BACKGROUND OF THE INVENTION High power ultrasound, when irradiated into a human subject, is known to produce energy at the second harmonic of the source of energy. Researchers are divided as to the physical basis for this effect. Some researchers believe that the effect is caused by non- linearity at a gradient in the material (scatter model). Other researchers believe that the effect occurs in homogeneous material and is caused by overpressure in the medium, giving rise to a non-linear pressure/deformation response (propagation model). Both effects may be present; however, most researchers believe that the effect is primarily explainable by the propagation model.

A paper of Fatemi and Greenleaf, entitled C-Scan Imagine boy Radiation Force Simulated Acoustic Emission Method (Proc. 1996 IEEE Ultrasonic Symposium, pp. 1459- 1462) reported that, when a point is irradiated by beams having different ultrasound frequencies, energy is generated at the point at a frequency which is the difference between the frequencies of the two beams. They also suggested sweeping the beams such that their juncture swept across a sheet of plastic to which a small bead was attached. They were then able to "image"the bead, by receiving a signal at the difference frequency with a non-directional hydrophone.

The position of the point at which the signal was generated was determined from a priori knowledge of the crossing points of the beams. While a signal at the difference frequency may also have been received when the beam crossing was not at the bead, this signal was apparently much smaller than the signal received when the crossed beams impinged the bead.

Other than a general statement that mechanical response of objects to external forces is of interest in applications such as medical diagnosis, non-destructive inspection of materials and material science, no practical utility for this effect was proposed.

In a later presentation entitled Ultrasound Stimulated Acoustic Emission Imaging by Confocal Beams, Fatemi and Greenleaf (Abstract published in"Ultrasonic Imaging,"Vol. 19, Number 1, January 1997) presented a confocal beam generator which generated two concentric

beams which are focused at the same, scanable point. Ultrasound energy at the difference frequency is imaged, presumably as in the earlier paper, to form an image of a scanned object.

Images of both normal and calcified human arteries were acquired.

It is also known in the art to focus relatively low frequency energy at a blood clot in a blood vessel. Since clots are absorptive of and sensitive to energy at these low frequencies, they can often be destroyed. However, due to the low frequencies (and long wavelengths) used, it is impossible to focus the beam to the small size of the clot. The long wavelength also makes it difficult to deliver the energy density required at the clot. Thus, in order to deliver high enough power to breakup the clot, heating the surrounding areas is unavoidable.

SUMMARY OF THE INVENTION One broad aspect of the invention is based on practical utility of the localization of low frequency acoustic energy generated by incident ultrasound energy at different frequencies.

In one preferred embodiment of the invention, ultrasound energy beams at different, relatively high frequencies are irradiated toward an obstruction (a clot, for example) blocking, for example, a blood vessel. At the obstruction, energy is converted from the high frequencies to energy at the difference frequency of the two beams. Preferably, the beams are focused onto the same point (i. e., the obstruction) and preferably the beams irradiate the point from different directions. The generated energy is made high enough so that cavitation of the obstruction occurs, destroying it. A similar technique can also be used to irradiate and ablate or otherwise heat-treat a tumor or other tissue. It is also believed that similar techniques may be suitable for the breaking-up of calcifications in blood vessels or in other parts of the body.

In a second preferred embodiment of the invention, a second set of ultrasound beams is utilized to irradiate a point in the blood vessel, which is downstream from the position of the obstruction to be destroyed. At the same time when power is applied to the obstruction, power is also applied at the downstream point in order to further break-up and destroy any pieces of the obstruction which are not completely destroyed by the first set of beams.

The cavitation effect is generally greater for low frequencies. Since the generation of energy at the difference frequency can, based on general principles, be expected to be more efficient for small differences in frequency, in this aspect of the invention, the frequencies of the two beams are generally close together.

In a second aspect of the invention, unlike the cited prior art, the generated energy is received and/or imaged by a directional receiver, such as a phased array. This allows for the rejection of spurious signals and noise.

In a third aspect of the invention, energy that is generated at a frequency that is the sum of the frequencies of the two beams, is used for imaging.

While, in the Fatemi et al. references cited above, energy at the difference frequency is used to image the region being irradiated, in a preferred embodiment of the invention, an image is formed of energy, which is at the sum of the two frequencies. It should be understood that, just as energy at twice the frequency is generated for a single high-energy beam, energy at the sum frequency is generated for a pair of beams at different frequencies.

In a preferred embodiment of the invention, utilizing the second aspect of the invention, the energy is detected by a preferably steerable, preferably focusable, directional antenna. In another embodiment of the invention, the detector is a relatively non-directional antenna. For imaging, the signal received is considered to be situated at the crossing of the beams at the time at which it is detected.

For this aspect of the invention, the sum of the frequencies are preferably chosen to be substantially different from harmonics of either of the irradiating frequencies in order to reduce spurious signals and to increase the ability of a receiver to reject the irradiating frequencies when receiving the generated sum frequency.

It should be understood that the first, second and/or third aspects can be combined, with the difference frequency being used to treat the relevant region and the sum (or difference) frequency being used for imaging.

In a fourth aspect of the invention, the beams are formed by novel acoustic structures.

In one preferred embodiment of the invention, each of the beams is formed by a linear phased array structure. Preferably, the two structures form a cross. This structure has a number of advantages.

First, while each of the beams have a high resolution in only one direction, the possible region of generation of acoustic energy at difference and sum frequencies has a high resolution in both cross-directions. In essence, it has the same resolution as a full matrix of elements would have.

Second, each beam is steerable and thus a large volume can be swept easily. Since one of the beams controls (for example) the azimuth of the possible region of interaction and the other controls (for example) the elevation, controlling the position is relatively simple. The point of focus is preferably controlled in the usual manner for such arrays. Addition of an extra row or a few rows of elements to each of the linear arrays, so that they form a cross of two long rectangular arrays can further refine and improve the focusing and steering abilities of

this structure. Such a structure can thus increase the power that can be brought to bear on a region by a substantial factor dependent on the number of rows added.

Third, a cross structure can be used for acquiring images at either the sum or difference frequencies, in accordance with the second aspect of the invention. The receive direction would be set to follow the direction of the crossed beams and would be set at a focus depth which is the same as that of the two structures. The receiving structure may be either the same structure or a similar structure tuned to the received frequency.

In preferred embodiments of the invention, this structure is used to generate the beams for the preferred embodiments of the first and third aspects of the invention.

In a fifth aspect of the invention a pair of beams is used to illuminate a region from which a biopsy is to be taken. A biopsy needle is then guided, utilizing the energy, at either the sum or difference frequency, which is generated at the site at which the biopsy is to be taken.

Thus, the pair of beams acts as an externally positionable marker beacon for guiding the biopsy needle.

In various preferred embodiments of the invention, a receiver of the energy may be mounted on a guiding structure of the needle and the needle may, for example, pass through the receiver. Alternatively, the receiver may be mounted on the needle itself. Preferably, the receiver is steerable so that the direction of the source of generated energy can be determined.

Additionally or alternatively, the receiver may have a variable focal depth to better determine the distance that the needle must travel. Alternatively or additionally, the needle's presence at the biopsy location is determined from an image formed by the one or both of the beams that illuminate the biopsy region.

In an alternative preferred embodiment of the invention, the region from which a biopsy is to be taken is illuminated by a first beam at a first frequency. Preferably, the beam is swept across the region such that a sector (fan) is illuminated. A second beam at a second frequency associated with the biopsy needle illuminates the sector and energy at the sum and difference frequencies is generated where the second beam crosses the fan. This energy is imaged and preferably superimposed on an image based on energy reflected from the swept beam. The direction of the biopsy needle is varied so that the image of the sum or difference energies is situated at the position at which the biopsy is to be taken. The needle is then inserted until it reaches the position as indicated, for example, by an image of the region.

In preferred embodiments of the invention, the distance which the needle must be inserted is determined before insertion by a measurement of the distance it is to be inserted

utilizing the various beams used to generate the sum and difference frequencies. Furthermore, in some preferred embodiments of the invention a warning is provided when a blood vessel is situated along the path of the biopsy needle.

In a sixth embodiment of the invention the response of tissue to the beams at the two frequencies is used to characterize the tissue type. It is believed that the low frequency energy, which is generated at points, remains localized at the point. This supposition is based on the fact that the point source of low-frequency energy comprises a radiator that is much smaller than the wavelength of the energy generated (generally by several orders of magnitude). Thus, the low frequency energy remains trapped near the site at which it is generated and is dissipated as heat. (Of course the high frequency energy does propagate and can be used for imaging as aforesaid.) The response of the tissue, both with respect to the efficiency of low frequency energy production and with respect to the decay time of the energy is believed to be characteristic of the tissue. Furthermore, while the amount of radiated low frequency energy is believed to be low, it is believed that it can be detected using the directional antenna of the second aspect of the invention.

There is thus provided, in accordance with the preferred embodiment of the invention, ultrasound imaging apparatus comprising: a first source of ultrasound energy at a first frequency which irradiates a region of a body; a second source of ultrasound energy at a second frequency which irradiates said region of the body, wherein the energy at the first and second frequencies at the region are sufficient to cause energy at at least one of a third frequency and a fourth frequency to be generated in the region, said third frequency being the difference between the first and second frequencies and the fourth frequency being the sum of the first and second frequencies; and a directional antenna which receives energy at one of the third and fourth frequencies from the region and provides an image signal in response thereto.

Preferably, the image signal is produced in response to the third frequency.

Alternatively or additionally the image signal is produced in response to the fourth frequency.

In a preferred embodiment of the invention, the energy at the region of the first and second frequencies is sufficient to produce a therapeutic amount of energy at the third frequency.

Preferably, the first source of energy comprises a phased array transducer. Preferably, the second source of energy comprises a phased array transducer.

In a preferred embodiment of the invention, the first and second sources of energy each comprise phased arrays having a long dimension and a short dimension and wherein the long dimensions of the two arrays are substantially perpendicular.

There is further provided, in accordance with a preferred embodiment of the invention, ultrasound imaging apparatus comprising: a first source of ultrasound energy at a first frequency which irradiates a region of a body; and a second source of ultrasound energy at a second frequency which irradiates said region of the body, wherein the energy at the first and second frequencies at the region are sufficient to cause energy at at least one of a third frequency and a fourth frequency to be generated in the region, said third frequency being the difference between the first and second frequencies and the fourth frequency being the sum of the first and second frequencies, wherein the first and second sources of energy each comprise phased arrays having a long dimension and a short dimension and wherein the long dimensions of the two arrays are substantially perpendicular.

In a preferred embodiment of the invention, the phase arrays are each focused at substantially the same depth such that said sufficient energy is present in a substantially limited region at which energy produced by both said first and second arrays is sufficient to produce acoustic energy at at least one of said third and fourth frequencies.

Preferably, the phased arrays which produce energy at said first and second frequencies each comprise a plurality of rows of elements lying along their respective lengths.

Preferably, the apparatus includes at least one additional phased array transducer comprising at least one additional row of elements arranged to produce an image signal at one of the third and fourth frequencies. Preferably, the at least one additional row of elements is arranged to produce an image signal at the third frequency. Alternatively or additionally, the at least one additional row of elements is arranged to produce an image signal at the fourth frequency.

In a preferred embodiment of the invention, the at least one additional row of elements comprises at least one row of elements and preferably a plurality of rows in each of the long directions of the first and second sources.

There is further provided, in accordance with a preferred embodiment of the invention, apparatus for treating a region of the body with low frequency ultrasound comprising: a first source of ultrasound energy at a first frequency which irradiates a region of a body; and a second source of ultrasound energy at a second frequency which irradiates said region of the body, wherein the energy at the first and second frequencies at the region are sufficient to cause energy at at least a third frequency to be generated in the region, said third frequency being the difference between the first and second frequencies and wherein said first and second energies at said portion is sufficient to cause a therapeutic amount of energy at said third frequency to be produced thereat.

Preferably the first and second sources of energy comprise any of the above described apparatus.

Preferably, the region includes a blood clot and wherein said therapeutic effect is cavitation or destruction of said clot.

In a preferred embodiment of the invention, the apparatus includes third and fourth sources of energy at fifth and sixth frequencies and which produce energy thereat at a frequency which is the difference between said fifth and sixth frequencies and including means for producing said energy at said difference frequency at a second region of the body downstream from said region of the body such that material which is dislodged from said region is cavitated or destroyed at said second region.

Preferably, the first and second sources of energy are produced by apparatus as described above.

There is further provided, in accordance with a preferred embodiment of the invention, apparatus for facilitating the taking of a biopsy, comprising: a biopsy guide which receives a biopsy needle and guides the needle toward a position at which a biopsy is to be taken; a first source of ultrasound energy at a first frequency which irradiates said position; a second source of ultrasound energy at a second frequency which irradiates said position, wherein the energy at the first and second frequencies at the position are sufficient to cause energy at at least one of a third frequency and a fourth frequency to be generated at the position, said third frequency being the difference between the first and second frequencies and the fourth frequency being the sum of the first and second frequencies,

a transducer associated with said biopsy guide which directionally receives energy at one of the third and fourth frequencies from the position; and a display which indicates the direction of guidance of the needle relative to an intercept position of the needle with the position.

Preferably the first and second sources of energy comprise any of the above described apparatus.

Preferably, the apparatus includes an additional ultrasonic imager which determines the presence of the needle at the position based on an image of a region including the position.

There is further provided, in accordance with a preferred embodiment of the invention, apparatus for facilitating the taking of a biopsy, comprising: a biopsy guide which receives a biopsy needle and guides the needle toward a position at which a biopsy is to be taken; a first source of ultrasound energy at a first frequency which irradiates said position; a transducer, associated with the guide, which generates ultrasound energy at a second frequency, wherein the energy at the first and second frequencies at the position are sufficient to cause energy at at least one of a third frequency and a fourth frequency to be generated at the position, said third frequency being the difference between the first and second frequencies and the fourth frequency being the sum of the first and second frequencies; and a second transducer associated with said first source of ultrasound energy which directionally receives energy at one of the third and fourth frequencies from the position and forms images based on said received energy.

Preferably, the apparatus includes a display which forms an image based on the received energy showing an image based on energy at the first frequency on which is superimposed an image based on the energy at the third or fourth frequencies.

Preferably, the first source of energy is swept across a region containing the position and and the second source of energy produces a beam associated with the direction of insertion of the biopsy needle.

Preferably, the second transducer comprises the first source of ultrasound energy.

Preferably, the apparatus includes means for determining the presence of a blood vessel in the path of the needle.

Preferably, the apparatus includes means for determining the distance to be traveled by the biopsy needle to the position.

In a preferred embodiment of the invention, the transducer is mounted on said biopsy guide. Alternatively or additionally the transducer is mounted on said biopsy needle.

In a preferred embodiment of the invention, the frequencies of the first and second energy sources are in a range of 500 kHz and 15 MHz, more preferably, between 500 kHz and 1 MHz. In a preferred embodiment of the invention the third frequency is less than 100 kHz <BR> <BR> <BR> more preferably, less than 50 kHz. In some preferred embodiments of the invention the third frequency is less than 20 kHz or less than 2 kHz There is further provided, in accordance with a preferred embodiment of the invention, ultrasound therapy apparatus comprising: a first source of ultrasound energy at a first frequency which irradiates said position; a second source of ultrasound energy at a second frequency which irradiates said position, wherein the energy at the first and second frequencies at the position are sufficient to cause energy at a third frequency, said third frequency being the difference between the first and second frequencies, wherein said third frequency is less than 2 kHz.

In a preferred embodiment of the invention the third frequency is less than 500 Hz or alternatively, less than 100 Hz.

There is further provided, in accordance with a preferred embodiment of the invention, ultrasound apparatus comprising: a first source of ultrasound energy at a first frequency which irradiates said position; a second source of ultrasound energy at a second frequency which irradiates said position, wherein the energy at the first and second frequencies at the position are sufficient to cause energy at a third frequency to be generated at the position, said third frequency being the difference between the first and second; a transducer which receives energy at the third frequency from the position and generates a time varying signal responsive thereto; and characterization circuit which characterizes at least one property of material at the position based on said time varying signal.

In a preferred embodiment of the invention, the characterization is based on a time response of said time varying signal to energy from said first and second sources of energy.

BRIEF DESCRIPTION OF THE DRAWINGS The invention will be more clearly understood from the following detailed description of non-limiting preferred embodiments thereof taken together with the following attached drawings in which: Fig. 1 is a schematic drawing of an ultrasound illumination system in accordance with a preferred embodiment of the invention; Fig. 2 is a schematic drawing of a irradiation and illumination system in accordance with a preferred embodiment of the invention; Fig. 3 is a schematic drawing illustrating a system and method for biopsy sampling in accordance with a preferred embodiment of the invention; and Fig. 4 is a schematic drawing illustrating an alternative system and method for biopsy sampling in accordance with a preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 1 shows a transmitter 10 (not to scale) which is especially suited for the accurate and precise irradiation of a region of a subject. Transmitter 10 comprises a first linear antenna array 12 and a second linear antenna array 14. Each array may be made up of one line of detector elements 16, as shown, or may be made up of a small number of rows of detector elements. The long axes of the arrays are preferably perpendicular to each other.

Transmitter 10 is activated and fed by a controller 18 which supplies signals to elements 16. Controller 18 is effective to supply signals to arrays 12 and 14 such as to cause them to generate beams which selectively illuminate a region being studied. As is well known in the art, signals supplied to the elements of a linear array control both the direction of the beam generated by the array and also the focus of the beam in the long direction of the array. If more than one line of elements is present in an array, signals to these additional lines cause the beam generated by the array to be better focused in the direction perpendicular to the long direction of the array and also to allow for varying the direction of the beam so that it is selectively directed in a direction out of the plane formed by the long direction of the array and the perpendicular to the array into the subject. Hereinafter, the plane formed by the long direction of an array and the perpendicular to the array will be referred to as the"array plane." Such activation of arrays is well known. However, while the resolution and focus in the array plane are relatively high, the resolution and focus in the direction perpendicular to the array plane are relatively poor. Adding a few additional rows of elements does improve both the focusing and the resolution (in addition to allowing for steering of the center of the beam

out of the array plane); however, unless a square matrix is used, the focus and resolution in the out of the array plane direction is still poorer than in the array plane direction.

In accordance with a preferred embodiment of the invention, array 12 is supplied with ultrasound signals at a frequency different from those supplied to array 14. Controller 18 supplies signals to the array elements which will cause the focus and direction of the beams of array 12 and array 14 to be such that they both selectively illuminate the same area. As indicated above, the beams do not illuminate the same volume since their focus and resolution is different for different directions, i. e., array 12 has poor focus and resolution in the direction that array 14 has good resolution, and vice versa.

For sufficiently high power levels of the two beams, acoustic energy will be generated in the material at the difference frequency of the beams (as reported by Fatemi et al.) and also at a frequency which is the sum of the two frequencies. The energy generated will be substantially confined to a small volume in which the beams generated by arrays 12 and 14 cross and in which they are focused. Thus, while each of the beams generated by array 12 and array 14 has relatively poor focusing in one direction, the energy will be generated with a relatively high resolution in both directions.

As indicated above, according to one aspect of the invention, and unlike Fatemi, et al., imaging may be done at a frequency that is the sum of the frequencies of the two beams.

Furthermore, in some aspects of the invention, and again unlike Fatemi. et al, the imaging may be performed utilizing a directional antenna.

In accordance with a preferred embodiment of the invention, transducer 10 is used to receive signals to form an image of the region irradiated at the crossing of the two fan planes.

In one preferred embodiment of the invention, the same array which is used for irradiating the region area are used to receive the image of the irradiation, utilizing the same elements as are used to irradiate the region. In an alternative preferred embodiment of the invention, additional elements or an additional array are provided for imaging the energy generated at the region selectively irradiated by both beams. Preferred methods and apparatus are described below.

Fig. 2 shows a transmitter/receiver 20 (not to scale) utilizing a first array 22 and a second array 24. Each array preferably includes three central rows of elements 26,28 and 30 on array 22 and 27,29 and 31 on array 24. Each of these arrays is designed to transmit energy at different frequencies such that a selectable portion of the body illuminated by both arrays at a high power density as described in conjunction with Fig. 1. Each of arrays 22 and 24 preferably includes two additional outer rows of elements, 32 and 34 on array 22 and 33 and

35 on array 24. In a preferred embodiment of the invention, these two rows form a steerable focusable array of elements which receives acoustic energy at a frequency which is the sum or difference of the frequencies of the beams generated by arrays 22 and 24 respectively.

In addition, rows 26,28 and 30 on the one hand and rows 27,29 and 31 may be used to receive energy at the transmitted frequencies in order to image the illuminated area at these frequencies as well.

It should be understood that while Fig. 2 shows a plurality of rows of transmitting elements and a plurality of rows of receiving elements, a single row can be used for one or both of transmitting or receiving (with loss of resolution and focusing ability) and that the same array can be used for transmission and receiving, at different frequencies and optionally at the same frequency as well. Furthermore, a greater number of rows of elements rnay be used for transmission and/or receiving. Also, the receiving array can be comprised in the center rows and the outer rows can form the transmitting array.

Fig. 2 also shows a controller 36 and user interface 38 which are used to control the power, focus, frequency and direction of the beams generated by arrays 22 and 24. Fig. 2 also shows a display 40 which displays one or more of the images received by the various arrays according to the various embodiments of the invention.

In other preferred embodiments of the invention, a separate system is used to image the low and/or high frequency energy generated by the two beams.

In a preferred embodiment of the invention, the low frequency energy which is generated at the difference between the frequencies of the illuminating beams is used to break up clots in blood vessels. As indicated above, it is known in the art to irradiate blood clots to break them up using low frequency energy. However, such low frequency energy can not be focused onto the clot and in order to break up the clot, a large area is unavoidably heated. In accordance with a preferred embodiment of the invention, such energy can be generated in a small volume. Furthermore, the position of the volume can be controlled by steering the beams used to generate the volume. Finally, the system as described above can also image the region being scanned, using one or more of the illuminating frequencies, the difference frequency or the sum frequency.

In a preferred method of the invention, the region of the blood clot is scanned to determine the position of the clot. When the position of the clot is determined beams of different frequencies (preferably frequencies which are very close together) illuminate the region of the clot, preferably using the apparatus shown in Figs. 1 or 2. While this apparatus is

preferred, any system for such illumination may be used. Power which is high enough to cavitate or break up the clot is applied to the clot. In a preferred embodiment of the invention, the high frequencies may be in the range of 650 kHz and the difference frequency may be in the range of 20-40 kHz. However, in other preferred embodiments of the invention, the high frequencies may be in the range between hundreds of kHz to tens of MHz and the difference frequency may be in range from a few Hz or tens of Hz to 100 kHz. The choice of high and low frequencies would depend on the application.

In a further preferred embodiment of the invention energy at the difference frequency is similarly generated downstream of the clot which is being broken up. In this way, any pieces of the clot which are not destroyed in situ are destroyed at the downstream region.

In further preferred embodiments of the invention, such difference frequency energy is utilized to provide localized heating to tumors, calcifications or other regions which are therapeutically sensitive to such heating.

In a further preferred embodiment of the invention, tissue is characterized by its response to simultaneous illumination by energy of two different frequencies. It is believed that the low frequency energy, which is generated at points, remains localized at the point.

This supposition is based on the fact that the point source of low-frequency energy comprises a radiator that is much smaller than the wavelength of the energy generated (generally by several orders of magnitude). Thus, the low frequency energy remains mainly trapped near the site at which it is generated and is dissipated as heat. (Of course the high frequency energy does propagate and can be used for imaging as aforesaid.) The response of the tissue, both with respect to the efficiency of low frequency energy production and with respect to the decay time of the energy is believed to be characteristic of the tissue. Furthermore, while the amount of radiated low frequency energy is believed to be low, it is believed that it can be detected using directional antennas, for example those shown in Figs. 1 and 2.

In accordance with a further preferred embodiment of the invention, a pair of beams is used to illuminate a region from which a biopsy is to be taken. A biopsy needle is then guided, utilizing the energy, at either the sum or difference frequency, which is generated at the site at which the biopsy is to be taken. Thus, the pair of beams acts as an externally positionable marker beacon for guiding the biopsy needle.

Fig. 3 shows one preferred embodiment of such a system. A transmitter receiver system such as transmitter 20 illustrated in Fig. 2 or transmitter 10 illustrated in Fig. 1 is used

to image a portion of the body including the place from which the biopsy is to be taken. The location for the biopsy is located on an image shown in display 40 and the two beams are directed so that they are both focused on the location. A needle guide 42, with an attached receiver 44 which is sensitive to either the difference or sum frequency (preferably the sum frequency) is placed on the body of the subject and is positioned such that the location at which energy at the sum frequency is generated is centered in the field of view of receiver 44.

Preferably, the field of view of receiver 44 and in particular, the region in which energy at the sum frequency is generated are preferably displayed on display 40.

Guide 42 is then locked in position so that a needle passed through the guide will pass directly to the location. In general, the image as generated by the crossed arrays will not include the path of the needle, however, when the needle nears or reaches the location which is illuminated by the crossed-array, it will be seen on the image, allowing for its proper placement when taking the sample.

Alternatively or additionally, the distance which the needle must travel to the location can be estimated by the following procedure. First the distance to the location from the illuminating transducer is estimated from the time an echo from the location takes to return to the transducer. Then, the sum of the distances--illuminating transducer-location-guide transducer--is determined by measuring the time between pulsed illumination of the location by the two frequencies and receipt of the sum frequency at the guide transducer. The difference is the distance from the guide transducer to the biopsy position.

In various preferred embodiments of the invention, a receiver of the energy may be mounted on a guiding structure of the needle and the needle may, for example, pass through the receiver. Alternatively, the receiver may be mounted on the needle may be mounted on the needle itself. Preferably, the receiver is steerable so that the direction of the source of generated energy can be determined. Additionally or alternatively, the receiver may have a variable focal depth to better determine the distance that the needle must travel. Alternatively or additionally, the needle's presence at the biopsy location is determined from an image formed by the one or both of the beams that illuminate the biopsy region.

In a further preferred embodiment of the invention, a biopsy needle may be tracked using the apparatus of Fig. 4. In Fig. 4 a fan 58 is swept at a first frequency by a transducer 58 including linear array 60. Array 60 may include only a single line of elements or will preferably include two or more rows of elements to better focus the ultrasound energy radiated by array 60. A biopsy guide 62 is fitted with a transmitter 64 which transmits a beam 66 of

acoustic energy at a second frequency, which is near the first frequency, toward the swept fan.

Beam 66 crosses fan 58 at a point 68 at which energy at the sum and difference of the first and second frequencies is generated.

In one preferred embodiment of the invention as shown in Fig. 4, array 60 produces a first image at the first frequency and a second image at the sum frequency. For this purpose, array 60 may include a second array which receives energy at the sum frequency or the system may utilize other imaging methods and structures as are known in the art. An image, comprising the second image superimposed on the first image, for example, in a distinctive color, is displayed on display 40. The operator will change the direction of the biopsy guide until the region from which a biopsy sample is desired is indicated by superposition of the image at the sum frequency at the region. At this point, the operator knows that the biopsy needle is aimed directly at the region from which a sample is to be taken.

As to the depth of the insertion of the needle, the operator can continue to monitor the superimposed image on display 40 until the tip of the biopsy needle enters the fan beam. At this point, a sample is taken and the biopsy needle is withdrawn.

In a further preferred embodiment of the embodiment shown in Fig. 4, transmitter 64 may also operate to receive energy at the second frequency.

Alternatively or additionally, the time delay (tl) for a signal from transmitter 64 (which is preferably at the point at which the needle enters the body) to the biopsy point may be determined by transmitting a beam from transmitter 64 to point of intersection and measuring the time delay for a signal to reach array 60. The time (2t2) it takes for energy transmitted from array 60 to be returned to array 60 is also determined. The time for energy to be transmitted from transmitter 64 to the intersection point (to) is then computed as to=tl-t2. The insertion distance to the point at which a biopsy is to be taken is then computed as d=to*v, where v is the velocity of sound in the body.

In a further preferred embodiment of the invention, transmitter 64 also comprises a receiver or operates as a transmitter/receiver. Under these conditions, if a blood vessel is present in the path of the needle it can be detected in one of a number of ways. One way is to determine the frequency of energy which is detected by the receiver portion of transmitter/receiver 64 when the transmitter portion transits energy. If this frequency is compared to the frequency of the transmitted energy the presence of a blood vessel will generally generate a reflection at a different frequency from that transmitted due to Doppler

shift. This difference in frequency can be detected and may serve to initiate a warning signal to the operator that he may be in danger of piercing a blood vessel.

It should be understood that many variations of the invention have not been described in the above non-limiting detailed description of the preferred embodiments. Many of these variations including combinations of features of the described embodiments and substitution of features from one embodiment to another will occur to a person of ordinary skill in the art.

The invention is thus not defined or limited by the above description but only by the following claims.