the sensor. Additionally or alternatively, the position sensor includes a communication coil, which is coupled to transmit the sensor signals to the system controller.  
         [0021]     In some embodiments, the system includes an implant that is a screw, which contains at least the one or more sensor coils of the position sensor, and which is adapted to be inserted into the bone. In some of these embodiments, the position sensor includes a power source, which is contained in the screw. In other embodiments, the position sensor includes an external unit, which includes at least a power source and is adapted to be positioned outside a body of the subject, and wires coupling the one or more sensor coils in the screw to the external unit.  
         [0022]     In a disclosed embodiment, the system includes a surgical tool, for operating on the bone, the tool including a tool position sensor, which is adapted to generate and transmit, responsively to the externally-applied magnetic fields, tool signals indicative of coordinates of the tool relative to the bone.  
         [0023]     There is also provided, in accordance with an embodiment of the present invention, a method for surgery, including: 
        implanting in a bone of a subject a wireless position sensor, which is adapted to generate and transmit sensor signals indicative of coordinates of the position sensor within the bone in response to externally-applied magnetic fields;     mounting a plurality of field generator coils in predetermined locations on a reference structure;     driving the field generator coils to generate the magnetic fields, thus defining a working volume for tracking the wireless position sensor;     moving the reference structure in order to position the working volume so as to intercept the bone; and     receiving and processing the sensor signals so as to determine the coordinates of the position sensor within the bone.        
 
         [0029]     The method may include performing a surgical procedure on the bone using a surgical tool including a tool position sensor, which is adapted to generate and transmit, responsively to the externally-applied magnetic fields, tool signals indicative of coordinates of the tool relative to the bone.  
         [0030]     The present invention will be more fully understood from the following detailed description of the embodiments thereof, taken together with the drawings in which:  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]      FIG. 1  is a schematic, pictorial illustration of a magnetic tracking system used in surgery, in accordance with an embodiment of the present invention;  
         [0032]      FIGS. 2A and 2B  are schematic, partly sectional illustrations, showing insertion of implantable position sensors into the bone of a patient, in accordance with an embodiment of the present invention;  
         [0033]      FIGS. 3A and 3B  are schematic, pictorial illustrations showing details of wireless position sensors, in accordance with embodiments of the present invention;  
         [0034]      FIG. 4  is a schematic, pictorial illustration showing details of a two-part position sensor, in accordance with an embodiment of the present invention;  
         [0035]      FIG. 5  is a schematic, pictorial illustration showing a surgical tool and a position sensor used to track coordinates of the tool, in accordance with an embodiment of the present invention;  
         [0036]      FIG. 6A  is a schematic, pictorial illustration showing an operating table and a location pad that is inserted into the table, in accordance with an embodiment of the present invention;  
         [0037]      FIG. 6B  is a schematic, pictorial illustration showing the location pad of  FIG. 6A  after insertion into the operating table, and showing the working volume of the location pad, in accordance with an embodiment of the present invention;  
         [0038]      FIG. 7  is a schematic, pictorial illustration showing adjustment of the magnetic tracking system of  FIG. 1  for use in a knee operation, in accordance with an embodiment of the present invention;  
         [0039]      FIG. 8  is a schematic, pictorial illustration of a magnetic tracking system for use in surgery, in accordance with another embodiment of the present invention;  
         [0040]      FIG. 9  is a schematic, pictorial illustration of a magnetic tracking system for use in surgery, in accordance with yet another embodiment of the present invention;  
         [0041]      FIGS. 10A and 10B  are schematic, pictorial illustrations of a magnetic tracking system for use in surgery, in accordance with still another embodiment of the present invention; and  
         [0042]      FIG. 11  is a schematic, pictorial illustration of a magnetic tracking system for use in surgery, in accordance with a further embodiment of the present invention.  
     
    
     DETAILED DESCRIPTION OF EMBODIMENTS  
       [0043]      FIG. 1  is a schematic, pictorial illustration of a magnetic tracking system  20  for use in surgery, in accordance with an embodiment of the present invention. In the pictured embodiment, a surgeon  22  is preparing to perform a procedure on a leg  24  of a patient  26 . The surgeon uses a tool  28  to implant or place an implant, generally designated  30 , (in this example a bone screw  30 ) in the bone of leg  24 . Both the tool and the implant, i.e. the screw, contain miniature, wireless position sensors, which are described in detail hereinbelow. Each sensor generates and transmits signals that are indicative of its location and orientation coordinates, in response to an external magnetic field produced by a set of field generator coils  32  (also referred to as radiator coils). Typically, multiple screws  30  with position sensors are implanted by surgeon  22  at key locations in the patient&#39;s bone. Additionally or alternatively, similar position sensors may be fixed to other implants  30 , such as a prosthetic joint or intramedullary insert or other implants  30  such as a nail, rod, pin, staple, bone or tissue anchor, or other orthopedic device, in order to permit the position of the implant  30  to be monitored, as well. For example, the use of such position sensors in a hip implant is shown in the above-mentioned U.S. patent application Ser. No. 10/029,473.  
         [0044]     Field generator coils  32  are driven by driver circuits  34  to generate electromagnetic fields at different, respective sets of frequencies {ω 1 }, {ω 2 } and {ω 3 }. Typically, the sets comprise frequencies in the approximate range of 100 Hz-30 kHz, although higher and lower frequencies may also be used. The sets of frequencies at which the coils radiate are set by a computer  36 , which serves as the system controller for system  20 . The respective sets of frequencies may all include the same frequencies, or they may include different frequencies. In any case, computer  36  controls circuits  34  according to a known multiplexing pattern, which provides that at any point in time, no more than one field generator coil is radiating at any given frequency. Typically, each driver circuit is controlled to scan cyclically over time through the frequencies in its respective set. Alternatively, each driver circuit may drive the respective coil  32  to radiate at multiple frequencies simultaneously.  
         [0045]     For the purposes of system  20 , coils  32  may be arranged in any convenient position and orientation, so long as they are fixed in respect to some reference frame, and so long as they are non-overlapping, that is, there are no two field generator coils with the exact, identical location and orientation. Typically, for surgical applications such as that shown in the figures, coils  32  comprise wound annular coils about 15-20 cm in outer diameter (O.D.) and about 1-2 cm thick, in a triangular arrangement, wherein the centers of the coils are about 80-100 cm apart. The coil axes may be parallel, as shown in this figure, or they may alternatively be inclined, as shown, for example, in  FIGS. 6A and 6B . Bar-shaped transmitters or even triangular or square-shaped coils could also be useful for such applications.  
         [0046]     In orthopedic and other surgical applications, it is desirable that coils  32  be positioned away from the surgical field, so as not to interfere with the surgeon&#39;s freedom of movement. On the other hand, the coils should be positioned so that the working volume of the tracking system includes the entire area in which the surgeon is operating. At the same time, the locations and orientations of coils  32  should be known relative to a given reference frame in order to permit the coordinates of tool  28  and screw  30  to be determined in that reference frame.  
         [0047]     In order to meet these potentially-conflicting requirements, coils  32  are mounted on a reference structure  40 . In the embodiment of  FIG. 1 , structure  40  comprises multiple arms  42 , which are fixed to an articulated base  0 . 44 . Alternative reference structures and configurations are shown in the figures that follow. Arms  42  hold coils  32  in known relative positions. Base  44 , however, is capable of tilting, turning and changing the elevations of arms  42 , so as to enable surgeon  22  to position coils  32  in convenient locations. The movement of base  44  may be controlled by computer  36 , so that the computer is also aware of the actual locations of coils  32 .  
         [0048]     Alternatively or additionally, an image registration procedure may be used to calibrate the positions of coils  32  relative to patient  26 . An exemplary registration procedure, based on X-ray imaging, is described in U.S. Pat. No. 6,314,310, whose disclosure is incorporated herein by reference. Further alternatively or additionally, a reference sensor, fixed to patient  26  or to the operating table in a known location, may be used for calibration. The use of reference sensors for this purpose is described, for example, in the above-mentioned U.S. Pat. No. 5,391,199.  
         [0049]     The position sensors in screw  30  and tool  28  typically comprise sensor coils, in which electrical currents are induced to flow in response to the magnetic fields produced by field generator coils  32 . An exemplary arrangement of the sensor coils is shown in  FIG. 3A  below. The sensor coils may be wound on either air cores or cores of magnetic material. Typically, each position sensor comprises three sensor coils, having mutually orthogonal axes, one of which is conveniently aligned with the longitudinal axis of tool  28  or of screw  30 . The three coils may be concentrically wound on a single core, or alternatively, the coils may be non-concentrically wound on separate cores, and spaced along the longitudinal axis of the tool or screw. The use of non-concentric coils is described, for example, in the above-mentioned PCT Patent Publication WO 96/05768 and in the corresponding U.S. patent application Ser. No. 09/414,875. Alternatively, the position sensors may each comprise only a single sensor coil or two sensor coils. Further alternatively, screw  30  and tool  28  may comprise magnetic position sensors based on sensing elements of other types known in the art, such as Hall effect sensors.  
         [0050]     At any instant in time, the currents induced in the sensor coils comprise components at the specific frequencies in sets {ω 1 }, {ω 2 } and {ω 3 } generated by field generator coils  32 . The respective amplitudes of these currents (or alternatively, of time-varying voltages that may be measured across the sensor coils) are dependent on the location and orientation of the position sensor relative to the locations and orientations of the field generator coils. In response to the induced currents or voltages, signal processing and transmitter circuits in each position sensor generate and transmit signals that are indicative of the location and orientation of the sensor. These signals are received by a receiving antenna (shown, for example, in  FIG. 6A ), which is coupled to computer  36 . The computer processes the received signals, together with a representation of the signals used to drive field generator coils  32 , in order to calculate location and orientation coordinates of screw  30  and tool  28 . The coordinates are used by the computer in driving a display  46 , which shows the relative locations and orientations of the tool, screw and other elements (such as prosthetic implants) to which position sensors have been fixed.  
         [0051]     Although in  FIG. 1 , system  20  is shown as comprising three field generator coils  32 , in other embodiments of the present invention, different numbers, types and configurations of field generators and sensors may used. A fixed frame of reference may be established, for example, using only two non-overlapping field generator coils to generate distinguishable magnetic fields. Two non-parallel sensor coils may be used to measure the magnetic field flux due to the field generator coils, in order to determine six location and orientation coordinates (X, Y, Z directions and pitch, yaw and roll orientations) of the sensor. Using three field generator coils and three sensor coils, however, tends to improve the accuracy and reliability of the position measurement.  
         [0052]     Alternatively, if only a single sensor coil is used, computer  36  can still determine five position and orientation coordinates (X, Y, Z directions and pitch and yaw orientations). Specific features and functions of a single coil system (also referred to as a single axis system) are described in U.S. Pat. No. 6,484,118, whose disclosure is incorporated herein by reference.  
         [0053]     When a metal or other magnetically-responsive article is brought into the vicinity of an object being tracked, such as screw  30  or tool  28 , the magnetic fields in this vicinity are distorted. In the surgical environment shown in  FIG. 1 , for example, there can be a substantial amount of conductive and permeable material, including basic and ancillary equipment (operating tables, carts, movable lamps, etc.), as well as invasive surgery apparatus (scalpels, scissors, etc., including tool  28  itself). The magnetic fields produced by field generator coils  32  may generate eddy currents in such articles, and the eddy currents then cause a parasitic magnetic field to be radiated. Such parasitic fields and other types of distortion can lead to errors in determining the position of the object being tracked.  
         [0054]     In order to alleviate this problem, the elements of tracking system  20  and other articles used in the vicinity of the tracking system are typically made of non-metallic materials when possible, or of metallic materials with low permeability and conductivity. For example, reference structure  40  may be constructed using plastic or non-magnetic composite materials, as may other articles in this vicinity, such as the operating table. In addition, computer  36  may be programmed to detect and compensate for the effects of metal objects in the vicinity of the surgical site. Exemplary methods for such detection and compensation are described in U.S. Pat. Nos. 6,147,480 and 6,373,240, as well as in U.S. patent application Ser. No. 10/448,289 filed May 29, 2003 and Ser. No. 10/632,217 filed Jul. 31, 2003, all of whose disclosures are incorporated herein by reference.  
         [0055]      FIG. 2A  is a schematic, sectional illustration showing implantation of screw  30  into a bone  50 , such as the femur of patient  26 , in accordance with an embodiment of the present invention. To insert the screw, surgeon  22  makes an incision through overlying soft tissue  52 , and then rotates the screw into bone  50  using tool  28 , for example. Alternatively, the screw may be inserted percutaneously, without prior incision. Note that in the embodiment of  FIG. 2A , screw  30  has no wired connection to elements outside the body. Typically, screw  30  is between about 5 and 15 mm long, and is about 2-4 mm in diameter. To avoid interfering with reception and transmission of signals by the sensor that it contains, screw  30  typically comprises a non-magnetic material, which may comprise metals, alloys, ceramics, plastics or a combination of such materials. The configuration and operation of the circuits in screw  30  are described hereinbelow with reference to  FIGS. 3A and 3B .  
         [0056]      FIG. 2B  is a schematic, sectional illustration showing another position sensor device  54 , in accordance with an alternative embodiment of the present invention. Device  54  comprises an implantable screw  56 , which is coupled by wires  58  to an external unit  60 . Screw  56  is inserted into bone  50  in substantially the same manner as is screw  30  (leaving wires  58  to pass out of the patient&#39;s body through soft tissue  52 ). In this case, however, because some elements of device  54  are contained in external unit  60 , screw  56  may generally be made smaller than screw  30 . For example, screw  56  may be between about 5 and 10 mm long, and between about 2 and 4 mm in diameter. The reduced screw size is helpful in reducing trauma and possible damage to bone  50 . Further details of device  54  are shown in  FIG. 4 .  
         [0057]      FIG. 3A  is a schematic, pictorial illustration of a wireless position sensor  70  that is contained in screw  30 , in accordance with an embodiment of the present invention. Sensor  70  in this embodiment comprises three sets of coils: sensor coils  72 , power coils  74 , and a communication coil  76 . Alternatively, the functions of the power and communication coils may be combined, as described in the above-mentioned U.S. patent application Ser. No. 10/029,473. Further alternatively, although communication coil  76  is shown in  FIG. 3A  to be wound in a plane that is perpendicular to the longitudinal axis of screw  30 , the communication coil or antenna may alternatively be arranged along the length of sensor  70 , roughly parallel to the longitudinal axis of the screw. Coils  72 ,  74  and  76  are coupled to electronic processing circuitry  78 , which is mounted on a suitable substrate  80 , such as a flexible printed circuit board (PCB). Details of the construction and operation of circuitry  78  are described in U.S. patent application Ser. No. 10/029,473 and in the above-mentioned U.S. patent application Ser. No. 10/706,298 which are incorporated herein by reference.  
         [0058]     Although for simplicity,  FIG. 3A  shows only a single sensor coil  72  and a single power coil  74 , in practice sensor  70  typically comprises multiple coils of each type, such as three sensor coils and three power coils. The sensor coils are wound together, in mutually-orthogonal directions, on a sensor core  82 , while the power coils are wound together, in mutually-orthogonal directions, on a power core  84 . Typically, each of the three power coils comprises about 30-40 turns of wire having a diameter of at least about 40 μm, while power core  84  is a ferrite cube of about 1.5-2 mm on a side. Each of the three sensor coils typically comprises between about 700 and 3000 turns of 11 μm diameter wire, while sensor core  82  is a ferrite cube of about 1.8-2.4 on a side. (It will be understood that these dimensions are given by way of example, and the dimensions may in practice vary over a considerable range.) Alternatively, the sensor and power coils may be overlapped on the same core, as described, for example in U.S. patent application Ser. No. 10/754,751, filed Jan. 9, 2004, whose disclosure is incorporated herein by reference. It is generally desirable to separate the coils one from another by means of a dielectric layer (or by interleaving the power and sensor coils when a common core is used for both) in order to reduce parasitic capacitance between the coils.  
         [0059]     In operation, power coils  74  serve as a power source for sensor  70 . The power coils receive energy by inductive coupling from an external driving antenna (shown, for example, in  FIG. 6A ). Typically, the driving antenna radiates an intense electromagnetic field at a relatively high radio frequency (RF), such as in the range of 13.5 MHz. The driving field causes currents to flow in coils  74 , which are rectified in order to power circuitry  78 . Meanwhile, field generator coils  32  ( FIG. 1 ) induce time-varying signal voltages to develop across sensor coils  72 , as described above. Circuitry  78  senses the signal voltages, and generates output signals in response thereto. The output signals may be either analog or digital in form. Circuitry  78  drives communication coil  76  to transmit the output signals to a receiving antenna (also shown in  FIG. 6A ) outside the patient&#39;s body. Typically, the output signals are transmitted at still higher radio frequencies, such as frequencies in the rage of 43 MHz or 915 MHz, using a frequency-modulation scheme, for example. Additionally or alternatively, coil  76  may be used to receive control signals, such as a clock signal, from a transmitting antenna (not shown) outside the patient&#39;s body. Although certain frequency ranges are cited above by way of example, those skilled in the art will appreciate that other frequency ranges may be used for the same purposes.  
         [0060]     In another embodiment, not shown in the figures, sensor coils  72  are non-concentric. In this embodiment, each of the sensor coils typically has an inner diameter of about 0.5-1.3 mm and comprises about 2000-3000 turns of 11 μm diameter wire, giving an overall coil diameter of about 1-1.9 mm. (As above, these dimensions are given only by way of example, and the actual dimensions may vary.) The wire size of the sensor coils can range from 10-31 μm, and the number of turns between 300 and more than 3000, depending on the maximum allowable size and the wire diameter. The effective capture area of the sensor coils is typically made as large as feasible, consistent with the overall size requirements. The sensor coils are typically cylindrical, but other shapes can also be used. For example, barrel-shaped or square coils may be useful, depending on the geometry of screw  30 .  
         [0061]      FIG. 3B  is a schematic, pictorial illustration of a wireless position sensor  90 , in accordance with another embodiment of the present invention. Sensor  90  differs from sensor  70 , in that sensor  90  comprises a battery  92  as its power source, instead of power coils  74 . Battery  92  may be of any suitable type, either single-use or rechargeable. In other respects, the operation of sensor  90  is substantially similar to that of sensor  70 , as described above. Use of battery  92  has the advantages of supplying higher operating power to electronic processing circuitry  78 , while avoiding the need to irradiate patient  26  with an intense electromagnetic field in order to provide inductive RF power to the sensor. On the other hand, incorporating battery  92  in sensor  90  typically increases the length of the sensor, by comparison to sensor  70 , and therefore may require the use of a longer screw  30  to contain the sensor. In addition, the operating lifetime of sensor  70  is effectively unlimited, while that of sensor  90  is limited by the lifetime of battery  92 .  
         [0062]      FIG. 4  is a schematic, pictorial illustration showing details of device  54 , in accordance with an embodiment of the present invention. The external features of device  54  and its implantation in bone  50  were described above with reference to  FIG. 2B . Device  54  comprises an internal sensing unit  94 , which is contained in screw  56 . Typically, sensing unit  94  contains only sensor coils  72 , and possibly or optionally elements of circuitry  78 . This arrangement allows the size of screw  56  to be minimized. External unit  60  typically contains a battery  96  and circuit elements  98 , which comprise some or all of circuitry  78  (depending on how much of circuitry  78  is located within sensing unit  94 ), as well as communication coil  76 . The battery may thus be replaced when necessary, without removing screw  56  from the bone. On the other hand, whereas sensors  70  and  90  are contained completely within screw  30 , and thus leave no elements protruding outside the patient&#39;s body, device  54  can operate only when external unit  60  is connected outside the body to wires  58  that are operatively connected to sensing unit  94  and communicate with sensing unit  94 .  
         [0063]      FIG. 5  is a schematic, pictorial illustration showing details of tool  28 , in accordance with an embodiment of the present invention. Tool  2 - 8  comprises a handle  100  and a shaft  102 . A tool sensor  104  fits snugly into a suitable receptacle inside handle  100 . Sensor  104  comprises sensing and communication circuits  106 , which are powered by a battery  108 . Typically, circuits  106  comprise three sensing coils, a communication coil and processing circuitry, as in sensor  90  ( FIG. 3B ). The sensing coils are similar to coils  72 , and sense the location and orientation of sensor  104  relative to the magnetic fields generated by field generator coils  32  ( FIG. 1 ). The communication coil conveys position signals to computer  36 . The operation of circuits  106  is thus similar to that of the circuits in sensors  70  and  90 , although elements of circuits  106  may be made larger and consume greater power than the corresponding elements in sensors  70  and  90 .  
         [0064]     Tool sensor  104  may be permanently housed inside tool  28 , or the sensor may alternatively be removable (to replace battery  108 , for example). Because the geometry of tool  28  is known, the location and orientation of handle  100 , as indicated by sensor  104 , indicates precisely the location and orientation of the distal tip of shaft  102 . Alternatively, the tool sensor  104  may be miniaturized and may thus be contained inside shaft  102 . Optionally, the tool sensor  104  may be calibrated before use in order to enhance the precision with which the shaft position is measured.  
         [0065]      FIGS. 6A and 6B  are schematic, pictorial illustrations showing insertion of a location pad  110  into an opening in an operating table  112 , in accordance with an embodiment of the present invention. Pad  110  may be used as the reference structure in system  20  ( FIG. 1 ), in place of structure  40 . Pad  110  comprises an integral unit, which holds three field generator coils  32  in fixed positions. The unit is typically made from non-magnetic material, such as carbon fiber, fiberglass, plastic or ceramic. The field generator coils in this case are angled diagonally inward. In  FIG. 6A  pad  110  is shown prior to insertion into the table, while in  FIG. 6B  the pad has been slid into place.  
         [0066]     Location pad  110  is also seen in  FIG. 6A  to comprise an optional power coil  114  and a communication coil  116 . Power coil  114  is coupled by wires (not shown) to driver circuits  34 , and generates an electromagnetic field to provide power inductively to power coils  74  in sensor  70  ( FIG. 3A ), as described above. (When a battery-powered sensor is used, the power coil is not required.) Communication coil  116  receives signals transmitted by communication coil  76  in sensors that are implanted in the patient&#39;s body, as well as from tool sensor  104 . Communication coil  116  may also be used to transmit control signals, such as a clock signal, to the implanted sensors and tool sensor. Communication coil  116  is coupled by wires (not shown) to computer  36 . The computer processes the signals received from communication coil  116  in order to determine the locations and orientations of the sensors. Coils  114  and  116  may be printed on the surface of pad  110 , as shown in  FIG. 6A , or they may alternatively comprise printed circuit traces or wire-wound coils contained inside pad  110 .  
         [0067]      FIG. 6B  schematically shows a working volume  118  created by field generator coils  32  when driven by driver circuits  34 . The surface of the working volume represents the outer limit of the region in which tracking system  20  is able to determine sensor coordinates to within a certain accuracy i.e. the location coordinates or position and orientation coordinates of the sensor. The required accuracy is determined by functional considerations, such as the degree of positioning precision required by surgeon  22  in performing the surgical procedure at hand. Typically, the outer surface of working volume  118  represents the limit in space at which tracking accuracy drops to the range of 1-2 mm. Tilting the field generator coils, as shown in  FIGS. 6A and 6B , typically lowers the centroid of the working volume. Because pad  110  is rigid, it cannot be raised and lowered or tilted, as can structure  40  in  FIG. 1 . Pad  110  may, however, be slid in and out of table  112  in order to shift the position of working volume  118  along the table, so that the working volume intercepts the bone  50  or portion of the bone  50  on which the surgeon in to operate.  
         [0068]      FIG. 7  is a schematic, pictorial illustration showing how reference structure  40  may be adjusted for use in surgery on a knee  120  of patient  26 , in accordance with an embodiment of the present invention. The patient lies on an operating table  122 , which folds as shown in the picture to give the surgeon convenient access to the patient&#39;s knee joint. Base  44  of structure  40  tilts accordingly, so that the working volume of field generator coils  32  encompasses the area of knee  120 , while still permitting the surgeon unimpeded access to the area.  
         [0069]      FIG. 8  is a schematic, pictorial illustration showing a reference structure  130  for supporting field generator coils  32 , in accordance with another embodiment of the present invention. Structure  130  comprises arms  132 , which hold coils  32 . The arms are fixed to an articulated boom  134 , which permits the height and angle of the field generator coils  32  to be adjusted relative to the position of the patient on an operating table  136 . Boom  134  may be carried by a wheeled cart  138 , so that structure  130  can be positioned at either side of table  136  or at the foot or head of the table. Cart  138  may also contain computer  36  and/or driver circuits  34 . To reduce clutter over operating table  136 , structure  130  may be integrated with an overhead lamp  140 , as shown in the figure. In this configuration, lamp  140  illuminates the area of the working volume of coils  32 . An additional lamp  142  is shown for completeness.  
         [0070]      FIG. 9  is a schematic, pictorial illustration showing a reference structure  150  supporting field generator coils  32 , in accordance with yet another embodiment of the present invention. Structure  150  comprises an articulated boom  154 , which holds arms  152  to which coils  32  are attached. In this embodiment, structure  150  is tilted and positioned over the area of the patient&#39;s knees, to provide functionality similar to that shown in  FIG. 7 .  
         [0071]      FIGS. 10A and 10B  are schematic, pictorial illustrations showing another reference structure  160 , in accordance with a further embodiment of the present invention. Structure  160  comprises a semicircular holder  162  for field generator coils  32 , which is mounted on a base  164 . Whereas the reference structures in the embodiments shown above are configured to position coils  32  in a plane that is roughly parallel to the long axis of the bone to be operated upon (such as the femur or the fibula), the plane of structure  160  is roughly perpendicular to this axis. Typically, for proper positioning of the working volume, structure  160  is placed so that the bone axis passes through the circle defined by the positions of coils  32 , i.e., so that holder  162  partly surrounds the bone axis.  
         [0072]     Structure  160  may be mounted on a cart  166  with wheels, enabling it to be positioned either at the foot ( FIG. 10A ) or head ( FIG. 10B ) of table  122 . An adjustment slot  167  or other mechanism in base  164  permits holder  162  to rotate about the patient. A hinge permits base  164  to tilt, while telescopic legs  170  permit the entire structure to be raised or lowered. Structure  160  may thus be positioned flexibly, at the convenience of the surgeon, depending on the type of procedure that is to be carried out. The configuration of  FIG. 10A , for example, may be convenient for hip surgery, while that of  FIG. 10B  is convenient for knee surgery.  
         [0073]      FIG. 11  is a schematic, pictorial illustration showing a magnetic tracking system  180  for use in surgery, in accordance with still another embodiment of the present invention. In this embodiment, the tracking system is integrated into an operating table  182 . The operating table may be custom-made for this purpose, and may thus comprise little or no magnetic material. A reference structure  184  is fixed to the underside of table  182  by an articulated mount that permits structure  184  to be rotated, tilted, raised and lowered, so as to position field generator coils  32  as required for the surgical procedure in question. A telescopic base  186  of table  182  contains driver circuits  34  and computer  36 . Positions and orientations of position sensors and tools are shown on display  46 , which is likewise integrated with table  182 . System  180  thus permits the surgeon to operate with only minimal added encumbrance due to the use of magnetic position tracking.  
         [0074]     Although the embodiments described hereinabove relate specifically to tracking systems that use time-varying magnetic fields, the principles of the present invention may also be applied, mutatis mutandis, in other sorts of tracking systems, such as ultrasonic tracking systems and tracking systems based on DC magnetic fields. It will thus be appreciated that the embodiments described above are cited by way of example, and 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 subcombinations of the various features described hereinabove, as well as variations and modifications thereof which would occur to persons skilled in the art upon reading the foregoing description and which are not disclosed in the prior art.