FIELD OF THE INVENTION [0002] The present invention relates to devices which utilize ultrasound to determine the direction and speed of a fluid flowing in a vessel, and more particularly to Doppler diagnostic medical systems and methods for measuring blood flow.

BACKGROUND OF THE INVENTION [0003] Doppler ultrasound measurements of flow, widely used for blood flow measurement in medical applications, and for the measurement of other scattering fluids in industrial applications, depend upon the Doppler effect, whereby a scatterer produces a change in the frequency of the ultrasound that it scatters. This change in frequency is proportional to two unknown quantities: the absolute magnitude of the velocity vector characterizing the motion of the scatterer, and the angle between the velocity vector and the insonating beam.

[0004] By simultaneously making two Doppler measurements of a velocity whose vector is coplanar with the transducer using two beams at known angles to each other, the resulting Doppler equations (each of which contains the unknown quantities of absolute value V and angle in the plane) can be solved simultaneously to calculate the velocity and angle to the transducer of that vector.

[0005] Determining three vector components of velocity by means of multiple Doppler equations has also been discussed, for example, U. S. Patent No. 5,738,097 issued to Beach et al, and as discussed in the referenced patents U. S. Patent No. 5,488,953 and U. S. Patent No. 5,5540,230. These patents taught apparatus and methods useful for pulsed Doppler, rather than CW Doppler. For certain applications, such as without imaging to provide guidance for placement of a sample gate, CW Doppler is required.

U. S. Patent No. 4,062,237, issued to Fox, utilizes crossed beams and multiple frequencies where pairs of transducers operate at different frequencies so as to set up a difference frequency standing wave in the region of interest (equivalent to sensitive volume in this disclosure) in order to detect a Doppler frequency.

[0006] The method of using multiple Doppler measurements to determine the vector components of the velocity has been used by Daigle (1974 Doctoral Dissertation, Colorado State University) and implemented in previous patents, such as U. S.

5,488,953"Doppler Diffracting Transducer"and 5,540,230 entitled"Doppler Diffracting Transducer", both issued to Vilkomerson, the inventor herein. These patents, in addition to co-pending, commonly assigned patent application 09/522,799, entitled"Angle Independent Continuous Wave Doppler Device"have disclosed means and methods of using special transducers, known as diffraction-grating-transducers (DGTs), to generate the multiple beams needed to effect this method. Patents 5,488,953 and 5,5540,230 teach the use of these transducers for pulsed operation, and pending patent application 09/522,799, incorporated herein by reference describes using these transducers for continuous wave (CW) operation. CW operation is often desirable for medical and some industrial uses because CW operation does not require adjustment of a"sample gate"to define the spatial region in which the Doppler system will measure the velocity. Instead, the region where the beams overlap define the"sensitive region".

Patent application 09/522,799, incorporated herein by reference, describes how this sensitive region is determined for CW operation.

[0007] In almost all of the systems described in those patents, a single transducer 10 produces multiple beams (1,2) at different angles in a plane, seen in Figs. 1A, 1B. For these beams to strike the blood vessel to be measured, the transducer (10) is oriented so that the axis of the blood vessel is perpendicular to the elements (10a, lOb,... lOn) in the diffraction-grating transducer. This coplanar geometry is illustrated in Figures 1A, 1B.

As the method disclosed in the cited patents depends upon the simultaneous solution to two Doppler equations arising from two measurements of the same blood flow, accurate measurement of the velocity requires the transducer to be oriented so that both beams intersect the blood vessel; therefore the vessel must be in the plane of the transducer. In addition, the vessel must be straight and not tapered (which would cause a change in the blood velocity) between the intersections of the beams and the vessel.

[0008] In conventional medical terms, this is an"angle-independent"measurement in that the transducer can be at any angle to the surface of the blood vessel. For the typical intra-operative case, for example, the vessel is easily visible so it is relatively easy to orient the transducer"along"the vessel. In the commercially-available EchoCath EchoFlow BVM-1 device, employing diffraction-grating transducers for angle- independent Doppler measurement, alignment is assured by requiring that the power backscattered from each of the beams is substantially equal. No velocity measurements are displayed unless comparable amounts of Doppler power exist in the two beams used for the measurement.

[0009] While orienting the transducer intra-operatively is relatively easy, it may not be so for transcutaneous measurement of blood flow, where the blood vessel may not be easily visible. One can"find"a vessel under the skin by moving the transducer over the skin surface until a Doppler signal is detected. However, orienting the transducer along the vessel as required to make a velocity measurement as described above, requires additional time. This is an undesirable characteristic for both physicians and patients.

In such circumstances an apparatus and method whereby the transducer, once over the vessel, would not need to be aligned to measure velocity, is highly desirable.

[0010] Co-pending patent application 09/522, 799 discloses a system 100, shown here as Figure 2 that eliminates the need for transducer alignment, once over the vessel for measuring velocity. By making four measurements as shown there, (that is, two measurements in the x-z plane and two measurements in the y-z plane) the absolute velocity in all three spatial axes may be determined.

[0011] The configuration disclosed in co-pending application 09/522,799 shown here as Figure 2 has the disadvantage of operating in a"wiggle mode"which limits its accuracy. As the sensitive volume (the overlap of the beams from the two transducers creating a volume from which moving particles such as blood will create a signal) may not change position more that the square root of the diameter of the vessel (as can be seen in Figure 2), the difference in angle between the two beam angles is limited, and therefore, as discussed in Proceedings of 1999 IEEE Ultrasonic Syrnposium, "Considerations in the Design of Angle-Independent Doppler Systems Using Diffraction Grating Transducers", by D. Vilkomerson et al the accuracy of the velocity calculation is limited. A system and method which overcomes the aforementioned problems and which provides for the simultaneous solution of three independent Doppler frequency signals containing three corresponding unknown velocity components in three spatial dimensions in order to obtain the velocity components in terms of the measured Doppler frequencies using the associated beam angles is highly desired.

SUMMARY OF THE INVENTION [0012] Using diffracting grating transducers (DGTs) oriented so that the vector components of the beams they generate include all three dimensions, and driven and connected to receive signals such that three or more Doppler measurements can be made with them, the velocity vector components of a fluid, such as blood, can be determined by simultaneous solution of the three Doppler equations obtained. Multiple configurations utilizing the characteristics of DGTs, including matrix operation and sensitive volume adjustment with frequency are disclosed herein, as well as non-DGT configurations for implementing the inventive method.

[0013] A method of determining the velocity of dynamic particles flowing through a lumen, comprising transmitting one or more insonifying beams onto a region of the lumen and receiving three corresponding Doppler frequency shifted signals reflected from the dynamic particles in the region of the lumen at corresponding beam angles.

The corresponding Doppler frequency signals in combination contain information about the velocity components in three spatial dimensions. The velocity of the dynamic particles in three spatial dimensions is obtained by processing the received Doppler frequency signals in three spatial dimensions using the corresponding beam angles associated with each of the received signals.

[0014] A method of determining the velocity of dynamic particles flowing through a lumen, comprises positioning at least three transducers oriented with respect to one another at given angles, over an area of the lumen to be analyzed; selectively driving at least some of the transducers in order to produce a plurality of insonifying beams onto the lumen; and selectively receiving by at least some of the transducers, Doppler shifted versions of the plurality of insonifying beams characteristic of velocity components associated with the particles undergoing velocity analysis in three spatial dimensions; and determining the velocity based on the Doppler shifted versions of the plurality of insonifying beams and the angles between the at least three transducers.

[0015] An apparatus for determining the velocity of dynamic particles flowing through a lumen comprising a first transducer driven to provide an insonifying beam onto a predetermined region of the lumen for insonifying particles undergoing velocity analysis; at least three diffracting grating transducers arranged at predetermined angles to one another and to the first transducer so as to provide cooperating intersecting beams which intersect within the predetermined region of the lumen for receiving at each of the diffracting transducers, Doppler signals associated with the insonifying beam, the Doppler signals having velocity components in at least three dimensions; and a processor responsive to the received Doppler signals and to the angle between the diffracting transducers for determining the velocity of the dynamic particles.

[0016] A probe useful for determining the velocity of dynamic particles flowing through a lumen comprising an array of diffraction grating transducers arranged in a matrix configuration and operable to be positioned over a region of the lumen, wherein at least some of the transducers within a row are driven to transmit overlapping beams at a given angle onto the region of the lumen defining a sensitive volume therein, and wherein another segment of the array is operative to receive Doppler signals indicative of the transmitted beams containing velocity components in three spatial dimensions; and a processor responsive to the Doppler signals containing the velocity components for determining the velocity of the particles and three dimensions using the Doppler frequencies associated with the received signals and the associated beam angles.

BRIEF DESCRIPTION OF THE DRAWINGS [0017] Figure 1A provides an exemplary illustration of the operation of a DGT in"flip" mode for sensing the velocity of a fluid using beams at a first angle and at a second angle, respectively.

[0018] Figure 1B provides an exemplary view of operation of a DGT in"wiggle"mode for sensing the velocity of a fluid via movement of the sensitive volume along the vessel according to a change in frequency of the DGTs.

[0019] Figure 2 shows a top down view of two sets of dihedral diffraction grating transducers perpendicular to one another for calculating blood velocity independent of the plane of the transducer or their orientation.

[0020] Figures 3A and 3B provide exemplary top and side views respectively of a transducer apparatus for determining velocity of a fluid flowing through a lumen in accordance with the present invention.

[0021] Figure 4A provides a top view of an alternative embodiment of a transducer apparatus for determining velocity of a fluid flowing through a lumen comprising a central transducer and three surrounding transducers.

[0022] Figure 4B provides an alternative embodiment to the configuration of Figure 4A of a transducer apparatus having equal weighting associated with the X and Y axes.

[0023] Figure 4C provides an alternative embodiment of a transducer apparatus which utilizes three transducers optimized for different frequencies for making continuous measurements of velocity vector components.

[0024] Figure 4D is a schematic illustration of a side view of the embodiment of Figure 4B showing the transmit and receive beam pattern forming a sensitive volume region according to the present invention.

[0025] Figure 5 provides an alternative embodiment of a transducer apparatus comprising a four transducer arrangement having an open central area therebetween.

[0026] Figure 6A depicts an alternative embodiment of an array configuration of transducers offset from one another according to an aspect of the present invention.

[0027] Figure 6B illustrates that an optimal angle of the individual transducer elements on each member of an array as shown in Figure 6A is dependent on the shape of the array.

[0028] Figure 7 provides an exemplary illustration of a mechanical motion adjustment mechanism for use with the transducer apparatus in accordance with an aspect of the present invention.

[0029] Figure 8 provides an exemplary illustration of a non-diffraction grating transducer for use in accordance with an aspect of the present invention.

[0030] Figure 9 provides an exemplary illustration of a diffraction grating transducer whereby higher frequencies are used to insonate shallow regions while low frequencies insonate deeper regions of a vessel.

[0031] Figure 10 is an exemplary embodiment of a probe having attached to an end thereof the transducer arrangement depicted schematically in FIG. 4B in accordance with an aspect of the present invention.

DETAILED DESCRIPTION OF THE INVENTION [0032] There is disclosed herein configurations and methods of using diffraction- grating transducers (DGTs) and non-DGTs for CW Doppler measurements to provide accurate three-dimensional measurement of velocity of dynamic particles such as blood cells flowing through a lumen such as a blood vessel. These configurations are applicable for transcutaneous use, where a need exists for measuring the velocity without seeing the orientation of the vessel. Moreover, certain of these configurations can be used in an array form. With such an array, blood vessels can be found as well as measured, and may be used in conjunction with the means and method previously described in commonly assigned U. S. Patent 5,669,388. Such configurations may be modified as disclosed herein to be used to measure the velocity in three-dimensions.

[0033] Referring now to Figures 3A and 3B, there is shown a transducer apparatus or probe comprising two DGTs, 10 and 20, set at right angles with respect to one another; a third transducer, 50, which need not be a DGT, fills the space between them. The probe is operable to be positioned over a region of a lumen so as to determine the velocity of dynamic particles flowing through the lumen. Each transducer has a width w and length l, with the elements of the DGT (lOa... 10n, 20a... 20n) as shown. Figure 3A shows a top view of the transducer apparatus while Figure 3B depicts a side view of the structure. The orientation of the transducers relative to the x, y, and z axes are as shown.

[0034] In accordance with the present invention, a Doppler measurement is performed by sequentially driving the transducers 10 and 20 with a continuous wave (CW) or quasi-CW energy source, while receiving on the other DGT (i. e. transducer 20 and 10) and on the non-DGT 50. As shown in Figure 3B, the intersection of the transmitting beam from DGT 10 with the receiving beam from non-DGT 50, i. e. the sensitive volume, is (using simple trigonometry) a solid region starting w/2*tanß below the surface and 1/tant high and w wide; the sensitive volume corresponding to the intersection of the DGT's 20 receiving beam is the intersection of two beams. The value of ß is determined by the characteristic frequency and spacing of the elements in the DGT, =arcsine (c/f *4p), where c is the velocity of sound, f is the frequency, and p is the element-to-element spacing, as disclosed in U. S. Patent No. 5,488,953.

[0035] If, as shown in Figure 3B, a blood vessel 25 is in the sensitive volume, a Doppler signal will be received on both transducers 50 and 20. The Doppler signal is known to be the sum of the Doppler signal found from the dot product of the velocity vector with the transmitted beam vector plus the dot product of the velocity vector with the receive beam vector (multiplied by minus one) as described for example, in A 3-D PW Ultrasonic Doppler Flowmeter : Tlieory and Experimental Characterization, by Calzolai, et al, IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, 46, pp. 108-113 (1999).

[0036] The generalized three-dimensional velocity will be considered to be V= vxi + vyj +vzk (where bold i, j, k represents a unit vector in the x, y, and z axes, respectively). Therefore, the frequency shift and thus, the Doppler signal, received by transducer 50 (receive beam of-k,) from the beam transmitted by transducer 10, for example, which if one takes the wavelength of unity for simplicity), -sim#i +-cosk is Fso.10-vz+sin#*vx+cos#*vz and received on DGT 20 (with a receive beam of-sin#j-cos#k) of F20 l0= sinß*vx+cosß*vz +sin#*vy+cos#*vz.

[0037] Next, by driving DGT 20 to transmit a signal beam and receiving the reflected beam on transducer 50, the Doppler signal is given as F50,20=vz+sin#*vy+cos#*vz [0038] The data output from the received Doppler signals is then processed using a signal processor, for example to simultaneously solve these three equations in the three unknowns of vx, vy, vz, using the Doppler frequencies F50,10,F20,10, and F50,20: vz = (F50, 2o+ F50,, o- F2olo) *. 5 vx = (Fso., 0- (1 +cos#) *vz) *sur8-' vy=(F50,20-(1+cos#) *vz) *sinus~ and V = (VX + vy2 + VZ2) m [0039] In this manner, the apparatus of the present invention utilizes transducers to produce 3 (or more) independent Doppler equations in the three unknowns of vx, vy, and vz, and a processor to solve these three equations simultaneously to determine the velocity (in its three components) from the measured Doppler signals. Through the above process, the method previously disclosed in U. S. Patent No. 5,488,953 for 2 dimensions has been extended to three dimensions.

[0040] In the examples shown below, various embodiments are illustrated and their relative advantages and disadvantages are described. Note, however, that each of these embodiments utilize the method of establishing three independent Doppler equations and solving these equations.

Multiple Configurations [0041] Continuous Measurement : While the embodiment shown in Figure 3 is simple and uses just 3 transducers, (two DGTs and one non-DGT), operation requires two successive measurements, one driving DGT 10 while receiving on transducer 50 and DGT 20, the second driving on DGT 20 and receiving on transducer 50. This requires switching, particularly for DGT 20, which is both a receiver and a transmitter. As two measurements are required for each reading, the velocity data can not be updated as quickly as if only one measurement were used.

[0042] Alternative embodiments depicted in Figures 4A, 4B eliminate the aforementioned problem. As shown in Figures 4A and 4B, a central transducer 90 is surrounded by three corresponding diffraction grating transducers 60,70, and 80.

DGTs 60,70 and 80 as shown in Figure 4A, are each disposed adjacent to the central transducer 90. Transducers 60 and 80 are disposed at right angles to DGT 70. Each of the DGTs are operative to transmit or receive at corresponding beam angles which intersect with one another defining the sensitive volume in which the Doppler signals are generated. Figure 4B illustrates the configuration of DGTs 60,70 and 80 disposed about central transducer 90 and at given angles A relative to one another so as to provide substantially equal weighting about the x and y axes. By driving the central transducer 90 and receiving on three surrounding DGTs 60,70, and 80, three Doppler signals, dependent on the three spatial components of the velocity vector, are developed, one for each DGT. While this configuration requires 3 signal channels instead of the 2 signal channels used in the configuration shown in Figure 3, it allows continuous measurement of the velocity without any switching between transmit and receive modes. Note that the embodiment depicted in Figure 4B gives equal weighting to the x and y axis, while the embodiment shown in Figure 4A provides greater signal- noise ratio for the x-axis than the y-axis, as there are two DGTs (60,80) receiving along that axis rather than one. It is to be understood that the central transducer 90 may instead be operable as the receiving transducer while DGTs 60,70 and 80 may operate as transmitters and placed substantially flat onto the surface of the skin, for example, for transmitting insonifying beams onto the lumen. As shown in Figure 4D, the region or volume 49 where all four beams (i. e transmit and receive beams from 60,70,80, and 90) intersect (i. e the"diamond"shaped region), vector Doppler measurements result.

[0043] Referring now to Figure 4C, there is depicted an embodiment of the present invention which uses three DGTs optimized for three different frequencies, fl, f2, f3, with DGT 65 driven at frequency 1, DGT 75 at frequency 2, and DGT 85 at frequency 3. The spacing between the elements of the DGTs is arranged so that the angle ß of the beams from each of the three is the same, (i. e. fl*dl = f2*d2 = f3*d3). Therefore the three beams overlap in the same sensitive volume as identical transducers using the same frequency. However, if the frequencies are separated by more than the peak Doppler shift frequencies, which is typically about 50 KHz, the signal from the plane transducer 95 will contain three separate signals, corresponding to the Doppler shift around each of the frequencies. As shown, when the signal is passed through the three mixers 33 and low-pass filtered via filters 43, three Doppler signals DOPI, DOP2, DOP3 equivalent to Doppler signals from each of the DGTs is produced, which can be used to make continuous measurement of the three components of the velocity vector.

[0044] As is clear to one skilled in the art, the same principle, that of using different frequencies with DGTs of different element spacing, can allow multiple beam measurements to be made simultaneously from one receiving transducer. It should be noted that the use of a single receiver, rather than three, also reduces the cost of the system, since only one sensitive, shielded transducer and low-noise coaxial cable and only one pre-amplifier may be required. The present invention also contemplates direct digital signal processing on the single signal to yield the three Doppler components, however, at present, such direct processing is much more expensive than the use of three analog channels as shown in Figure 4C.

[0045] Figure 10 illustrates an exemplary embodiment of a probe having attached to an end thereof the transducer arrangement depicted schematically in FIG. 4B in accordance with an aspect of the present invention. As shown in Figure 10, probe 8 has formed on an end thereof three PVDF transmitting DGTs 60,70 and 80 surrounding an 8 millimeter (mm) flat receiving central transducer 90. The transducers were fabricated from P (VF2/TrFE) by spin coating a 70-micron thick film onto 1.75 mm thick FR4 circuit board onto which had been photo-etched the transducer pattern of thirty-six 100-micron wide elements on 215 micron pitch. The transducers are each about 7.5 mm long and 9 mm high.

[0046] Clear Central Area Transducer : In figure 5 there is shown a top view of a transducer configuration that allows the central area A to remain clear of transducers.

This is advantageous for finding the appropriate spot-to draw blood, for example. In this configuration, DGT 130 is driven to transmit while DGTs 100,110, and 120 would receive, thereby providing the three independent Doppler measurements needed to calculate the three components of velocity. As should be understood, any of the four transducers could be used as the transmitters with the other DGTs are operable as receivers. Further, one skilled in the art recognizes the symmetry between the transmit and receive functions, such that various combinations of transmitters and receivers could be used, (for example DGT 100 is driven to transmit an insonifying beam, with Doppler signals received on DGT 110 and DGT 120; transmitting on DGT 120 and receiving on DGT 100 and DGT 110, etc.) The key requirement is that the transmit and receive vectors provide three Doppler measurements such that there are three independent equations in the three unknown velocity components; these equations can then be solved simultaneously to obtain the velocity components in terms of the measured Doppler frequencies.

[0047] In addition to the configurations described herein, other combinations and sequences are also contemplated. For example, in the above configuration, transmitting on three of the transducers in turn and receiving on the fourth, will also produce the three Doppler equations containing the three velocity vectors, and will serve to provide all-angle Doppler measurements. In general, any configuration of transducers whose beams are in x, y, z directions and which produce three independent Doppler frequencies dependent on the components of the velocity to be measured, can be used for any-angle Doppler. Which configuration is optimal often depends on the particular measurement situation.

[0048] Matrix operation : An array of DGT transducers can also be used to cover an area and detect blood motion below it. In an array configuration use, we use the fact that the DGT can operate to transmit a beam at by driving or receiving on half the elements or simulate an ordinary transducer transmitting or receiving at 0 by driving or receiving with all the elements without any phase shift.

[0049] Figure 6 (a) depicts a preferred embodiment array configuration where each row (a, b, c, d, e) represents an alternating sequence of DGTs, with each row offset by half an element width we with respect to the previous row. This is shown in greater detail in Figure 6 (b). The two elements al and a2, shown as DGTs in Fig. 6B, when driven as DGTs, will produce overlapping beams 31 and 32, below the element shown as a non- DGT b2. Note that while transducer B2 is shown as a non-diffraction grating transducer, it is understood that this may be implemented via a DGT transducer that simulates operation of a plane transducer, for example by transmitting or receiving at zero degrees without any phase shift. As the beams from the DGTs are angled to each other, a sequence wherein the non-DGT-acting element transmits and the two DGTs receive, followed by one or both of the DGTs transmitting and the non-DGT-acting element receiving, would produce the required three Doppler equations containing the three required velocity vectors, i. e. function exactly as described for Figure 3. Thus any point on the array in Figure 6A could be selected for receiving., and therefore detect blood flow at any point below the array by appropriately driving the elements of the array as described.

[0050] This array configuration lends itself to the automatic placement procedure described in commonly assigned patent U. S. 5,669,388 issued in the name of Vilkomerson, the inventor herein. In that patent, incorporated herein by reference, a method of finding the Doppler signal by comparing the Doppler signals arising from various combinations of lines of sensitive elements is disclosed. The array disclosed herein can be used, as described below.

[0051] If the row b of the array in Figure 6A is driven, with the elements acting as a transmitting element, the b-row elements will produce"sensitive volumes"for detecting Doppler below certain of the elements in row c, namely cl (i. e. row c, column 1), c3, c5 ; if row d is driven similarly to row b, sensitive volumes will also exist below c2 and c4, i. e. every element in row c, which transducers can be connected so as to act as a non-DGT, will be over a sensitive volume. If row c is configured as a row of receivers, any blood vessel with velocity components in the x or y direction will produce a Doppler signal from that row.

[0052] If every other row, e. g. b, d, f, (not shown) h (not shown) and so on is set up as a DGT row (by the appropriate switching matrix, well-known to those skilled in the art) and driven to transmit, the other rows, in between the driven rows, can be configured an non-DGT elements, and the entire row will act as a sensor for any blood flowing in the x-y directions below those rows.

[0053] By interchanging the driving and receiving rows, e. g. driving rows a, c, e as DGTs and switching rows b, d, f as non-DGT receivers, complete coverage of the area below the array is achieved. As discussed in U. S. Patent No. 5,669,388, by the appropriate procedure the sensitive lines can be examined, and certain portions of the driving lines used, to find the spot that produces the best Doppler signal. That spot can then be interrogated as described above, in the fashion of Figure 3, to determine the vector velocity of the blood therein.

[0054] It should be further noted that the optimal angle of the individual transducer elements, e. g. shown as 45° in Figure 6B, on each member of the array depends upon the particular shape of the array member. If, for example, the array member is tall and thin, the appropriate angle is closer to horizontal for proper illumination of the region below the row below the element, as can be seen by drawing that geometry.

Adjustable Sensitive Volume Using DGTs [0055] As shown in Figure 3 (b), the sensitive volume may be controlled by: 1) the length, 1, and width w, of the elements; 2) the spacing between the elements; and 3) the angle of the beam, shown there as .

[0056] The distance below the transducer where the sensitive volume (where the midpoint of the beams intersect) is: dl = (g+w/2)/tan (1) and the width of the sensitive volume bw=w (2) and the height of the sensitive volume, i. e. the distance from dl where it starts to d2 where it ends, is h = 1/tan ß. (3) [0057] Therefore, to design the location and size of the sensitive volume, the dimensions can be adjusted, e. g. the width and length of the elements. Further adjustments can be implemented including how they are arranged, (e. g. by the size of the gap g and any padding below the transducers), by way of operation of a DGT, as described in patents U. S. 5,488,953 and U. S. 5,540,230, by = arcsin (c/4fp) (4) where c is the velocity of sound, p is the spacing between the elements of the DGT, and f the frequency of operation.

[0058] Figure 7 provides an example of how mechanical motion can be used to adjust the depth of the sensitive volume. As shown in Figure 7, the transducer apparatus or probe includes an adjusting knob 13 in mechanical communication with a set of pinion gears 16,18 coupled via belt 17 and reversing belt 19 for adjusting the gap g between transducers. That is, by turning adjusting knob, 13 mounted onto the transducer apparatus, the rack and pinion elements on the DGTs, 23 and 24, will move the DGTs away from each other (the reversing belt connecting the knob with the rack and pinion 22 ensures this) increasing"g"in equation (1). The depth of the sensitive volume will move down by the amount of the increase in the gap, divided by tanks, which for ß below 45 where tanß is less than one, acts to make the sensitive volume move down more than the gap is increased. It is, of course, contemplated that there exists many ways of providing such motion with the above provided as an example of such an embodiment, including electrical stimulation to cause mechanical motion of the transducers, for example.

[0059] Note that using a DGT, which allows a change in frequency of operation, provides more flexibility than non-DGT devices. Ultrasound transducers typically have 80% bandwidth, meaning that the frequency can be adjusted from. 6 of the center frequency to 1.4 of the center frequency, i. e. providing a factor of two in frequency range. By equations (1) and (3), halving the frequency would not only move the depth down by a factor of two (if 13 is in the range where tanft-ft), but increase the length of the sensitive volume by this factor.

[0060] For example, assume the configuration of Figure 4A, with w = 2 mm, g = 1 mm, a length of the transducer of 6 mm, and the element spacing (p of equation 4) of. 075 mm. Using equations 1,3, and 4, above, at 6 MHz the sensitive volume will start at 1.3 mm from the transducer surface and end at 5.3 mm; at 14 MHz, the same equations will yield a sensitive volume starting at 5.2 mm below the surface and ending at 20.6 mm.

Thus, the change in frequency can adjust the sensitive volumes so that they overlap and extend the region of operation of these configurations.

[0061] Combining mechanical motion with changes in the frequency of operation of the DGT provides for significant flexibility in adjusting the sensitive volume within which the velocity can be measured.

DGT and Non-DGT Transducers [0062] It should be noted that for many of the configurations described herein, non- DGT transducers structured as shown in Figure 8 can be used in place of the DGTs described. These transducers, being thicker, less flexible in operation (their ß cannot be changed), and heavier than DGTs used in these configurations, are not as desirable, but can be used if the particular situation allows their sub-optimal physical attributes and requires less expensive transducer elements.

[0063] However, such non-DGT transducers may not be used in the matrix embodiments described herein, as the transducers must be switchable between DGT and non-DGT operation, or where the sensitive volume is to be adjusted by means of adjusting the frequency of operation.

Extended Sensitive Volumes by Sequential or Simultaneous Multi-Frequency Operation [0064] It is understood that the beam from a DGT occurs at an angle to the perpendicular given by equation (3), the arcsine of (c/fd). Thus, the higher the frequency, the smaller the angle. With the DGT parallel to the surface, as shown in Figures 4 and 5 for example, a higher frequency, making the angle of the beam ß less, would lower the position of the sensitive volume. As tissue attenuates higher frequencies more rapidly than lower frequencies (typically. 6 dB/cm/Mhz), a sensitive volume being further below the surface, where the travel path for the ultrasound is longer and therefore attenuation increased, is not useful.

[0065] The configuration of Fig. 9, while sharing the physical drawbacks of the non- DGT transducer arrangement shown in Fig. 8, has distinct advantages. Because the perpendicular (0 angle) beam is at a high angle due to the orientation of the transducer, the high frequency beams will intersect at a shallow distance below the surface, (as determined by the angle of the transducer and the frequency chosen) where the high attenuation that comes with high frequency does not reduce the signal-noise ratio below usefulness, and low frequencies, which, in accordance with equation 4, produce a large angle to the perpendicular, will now form sensitive volumes much below the surface.

Because the attenuation for low frequencies is so much lower, these deep sensitive volumes will still allow the detection of any blood vessels therein. In this manner, the higher frequencies are used to insonate the more shallow regions, while the low frequencies insonate the deeper regions.

[0066] Operation at multiple frequencies simultaneously is also possible, and will produce multiple overlapping sensitive regions. As long as the transmitting frequencies, normally in the MHz, are separated by more than the maximum Doppler shift (< 50 KHz, typically), Doppler signals from the differing sensitive regions can be easily separated.

[0067] Any of the previous configurations using DGT transducers can be operated using multiple frequencies for multiple sensitive volumes, using these frequencies sequentially or in parallel. The only limitation on using the frequencies in parallel is the requirement, in medical applications, of limiting the acoustic power, and requiring the sum of the powers in all the frequencies to be no more than would be in one for non- parallel operation.

[0068] While the foregoing invention has been described with reference to the above embodiments, various modifications and changes can be made without departing from the spirit and scope of the invention.