AU2007201581B2 - Implantable and insertable passive tags - Google Patents

Description

PlO 010I I Regulation 3.2 AUSTRALIA Patents Act 1990 COMPLETE SPECIFICATION STANDARD PATENT Invention Title: Implantable and insertable passive tags The following statement is a full description of this invention, including the best method of performing it known to us: 1 A IMPLANTABLE AND INSERTABLE PASSIVE TAGS CROSS-REFERENCE TO RELATED APPLICATIONS This application is a divisional of Australian specification 2002318855 and the reader should refer to that earlier specification for aspects described herein but not 5 claimed. FIELD OF THE INVENTION The present invention relates generally to position sensing systems, and specifically to systems for determining the position of an object inside a human body. BACKGROUND OF THE INVENTION 10 In many medical procedures, probes, such as endoscopes and catheters, are temporarily inserted into a patient's body. There are also procedures in which devices, such as implants, are inserted into the body permanently or for extended periods. Various methods of determining the location of these inserted medical devices are known in the art. X-ray imaging is the most commonly used location confirmation system. Position 15 sensing systems can also be used for this purpose, and are preferable in particular when the location of the device must be tracked over an extended period. Ultrasound intrabody position sensing is well known. Such systems commonly require an active transducer in the device that is inserted into the body, connected by wires to a console outside the body. The transducer either receives ultrasonic waves from 20 emitters outside the body or radiates ultrasonic waves to receivers outside the body. Other ultrasonic systems use a passive ultrasound reflector in the inserted device which gives a strong reflection of ultrasonic waves irradiating the body without the necessity of running wires through the catheter. These passive systems necessarily create a strong background of ultrasonic radiation against which the position of the reflector must be found. Other position sensing systems use electromagnetic fields. For example, PCT Patent Publication WO 96/05768 to 5 Ben-Haim et al., whose disclosure is incorporated herein by reference, describes a locating system for determining the location and orientation of an invasive medical instrument whereby an externally applied RF field induces a current in three coils located within the invasive medical instrument. 10 Wires or some other form of physical leads are required to carry this induced signal from the catheter to a signal processor in the extrabody space. The processor analyzes the signal so as to calculate the location and orientation of the invasive medical instrument. 15 In many applications, it is advantageous to fix a wireless passive emitter, or "tag," to the device that is inserted into the body. Such a tag contains no internal power source, but is rather actuated by an external energy field, typically applied from outside the body. The tag then emits 20 ultrasonic or electromagnetic energy, which is detected by antennas or other sensors outside the body. The detected signals are generally used to simply to ascertain the presence of the tag within a given region (such as the abdominal cavity), although some tags may also be used to determine 25 position coordinates. Passive ultrasonic reflectors, mentioned above, are one simple example of such tags. Other passive tags receive and re-emit electromagnetic radiation, typically with a frequency and/or phase shift. Hybrid tags, combining ultrasonic and electromagnetic interactions, are 30 also known in the art. For example, U.S. Patent 6,026,818 to Blair et al., whose disclosure is incorporated herein by reference, describes a 2 method and device for the detection of unwanted objects in surgical sites, based on a medically inert detection tag which is affixed to objects such as medical sponges or other items used in body cavities during surgery. The detection tag 5 contains a single signal emitter, such as a miniature ferrite rod and coil and capacitor element embedded therein. Alternatively, the tag includes a flexible thread composed of a single loop wire and capacitor element. A detection device is utilized to locate the tag by pulsed emission of a wide 10 band transmission signal. The tag resonates with a radiated signal, in response to the wide-band transmission, at its own single non-predetermined frequency, within the wide-band range. The return signals build up in intensity at a single (though not predefined) detectable frequency over ambient 15 noise, so as to provide recognizable detection signals. U.S. Patent 5,057,095 to Fabian, whose disclosure is incorporated herein by reference, describes apparatus for detecting a surgical implement in human or animal tissue, comprising a detector responsive to the presence, within an 20 interrogation zone, of a surgical implement to which a marker is secured. The marker is adapted to produce identifying signal characteristics within a frequency band generated in the interrogation zone. Variations in the phase and or direction of the interrogating field and changes in the 25 electromagnetic coupling between markers and receiver are intended to optimize coupling therebetween. U.S. Patent 6,076,007 to England et al., whose disclosure is incorporated herein by reference, describes a method for determining the position and orientation of a surgical device 30 within a human body. In one application, a catheter or prosthesis is characterized in that it carries, at a predetermined location, a tag formed of a high permeability, 3 low coercivity magnetic material. The position of the tag (and hence of the surgical device) is sensed by remotely detecting its magnetic response to an interrogating signal. U.S. Patent 5,325,873 to Hirschi et al., whose disclosure 5 is incorporated herein by reference, describes a system to verify the location of a tube or other object inserted into the body. It incorporates a resonant electrical circuit attached to the object which resonates upon stimulation by a hand-held RF transmitter/receiver external to the body. The 10 electromagnetic field generated due to resonance of the circuit is detected by the hand-held device, which subsequently turns on a series of LEDs to indicate to the user the direction to the target. An additional visual display indicates when the transmitter/receiver is directly above the 15 object. In a non-medical context, U.S. Patent 3,713,133 to Nathans et al., whose disclosure is incorporated herein by reference, describes a theft-prevention system in which a piezoelectric crystal having. a resonant frequency is 20 incorporated into a device which is then attached to individual items within a store. When a radio frequency (RF) signal having a frequency equal to the resonant frequency of the crystal strikes the crystal, an oscillating electrical field gradient is produced across the face of the crystal at 25 the radiated RF frequency, and two tin foil members mounted on the crystal vibrate, emitting ultrasound. Detection of the ultrasound under appropriate conditions produces an alarm, indicative of an attempt to remove the item from the store without authorization. In another embodiment, a small, thin 30 metal diaphragm vibrates when irradiated with an ultrasound field at or near the resonant frequency. The vibration of the diaphragm induced by the ultrasound field modulates an 4 5 incident RF field, and the modulation is detected by an RF transducer to activate the alarm. These systems do not provide specific information describing the location of the item, but only that the item has entered a detection area (typically near an exit from the store). 5 Passive sensors and transponders, fixed to implanted devices, can also be used for conveying other diagnostic information to receivers outside the body. For example, US Patent 6,053,873 to Govari et al., whose disclosure is incorporated herein by reference, describes a stent adapted for measuring a fluid flow in the body of a subject. The stent contains a coil, which receives energy from an electromagnetic field irradiating the body so 10 as to power a transmitter for transmitting a pressure-dependent signal to a receiver outside the body. In one embodiment, the transmitter is based on a tunnel diode oscillator circuit, suitably biased so as to operate in a negative resistance regime, as is known in the art. As another example, US Patent 6,206,835 to Spillman et al., whose disclosure is 15 incorporated herein by reference, describes an implant device that includes an integral, electrically-passive sensing circuit, communicating with an external interrogation circuit. The sensing circuit includes an inductive element and has a frequency-dependent variable impedance loading effect on the interrogation circuit, varying in relation to the sensed parameter. 20 Reference to any prior art in the specification is not, and should not be taken as, an acknowledgment or any form of suggestion that this prior art forms part of the common general knowledge in Australia or any other jurisdiction or that this prior art could reasonably be expected to be ascertained, understood and regarded as relevant by a person skilled in the art.

6 SUMMARY OF THE INVENTION It is an object of at least a preferred embodiment of the present invention to provide improved wireless tags, which are fixed to devices that are inserted into or implanted in the body of a patient for use in determining coordinates of the device inside 5 the body. It is a particular object of some embodiments of the present invention to provide tags that enable both position and orientation coordinates of the device inside the body to be determined accurately. In some preferred embodiments of the present invention, a wireless tag 10 comprises an ultrasonic reflector or transducer, having a nonlinear and/or anisotropic response to incident ultrasonic radiation. The ultrasonic radiation is directed toward the area of the tag by one acoustic radiators outside the body. In one of these embodiments, the radiation reflected from the tag is shifted in frequency, so that acoustic detectors outside the body can distinguish the reflected signal easily from the 15 background radiation generated by the acoustic radiators. In some of these preferred embodiments, the tag is not spherically symmetrical, causing the intensity and, preferably, the frequency shift of the reflected radiation to vary as a function of angle. The differences in the frequency spectrum of the reflected signal detected at the different positions of the acoustic detectors can then be used to determine not only the 20 position, but also the orientation, of the tag, and hence of the device to which the tag is attached. In other preferred embodiments in which ultrasonic irradiation is used, the tag comprises a piezoelectric or magnetoacoustic transducer, having an anisotropic response to the incident ultrasonic radiation. In response to the ultrasonic irradiation, 25 the transducer emits electromagnetic radiation, which is detected by antennas or other electrical sensors outside the body. The spatial variations of the detected radiation are analysed to determine the position and orientation of the tag. In the piezoelectric case, 7 the piezoelectric crystal is preferably fabricated and polarised so that its voltage response to the ultrasonic radiation is anisotropic. Different resonant circuits are coupled across different faces of the crystal so that the frequency of the emitted radiation varies as a function of angle relative to the axis of the crystal. In the 5 magnetoacoustic case, the transducer preferably comprises a magnetostrictive material, which is shaped asymmetrically so as to have different resonant vibrational frequencies along different axes. The magnetic field generated by the transducer will thus also have a frequency that varies as a function of angle. In still other preferred embodiments of the present invention, the tag comprises a 10 transducer, which emits ultrasonic radiation in response to electromagnetic radiation directed toward the tag from the outside the body. Some tags of this type are described in the abovementioned US patent application 09/265,715. Another tag of this type comprises a magnetoacoustic transducer, which is excited by the incident electromagnetic field to vibrate at a characteristic resonant vibrational frequency. As 15 described above, the transducer is preferably shaped so that the frequency varies as a function of angle. The emitted ultrasonic radiation is detected by acoustic sensors outside the body, and its spatial variations are analysed to determine the position and orientation of the tab. A piezoelectric transducer may be used in like fashion. Described in the present specification, but not claimed, is an apparatus for 20 determining the position of an object within a body of a subject, including: at least one acoustic wave generator, adapted to direct a first acoustic wave toward the body at a first frequency; an acoustic tag adapted to be fixed to the object, the tag including a shell defining a cavity therein and a medium contained within the shell, such that responsive 25 to incidence thereon of the first acoustic wave, the tag emits a second acoustic wave at a second frequency, different from the first frequency; 8 one or more detectors, adapted to detect the second acoustic wave and to generate signals responsive thereto; and a signal processor, coupled to process the signals so as to determine coordinates of the object in the body. 5 Preferably, there is substantially no wired connection to the tag. In a preferred embodiment, the tag has an axis and is constructed so that responsive to incidence thereon of the first acoustic wave, the tag emits the second acoustic wave at the second frequency with a first pattern of intensity variation relative to the axis, and a third acoustic wave at a third frequency, different from the first and 10 second frequencies, with a second pattern of intensity variation relative to the axis, and responsive to detection of the second and third acoustic waves by the one or more detectors, the signal processor is adapted to determine an angular orientation of the object responsive to a difference between the first and second patterns. There is also described, but not claimed, apparatus for determining the position 15 of an object within a body of a subject, including: at least one acoustic wave generator, adapted to direct acoustic waves toward the body over a range of frequencies, including at least first and second frequencies; an acoustic tag adapted to be fixed to the object, the tag being constructed so as to reflect the acoustic waves at the first frequency with a first spatial pattern of intensity 20 variation, and to reflect the acoustic waves at the second frequency with a second spatial pattern of intensity variation; one or more detectors, adapted to detect the reflected acoustic waves and to generate signals responsive thereto; and 9 a signal processor, coupled to process the signals so as to determine an angular orientation coordinate of the object in the body responsive to a difference between the first and second spatial patterns. Preferably, the signal processor is further adapted to determine position 5 coordinates of the object responsive to the signals. Further preferably, the tag has an axis, and the tag is constructed so that in the first spatial pattern, the acoustic waves are reflected predominantly in a first direction relative to the axis, while in the second spatial pattern, the acoustic waves are reflected predominantly in a second direction relative to the axis, different from the first direction. 10 There is additionally provided, in accordance with a first aspect of the present invention, apparatus for determining the position of an object within a body of a subject, including: at least one acoustic wave generator, adapted to direct acoustic waves toward the body; 15 a transducer adapted to be fixed to the object and constructed to emit electromagnetic radiation responsive to the acoustic waves with a response that varies depending on an orientation angle of the transducer relative to the at least one acoustic wave generator; one or more detectors, adapted to detect the electromagnetic radiation emitted 20 by the transducer and to generate signals responsive thereto; and a signal processor, coupled to process the signals so as to determine an angular orientation coordinate of the object in the body.

10 In a preferred embodiment, the transducer includes a piezoelectric crystal, which is polarised so as to respond anisotropically to the acoustic waves. Preferably, the piezoelectric crystal has multiple opposing faces, and the transducer further includes a plurality of resonant circuit elements having different, respective resonant frequencies, 5 the circuit elements being coupled between respective pairs of the faces of the crystal so as to emit the electromagnetic radiation at the different resonant frequencies with respective amplitudes that vary responsive to the orientation angle of the transducer. Most preferably, the circuit elements include coils having different, respective values of inductance. 10 In another preferred embodiment, the transducer includes a magnetostrictive element, which is shaped so as to respond anisotropically to the acoustic waves. Preferably, the magnetostrictive element is shaped to as to focus the electromagnetic radiation that it emits. There is further provided, in accordance with a second aspect of the present 15 invention, apparatus for determining the position of an object within a body of a subject, including: at least one field generator, adapted to generate an electromagnetic field within the body; a transducer adapted to be fixed to the object and constructed to emit acoustic 20 waves responsive to the electromagnetic field; one or more acoustic detectors, adapted to detect the acoustic waves emitted by the transducer and to generate signals responsive thereto; and a signal processor, coupled to process the signals so as to determine coordinates of the object in the body.

11 In a preferred embodiment, the transducer includes a magnetoacoustic transducer, preferably including a magnetostrictive material. Preferably, the magnetoacoustic transducer is shaped so as to respond anisotropically to the electromagnetic field, so that the acoustic waves emitted thereby vary as a function of 5 an orientation angle of the transducer relative to the at least one field generator, and the signal processor is adapted to determine the orientation angle of the object responsive to the signals. Most preferably, the magnetoacoustic element is shaped to as to focus the electromagnetic radiation that it emits. There is moreover described, but not claimed, a method for determining the 10 position of an object within a body of a subject, including: fixing an acoustic tag to the object, the tag including a shell defining a cavity therein and a medium contained within the shell, such that responsive to incidence thereon of a first acoustic wave at a first frequency, the tag emits a second acoustic wave at a second frequency, different from the first frequency; 15 inserting the object into the body of the subject; directing the first acoustic wave toward the body at the first frequency, causing the tag to emit the second acoustic wave at the second frequency; detecting the second acoustic wave and generating signals responsive thereto; and 20 processing the signals so as to determine coordinates of the object in the body. There is furthermore described, but not claimed, a method for determining the position of an object within a body of a subject, including: 12 fixing an acoustic tag to the object, the tag being constructed so as to reflect acoustic waves at a first frequency with a first spatial pattern of intensity variation, and to reflect acoustic waves at a second frequency with a second spatial pattern of intensity variation; 5 inserting the object into the body of the subject; directing the acoustic waves toward the body over a range of frequencies, including at least the first and second frequencies; detecting the reflected acoustic waves and generating signals responsive thereto; and 10 processing the signals so as to determine an angular orientation coordinate of the object in the body responsive to a difference between the first and second spatial patterns. There is also provided, in accordance with a third aspect of the present invention, a method for determining the position of an object within a body of a subject, including: 15 fixing a transducer to the object, the transducer being configured to emit electromagnetic radiation responsive to acoustic waves incident thereon with a response that varies depending on an orientation angle of the transducer relative to a source of the acoustic waves; inserting the object into the body of the subject; 20 directing the acoustic waves toward the body; detecting the electromagnetic radiation emitted by the transducer responsive to the acoustic waves, and generating signals responsive thereto; and processing the signals so as to determine an angular orientation coordinate of the object in the body.

13 There is additionally provided, in accordance with a fourth aspect of the present invention, a method for determining the position of an object with a body of a subject, including: fixing a transducer to the object, the transducer being configured to emit acoustic 5 waves responsive to an electromagnetic field that is incident thereon; inserting the object into the body of the subject; generating the electromagnetic field within the body; detecting the acoustic waves emitted by the transducer and generating signals responsive thereto; and 10 processing the signals so as to determine coordinates of the object in the body. As used herein, the term "comprise" and variations of the term, such as "comprising", "comprises" and "comprised", are not intended to exclude other additives, components, integers or steps. The present invention will be more fully understood from the following detailed 15 description of the preferred embodiments thereof, taken together with the drawings in which: BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic illustration of a catheter tracking system, in accordance with a preferred embodiment of the present invention; 20 Fig. 2 is a schematic pictorial illustration showing a cutaway view of an ultrasonic reflector, in accordance with a preferred embodiment of the present invention; Fig. 3 is a schematic pictorial illustration showing 5 another ultrasonic reflector, in partial cutaway view, in accordance with a preferred embodiment of the present invention; Fig. 4 is a schematic pictorial illustration of a piezoelectric tag, in accordance with a preferred embodiment 10 of the present invention; Fig. 5A is a schematic pictorial illustration of a magnetoacoustic tag, in accordance with a preferred embodiment of the present invention; and Fig. 5B is a schematic, cross-sectional view of the tag 15 of Fig. 5A, taken along a line marked VB-VB. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 is a schematic representation of a system 20 for tracking the position of a catheter 22 in the body of a human 20 or non-human subject, in accordance with a preferred embodiment of the present invention. In this application, catheter 22 is inserted through a blood vessel 18 of a patient using standard techniques. Catheter 22 comprises a body 14, a wireless locating tag 12, and an active portion 10 at the 25 distal end of the catheter. The active portion may include, for example, an electrical sensor, an ultrasound head, a fiber optic viewing head, an electrical stimulator, an electrical or laser ablator, an ionic sensor, an oxygen or carbon dioxide sensor, an accelerometer, a blood pressure or temperature 30 sensor, or a cryogenic probe, as are known in the art. In general, the catheter will include leads, light guides, wave 14 guides, etc., for energizing the active portion in response to commands of an operator, and may also include a tip deflection mechanism, for steering the catheter inside the body. The position and/or orientation of the distal end of the 5 catheter is obtained by determining the position and/or orientation of tag 12. Different possible implementations of tag 12 are shown below in Figs. 2, 3, 4 and SA/SB and are described in detail with reference thereto. In some embodiments, tag 12 emits ultrasonic radiation when subjected 10 to ultrasonic irradiation by acoustic generators 11, 13 and 15. The ultrasonic emission of the tag is sensed by acoustic detectors 34, 36 and 38. The acoustic generators and detectors typically comprise ultrasound transducers, as are known in the art, and it is possible to use the same 15 transducers from both irradiation and detection. In alternative embodiments, tag 12 emits electromagnetic radiation when subjected to the ultrasonic irradiation, and this electromagnetic emission is detected by at least one radio frequency (RF) detector 17. In still other embodiments, 20 tag 12 emits ultrasonic radiation when subjected to electromagnetic irradiation by RF radiators 40, 42 and 44. The RF radiators and detectors preferably comprise coils, or antennas of other types, as are known in the art. For convenience and compactness of illustration, Fig. 1 25 shows the entire gamut of irradiators - including both acoustic generators 11, 13 and 15, and RF radiators 40, 42 and 44 - as well as showing both acoustic detectors 34, 36 and 38 and RF detector 17. In practice, only one type of irradiator and one type of detector are typically used in any given 30 embodiment, depending on the type of tag 12 that is used. Acoustic generators 11, 13 and 15 and acoustic detectors 34, 36 and 38 are preferably positioned against a body surface 24 15 of the patient, as is known in the art, while the RF radiators and detector may be positioned a short distance away from surface 24. The RF or acoustic irradiation of the area of tag 12 is 5 initiated by control signals from a control unit 32. When RF irradiation is used, these control signals cause an RF radiator driver 26 to generate driving signals to RF radiators 40, 42 and 44. A representation of the driving signals to the RF radiators or acoustic generators is also sent to a signal 10 processor 30. The RF or acoustic detectors are arranged such that signal processor 30 can utilize inputs representing the driving signals and measurements from the detectors in order to calculate the position and orientation of locating transducer 12. The position measurement is preferably based 15 on a triangulation algorithm, as is known in the art. The detectors may be arranged in any convenient position and orientation, but it is preferable that (a) they are fixed in respect to some reference frame; (b) they are non-overlapping, that is, there are no two detectors with the exact, identical 20 location and orientation; (c) the detectors are not placed collinearly; and (d) two detectors and the locating transducer are at no time all collinear. The numbers and positions of the different type of irradiators and detectors shown in the figure are chosen for 25 illustration only, and greater or lesser numbers of each item may be used, depending on specific application requirements. In practice, the active end of the catheter may be used to gather information, such as ultrasound echo information, electrical activity information, etc., and optionally to 30 perform certain procedures on the arteries (or veins) or other tissue within an organ chamber 16 to which the artery (or vein) leads. Particular examples of organ chambers are the 16 chambers of the heart, brain, or gastrointestinal tract. Depending on the application, system 20 may be configured to provide precise knowledge of the orientation of catheter 22 (e.g., for laser ablation in the heart), or to provide only 5 knowledge of the position of the catheter or other probe (e.g., for gastrointestinal tract tube placement). Although the embodiment of Fig. 1 shows specifically the use of tag 12 in intravascular catheter 22, the tags shown and described hereinbelow may likewise be used in medical probes of other 10 types, as well as in implantable devices. In response to the ultrasonic or electromagnetic radiation emitted by tag 12, acoustic detectors 34, 36, and 38 or RF detector 17 produce electrical signals which are passed to signal processor 30, in either analog or digital form. 15 Signal processor 30 processes the outputs of the detectors to calculate the position and/or orientation of tag 12, and transmits this information to a display monitor 28 and/or control unit 32. Methods of calculating the coordinates of catheter 22 using system 20 are described in detail in the 20 above-mentioned patent application 09/265,715. Fig. 2 is a schematic, pictorial, partly cutaway illustration of an ultrasonic reflecting tag 50, in accordance with a preferred embodiment of the present invention. Tag 50 comprises a shell 52 that is struck by ultrasound waves 25 generated by one or more of acoustic generators 11, 13, 15 outside the patient's body The incident waves induce the tag to resonate and to emit a detectable ultrasound echo. If shell 52 is spherical (as shown), then the emitted echo is generally isotropic, and triangulation of the echo yields the 30 location of the target in the body. Preferably, shell 52 contains a medium 54, and the shell and medium are configured so that tag 50 has a nonlinear 17 vibrational response to incident ultrasonic radiation. Ultrasound waves having a frequency fl, emitted by the acoustic generators outside the patient's body, strike the shell, imparting energy to the shell and/or the contained 5 medium. The shell then emits ultrasound waves at its resonant frequency f2, which is different from fl. The resonant frequency is determined by parameters such as the shell radius, Young modulus and thickness, as is known in the art. The power of the signal emitted by the tag at frequency f2 is 10 preferably measured by detectors 34, 36, 38 at three or more sites outside the patient's body, in order to allow the determination of the target's location by triangulation. Preferably, the material of shell 52 is selected so as to be clearly visible using standard imaging techniques. 15 Fig. 3 is a schematic, pictorial, partly cutaway view of a tag 60, which reflects incident ultrasound waves, in accordance with another preferred embodiment of the present invention. As in the preceding embodiment, tag 60 comprises a shell containing medium 54. In this case, however, the tag 20 has the form of a rectangular prism with unequal faces 62, 64 and 66, making up the shell. Alternatively, the tag may be cylindrical or have some other non-spherical shape. Because of the asymmetry of tag 60, the ultrasound radiation emitted by tag 60 is typically anisotropic. If multiple acoustic 25 generators 11, 13 and 15 are used in alternation to irradiate tag 60 from different angles, and detectors 34, 36 and 38 then measure the strength of the reflected waves at these different angles, signal processor 30 can determine both the position and orientation coordinates of the tag inside the patient's 30 body. As noted earlier, methods of calculation for this purpose are described in the above-mentioned patent application 09/265,715. 18 Furthermore, because of the differing dimensions of faces 62, 64 and 66, each axis of tag 60 typically has its own characteristic resonant frequency or frequency shift. As a result, the reflected ultrasound waves received by detectors 5 34, 36 and 38 vary as a function of detector position and orientation not only in intensity, but also in frequency shift. The frequency shifts are preferably measured and used in determining the orientation angle of the tag. Fig. 4 is a schematic illustration of a tag 70, which 10 comprises a polarized piezoelectric crystal 72, in accordance with a preferred embodiment of the present invention. Inductors (or coils) 74, 78 and 82 are connected across crystal 72, each inductor contacting a respective face 76, 80, 84 and its opposing face on the other side of the crystal. 15 When tag 70 is irradiated with ultrasound waves by any of acoustic generators 11, 13 and 15, crystal 72 vibrates, and the voltages thus created between its opposing faces cause currents to flow in the respective inductors. As a result, the inductors radiate electromagnetic fields, which are 20 detected by RF detector 17. Tag 70 is preferably configured so that its response to the incident ultrasound waves is anisotropic. Preferably, crystal 72 is polarized at the time of its fabrication, so that its voltage response to the mechanical stimulus of the 25 waves differs for each of its axes. Further preferably, each of inductors 76, 80 and 84 forms a part of a resonant circuit, each with a different resonant frequency. (Typically, each inductor is selected to have a different inductance.) Then, in response to the incident ultrasound irradiation, each 30 inductor emits electromagnetic radiation at its particular frequency, with a magnitude that varies as a function of the relative angular orientation of the tag with respect to the 19 source of the ultrasound radiation. Thus, signal processor 30 can analyze the electromagnetic waves emitted by tag 70 in order to yield both the location and the complete angular orientation of the tag. 5 Alternatively, tag 70 may be irradiated by electromagnetic waves generated by RF radiators 40, 42, 44. The waves induce currents to flow in inductors 74, 78 and 82, causing voltages to develop between the respective faces of crystal 72. As a result, the crystal vibrates, emitting 10 ultrasonic waves, which are detected by acoustic detectors 34, 36, 38. Preferably, the inductors comprise coils with a large numbers of turns, so that high enough voltages are applied to the faces of the crystal to cause substantial vibrations. A voltage doubler circuit, as is known in the art, may also be 15 used. If each of the inductors has a different frequency response, as described above, the variation of the emitted ultrasonic waves as a function of RF excitation frequency can be used by processor 30 to determine both the position and the orientation of tag 70. 20 Figs. 5A and 5B schematically illustrate a tag 90 comprising a magnetoacoustic transducer, in accordance with a preferred embodiment of the present invention. Fig. SA is a pictorial illustration, while Fig. 5B is a cross-sectional illustration, taken along line VB-VB in Fig. 5A. Tag 90 25 preferably comprises a magnetostrictive material, as is known in the art, such as Terfenol-D. RF radiators 40, 42, 44 transmit electromagnetic radiation towards tag 90, preferably in the range of about 100-1000 kHz. The time-varying magnetic field of the 30 radiation at the tag causes the tag to expand and contract, so that some of the electromagnetic energy is converted into mechanical vibrations. As a result, tag 90 emits ultrasound 20 waves, typically at the frequency of the exciting magnetic field. These ultrasound waves are sensed by acoustic detectors 34, 36 and 38, and the resultant signals are analyzed by signal processor 30 to determine the location of 5 the tag by triangulation. Alternatively or additionally, acoustic generators 11, 13, 15 transmit ultrasound waves at tag 90, causing the tag to vibrate. As a result of the vibration, tag 90 generates an electromagnetic wave having a frequency determined by the 10 properties of the transducer and by the frequency of the exciting ultrasound waves. The emitted electromagnetic wave is detected by RF detector 17. Preferably, tag 90 is asymmetrical, and is shaped so that its vibrational frequency response is anisotropic. As a 15 result, both the response of the tag to incident ultrasound waves (when the tag is excited by acoustic irradiation) and the pattern of emitted ultrasound waves (when the tag is excited by RF irradiation) vary as a function of the orientation angle of the tag. The spatial variation sensed by 20 the RF or acoustic detectors can then be used to determine both the position and the orientation of tag 90, as described above. Optionally, tag 90 is shaped so as to concentrate the emitted ultrasound or electromagnetic radiation in a 25 particular direction, or at a particular location, with respect to the tag. For example, the tag may be plano-concave (as shown), causing the emitted radiation to be focused generally to a point outside the patient's body. An array of detectors, suitably positioned, can be used to determine the 30 location of that focal point, and, consequently, the location and orientation of the tag within the patient's body. As a further option, several such shaped transducers may be 21 included in a single tag. The ultrasound waves emitted by tag 90 may also be used for imaging, or to analyze properties of tissue in which the tag is placed. Alternative, useful shapes for tag 90, suitable for 5 enabling identification of the orientation of the tag, will be apparent to those skilled in the art. Cylindrical and disk shapes are particularly useful in generating directional ultrasonic pulses. In a further embodiment of the present invention, not 10 shown in the figures, a passive tag comprises a tunnel diode coupled to a resonant circuit. Circuits of this type and their use as passive transponders are described in detail in the above-mentioned U.S. Patent 6,053,873. The circuit is excited by an electromagnetic wave generated one or more of 15 radiators 40, 42, 44 at a first frequency (fl) , and emits an electromagnetic wave of another frequency (f2) . Tunnel diodes are particularly well suited for this purpose, because the characteristic I-V curve of a tunnel diode includes a portion in which the diode demonstrates "negative" resistance, i.e., 20 as the voltage applied across the diode decreases, the current through the diode increases, causing oscillations to occur in the circuit. The oscillation frequency (f2) differs from the normal resonant frequency of the circuit because of the effective capacitance of the tunnel diode. Typically, 25 frequency f2 differs from the excitation frequency fl by about 10% - 40%. For example, an excitation frequency fl of 88 MHz may yield a waveform (detectable by the external apparatus) having a frequency f2 of 120 MHz. Triangulation of the power of the electromagnetic wave 30 emitted by the circuit yields the location of the tag. Typically, three or more RF detectors at respective sites are used for this purpose. The angular position of the tag can 22 also be determined, at least in part, based on the angular position of the inductor in the resonant circuit. Optionally, the tag comprises multiple resonant circuits with mutually orthogonal inductors, in order to allow more precise 5 determination of the angular orientation of the tag. It will be appreciated that the preferred 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 10 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. 15 23

Claims (34)

1. Apparatus for determining the position of an object within a body of a subject, comprising: at least one acoustic wave generator, adapted to direct acoustic waves toward 5 the body; a transducer adapted to be fixed to the object and constructed to emit electromagnetic radiation responsive to the acoustic waves with a response that varies depending on an orientation angle of the transducer relative to the at least one acoustic wave generator; 10 one or more detectors, adapted to detect the electromagnetic radiation emitted by the transducer and to generate signals responsive thereto; and a signal processor, coupled to process the signals so as to determine an angular orientation coordinate of the object in the body.

2. Apparatus according to claim 1, wherein the transducer comprises a piezoelectric 15 crystal, which is polarised so as to respond anisotropically to the acoustic waves.

3. Apparatus according to claim 2, wherein the piezoelectric crystal has multiple opposing faces, and wherein the transducer further comprises a plurality of resonant circuit elements having different, respective resonant frequencies, the circuit elements being coupled between respective pairs of the faces of the crystal so as to emit the 20 electromagnetic radiation at the different resonant frequencies with respective amplitudes that vary responsive to the orientation angle of the transducer.

4. Apparatus according to claim 3, wherein the circuit elements comprise coils having different, respective values of inductance.

5. Apparatus according to any one of the preceding claims, wherein the signal 25 processor is further adapted to determine position coordinates of the object responsive to the signals. 25

6. Apparatus according to any one of the preceding claims, wherein the transducer comprises a magnetostrictive element, which is shaped so as to respond anisotropically to the acoustic waves.

7. Apparatus according to claim 6, wherein the magnetostrictive element is shaped 5 to as to focus the electromagnetic radiation that it emits.

8. Apparatus according to any one of the preceding claims, wherein there is substantially no wired connection to the transducer.

9. Apparatus for determining the position of an object within a body of a subject, comprising: 10 at least one field generator, adapted to generate an electromagnetic field within the body; a transducer adapted to be fixed to the object and constructed to emit acoustic waves responsive to the electromagnetic field; one or more acoustic detectors, adapted to detect the acoustic waves emitted by 15 the transducer and to generate signals responsive thereto; and a signal processor, coupled to process the signals so as to determine coordinates of the object in the body.

10. Apparatus according to claim 9, wherein the transducer comprises a piezoelectric crystal, which is polarised so as to respond anisotropically to the electromagnetic field. 20

11. Apparatus according to claim 10, wherein the piezoelectric crystal has multiple opposing faces, and wherein the transducer further comprises a plurality of resonant circuit elements having different, respective resonant frequencies, the circuit elements being coupled between respective pairs of the faces of the crystal so as to cause the crystal to emit the acoustic waves at the different resonant frequencies with respective 25 amplitudes that vary responsive to the orientation angle of the transducer. 26

12. Apparatus according to claim 11, wherein the circuit elements comprise coils having different, respective values of inductance.

13. Apparatus according to any one of claims 9 to 12, wherein the transducer comprises a magnetoacoustic transducer. 5

14. Apparatus according to claim 13, wherein the transducer comprises a magnetostrictive material.

15. Apparatus according to claim 13, wherein the magnetoacoustic transducer is shaped so as to respond anisotropically to the electromagnetic field, so that the acoustic waves emitted thereby vary as a function of an orientation angle of the transducer 10 relative to the at least one field generator, and wherein the signal processor is adapted to determine the orientation angle of the object responsive to the signals.

16. Apparatus according to claim 15, wherein the magnetoacoustic element is shaped to as to focus the electromagnetic radiation that it emits.

17. Apparatus according to any one of claims 9 to 16, wherein there is substantially 15 no wired connection to the transducer.

18. A method for determining the position of an object within a body of a subject, comprising: fixing a transducer to the object, the transducer being configured to emit electromagnetic radiation responsive to acoustic waves incident thereon with a 20 response that varies depending on an orientation angle of the transducer relative to a source of the acoustic waves; inserting the object into the body of the subject; directing the acoustic waves toward the body; detecting the electromagnetic radiation emitted by the transducer responsive to 25 the acoustic waves, and generating signals responsive thereto; and 27 processing the signals so as to determine an angular orientation coordinate of the object in the body.

19. A method according to claim 18, wherein the transducer comprises a piezoelectric crystal, which is polarised so as to respond anisotropically to the acoustic 5 waves.

20. A method according to claim 19, wherein the piezoelectric crystal has multiple opposing faces, and wherein the transducer further comprises a plurality of resonant circuit elements having different, respective resonant frequencies, the circuit elements being coupled between respective pairs of the faces of the crystal so as to emit the 10 electromagnetic radiation at the different resonant frequencies with respective amplitudes that vary responsive to the orientation angle of the transducer.

21. A method according to claim 20, wherein the circuit elements comprise coils having different, respective values of inductance.

22. A method according to any one of claims 18 to 21, wherein processing the 15 signals further comprises determining position coordinates of the object responsive to the signals.

23. A method according to any one of claims 18 to 22, wherein the transducer comprises a magnetostrictive element, which is shaped so as to respond anisotropically to the acoustic waves. 20

24. A method according to claim 23, wherein the magnetostrictive element is shaped to as to focus the electromagnetic radiation that it emits.

25. A method according to any one of claims 18 to 24, wherein fixing the transducer to the object comprises fixing the transducer so that when the objected is inserted into the body, there is substantially no wired connection between the transducer and 25 circuitry outside the body.

26. A method for determining the position of an object within a body of a subject, comprising: 28 fixing a transducer to the object, the transducer being configured to emit acoustic waves responsive to an electromagnetic field that is incident thereon; inserting the object into the body of the subject; generating the electromagnetic field within the body; 5 detecting the acoustic waves emitted by the transducer and generating signals responsive thereto; and processing the signals so as to determine coordinates of the object in the body.

27. A method according to claim 26, wherein the transducer comprises a piezoelectric crystal, which is polarised so as to respond anisotropically to the 10 electromagnetic field.

28. A method according to claim 27, wherein the piezoelectric crystal has multiple opposing faces, and wherein the transducer further comprises a plurality of resonant circuit elements having different, respective resonant frequencies, the circuit elements being coupled between respective pairs of the faces of the crystal so as to cause the 15 crystal to emit the acoustic waves at the different resonant frequencies with respective amplitudes that vary responsive to the orientation angle of the transducer.

29. A method according to claim 28, wherein the circuit elements comprise coils having different, respective values of inductance.

30. A method according to any one of claims 26 to 29, wherein the transducer 20 comprises a magnetoacoustic transducer.

31. A method according to claim 27, wherein the transducer comprises a magnetostrictive material.

32. A method according to claim 27, wherein the magnetoacoustic transducer is shaped so as to respond anisotropically to the electromagnetic field, so that the acoustic 25 waves emitted thereby vary as a function of an orientation angle of the transducer 29 32. A method according to claim 27, wherein the magnetoacoustic transducer is shaped so as to respond anisotropically to the electromagnetic field, so that the acoustic waves emitted thereby vary as a function of an orientation angle of the transducer relative to the at least one field generator, and wherein processing the signals 5 comprises determining the orientation angle of the object responsive to the signals.

33. A method according to claim 32, wherein the magnetoacoustic element is shaped to as to focus the electromagnetic radiation that it emits.

34. A method according to any one of claims 26 to 33, wherein fixing the transducer to the object comprises fixing the transducer so that when the objected is inserted into 10 the body, there is substantially no wired connection between the transducer and circuitry outside the body.