Ultrasound blood flow/tissue imaging system

• US 5,477,858 A
• Filed: 07/09/1991
• Issued: 12/26/1995
• Est. Priority Date: 07/30/1986
• Status: Expired due to Term

• Current Assignee: Siemens Quantum, Inc. (Siemens AG)
• Original Assignee: Siemens Medical Systems, Inc. (Siemens AG)
• Inventors: Lazenby, John C., Crone, William E., Dunbar, Lee D., Norris, Paul R., Johnson, Richard K., Revell, Richard A., Axness, David R., Sargent, Brian J., Pirie, Bruce M., Veraya, Michael J., Polakowski, David M., Folline, John, Gardner, Donald R., Myrick, Charles C., Seader, Leonard D., Barrere, W. Gerrit, Heaton, Louis A., Chekerylla, James R., Clary, Thomas R., Chesarek, Richard H.
• Primary Examiner(s): JAWORSKI, FRANCIS J
• Application Number:
  US07/728,830
• Time in Patent Office: 1,631 Days
• Field of Search: 128/660.04, 128/660.05, 128/660.07, 128/661.09, 128/661.1, 73/625, 73/626, 73/631, 333/138
• US Class Current: 600/441
• CPC Class Codes: A61B 8/06 Measuring blood flow measuring volume flow in general G01F, e.g. G01F1/66, G01F1/72; measuring speed of fluids in general G01P5/00
• CPC Class Codes: A61B 8/4281 characterised by sound-transmitting media or devices for coupling the transducer to the tissue
• CPC Class Codes: A61B 8/467 characterised by special input means
• CPC Class Codes: G01S 15/8918 the array being linear
• CPC Class Codes: G01S 15/8981 Discriminating between fixed and moving objects or between objects moving at different speeds, e.g. wall clutter filter
• CPC Class Codes: G01S 15/8988 Colour Doppler imaging
• CPC Class Codes: G01S 15/899 Combination of imaging systems with ancillary equipment
• CPC Class Codes: G01S 7/52026 Extracting wanted echo signals Doppler systems G01S15/50; Doppler short range imaging systems G01S15/8979
• CPC Class Codes: G01S 7/52033 Gain control of receivers for seismic signals G01V1/245
• CPC Class Codes: G01S 7/52034 Data rate converters
• CPC Class Codes: G01S 7/52069 Grey-scale displays
• CPC Class Codes: G01S 7/52073 Production of cursor lines, markers or indicia by electronic means
• CPC Class Codes: G01S 7/52084 related to particular user interfaces special user input means for ultrasonic medical diagnostic devices A61B8/467
• CPC Class Codes: G10K 11/002 Devices for damping, suppressing, obstructing or conducting sound in acoustic devices G10K1/06 - G10K1/10 take precedence; for electro-mechanical transducers for communication H04R3/002
• CPC Class Codes: G10K 11/02 Mechanical acoustic impedances; Impedance matching, e.g. by horns; Acoustic resonators
• CPC Class Codes: G10K 11/346 using phase variation
• CPC Class Codes: G16H 40/40 for the management of medical equipment or devices, e.g. scheduling maintenance or upgrades
• CPC Class Codes: G16H 40/63 for local operation
• CPC Class Codes: G16Z 99/00 Subject matter not provided for in other main groups of this subclass

Abstract

An ultrasound imaging system is provided which produces gray scale images of the anatomy simultaneous with Doppler signal processing so as to provide a single real-time image that shows not only the vascular anatomy and surrounding tissue but also flow events in the vascular compartment. A phased linear array having near-continuous focus over the scanning field provides high resolution and very small Doppler sample volumes. The system tests incoming signals from throughout the scanning field for amplitude, phase and frequency so as to determine B-mode data, motion presence and direction, and echo source velocity.

Claims

1. A system for ultrasound blood flow imaging, comprising:

a transducer having a plurality of transducer elements arranged in a linear array;

transmitter means having a plurality of respective transmitters connected to a plurality of elements of said transducer, said transmitters selectively generating an ultrasound electric signal in synchronism with each other to cause said transducer to output a beam of ultrasound energy;

a switch having an input terminal connected to each of said transducer elements, said switch selectively connecting a subset of adjacent transducer elements to respective output ports;

ultrasound processing means connected to the output ports of said switch, said ultrasound processing means summing the signals from ultrasound elements that are substantially equidistant from the center transducer element in said beam and outputting said sum signals on respective output lines;

delay line means connected to each output line of said ultrasound processing means, said delay lines of said ultrasound processing means by amounts which increase for delay line means connected to transducer elements toward the center of said beam, thereby focusing said ultrasound return signals at the center of said beam;

summing means adding the outputs of each channel of said delay line means to each other to generate a delayed output signal;

clutter canceller means removing components of said delayed output signal corresponding to ultrasound returns from non-moving ultrasound reflectors;

flow processor means receiving said delayed output signal from said summing means and calculating a single blood flow velocity approximating a range of blood flow velocities that correspond to the frequency spectrum of said delayed output signal;

image memory means storing each of said data indicative of a single blood flow velocity for a plurality of discrete sample sites in each of a plurality of ultrasound beams;

display means receiving the output of said image memory means, said display means providing a visual indication of the blood flow at the sample sites beneath said transducer as a function of depth; and

control means controlling and coordinating the operation of said ultrasound imaging system.

2. The ultrasound imaging system of claim 1 wherein said transducer further comprises an ultrasound coupling wedge having a first coupling face in contact with said transducer elements, a second coupling face positioned at an angle with respect to said first face, and an ultrasound coupling medium positioned therebetween, said second face being adapted for contact with the skin of a patient in order to position said transducer elements at a fixed predetermined angle with respect to the skin of said patient.

3. The ultrasound imaging system of claim 2 wherein said coupling wedge comprises a hollow, wedge-shaped body defining a cavity filled with a fluid having low ultrasound attenuation and an acoustic impedance approximating the acoustic impedance of said patient, said wedge-shaped body having a pair of generally triangularly shaped side walls, an end wall, and two opposed, non-parallel, flexible membranes, each extending between said side surfaces and defining said first and second faces, respectively.

4. The ultrasound imaging system of claim 3 wherein said membranes have a thickness that is a multiple of one-quarter wavelength of the ultrasound signal.

5. The ultrasound imaging system of claim 3, further including a sound-absorbing material linking the inner surface of said end wall to absorb ultrasound signals reflected from the membrane covering the second face of said wedge-shaped body.

6. The ultrasound imaging system of claim 5 wherein the inner surface of said sound-absorbing material is angled toward one of said side walls so that any ultrasound reflected from said membrane and from the inner surface of said sound-absorbing material is reflected toward one of said side walls rather than toward said first or second faces.

7. The ultrasound imaging system of claim 2 wherein said switch disconnects said input terminal and said output port for a period of time that is proportional to the thickness of said wedge so that the portion of said system connected to said switches is blanked until ultrasound returns are received from sample sites beneath said wedge.

8. The ultrasound imaging system of claim 1 wherein said transmitter means comprises:

decoder means receiving serial data from said control means indicative of the power level of the transmitted ultrasound and the identity of the transmitters to be energized in each of a plurality of beams, said decoder means generating a first multi-bit word indicative of the power level of the ultrasound signal and a second multi-bit word identifying the transmitters to be energized;

power supply means generating a transmit voltage determined by said first multi-bit word;

signal-generating means outputting a predetermined number of transmit switching pulses having a predetermined repetition rate and pulse width characteristic; and

respective pulse-generating means connected to each of said transducer elements, said pulse-generating means being selectively enabled by said second multi-bit word and, when enabled, outputting said ultrasound electric signal to its respective transducer element during said transmit switching pulses.

9. The ultrasound imaging system of claim 1 wherein ultrasound reflected from sample sites beneath said transducer elements is received in a plurality of beams identified by respective beam numbers, each of said beams being formed by N adjacent transducer elements, and wherein said switch further comprises:

transducer element selector means receiving a digital word from said control means indicative of one of said beam numbers and outputting a plurality of transducer element enable signals designating the transducer elements forming said beam; and

N switching circuits, each of which has a plurality of inputs connected to one of said transducer elements and all transducer elements spaced multiples of N transducer elements from said transducer element, respectively, each of said switching circuits outputting the ultrasound signal from one of the transducer elements as designated by said transducer element selector means, whereby the ultrasound signals from all of said transducer elements are process in N channels.

10. The ultrasound imaging system of claim 1 wherein said delay line means comprise:

a tapped delay line receiving the output from said ultrasound processing means, said tapped delay line delaying the output of said ultrasound processing means by one of a plurality of fixed delay periods, each corresponding to respective delay line taps;

a variable delay line receiving an input from a tap of said fixed delay line, said variable delay line further delaying the output of said multiplexer by an amount determined by a plurality of control signals received from said control means in order to generate a delayed output signal; and

delay control means generating said delay control signal as a function of both the depth from which an ultrasound return is being received and the distance between the center element of each beam and the element associated with the delay line channel.

11. The ultrasound imaging system of claim 10 wherein said variable delay line further includes impedance control means for varying the input and output impedance of said variable delay line to prevent signals from being reflected from the input and output of said delay line.

12. The ultrasound imaging system of claim 10 wherein said delay lines comprise:

a plurality of inductors connected in series; and

a plurality of varactors connected between the junction between adjacent inductors and a control input line.

13. The ultrasound imaging system of claim 12 wherein said varactors are arranged in alternating polarity, with varactors connected with one polarity connected to a first control input line and varactors connected with the other polarity connected to a second control input line, said control input lines receiving respective control input signals having opposite polarities and equal absolute values.

14. The ultrasound imaging system of claim 10 wherein said tapped and variable delay lines are mounted on a circuit board, and wherein the taps of said tapped delay line are selectively applied to said variable delay lines by multiplexer means, said multiplexer means being controlled by the location that said circuit board is mounted in a larger circuit board, whereby the delay line channel associated with said tapped delay line is determined by the location of the circuit board containing said tapped delay line in said larger circuit board.

15. The ultrasound imaging system of claim 1 wherein said ultrasound processing means further includes a dynamic aperture circuit, comprising:

a plurality of variable gain circuits receiving respective inputs from each channel of said delay line means, each of said variable gain circuits generating an output bearing a ratio to the input that is determined by a respective gain control signal; and

gain control means generating said gain control signals, said gain control means increasing the ratio between their output and input for variable gain circuits receiving inputs from delay line channels further away from the center of each beam as ultrasound returns from deeper sample sites are processed, whereby the aperture of said transducer becomes larger for increasingly deeper sample sites.

16. The ultrasound imaging system of claim 15 wherein said variable gain circuits are voltage-controlled attenuators.

17. The ultrasound imaging system of claim 15 wherein a plurality of variable gain circuits receive respective inputs from the outputs of the channels of said delay line means, each of said variable gain circuits generating an output bearing a ratio to the input that is determined by a respective gain control signal;

and gain control means generating said gain control signals, said gain control means increasing the ratio between their output and input as a function of the elapse in time from the transmission of each ultrasound pulse, whereby the gain is increased for deeper sample sites.

18. The ultrasound imaging system of claim 15 further comprising:

an ultrasound coupling wedge having a first coupling face in contact with said transducer elements, a second coupling face positioned at an angle with respect to said first face, and an ultrasound coupling medium positioned therebetween, said second face being adapted for contact with the skin of a patient in order to position said transducer elements at a fixed predetermined angle with respect to the skin of a patient;

a plurality of variable gain circuits receiving respective inputs from the signals output by the channels of said delay line means, each of said variable gain circuits generating an output bearing a ratio to the input that is determined by a respective gain control signal; and

gain control means generating gain control signals that make said ratio substantially zero for a period that is proportional to the thickness of said wedge, whereby the components of said system connected to said variable gain circuits are effectively blanked until ultrasound returns are received from sample sites beneath said wedge.

19. The ultrasound imaging system of claim 1 wherein said system further comprises memory means for reordering the data received from said clutter canceller means, said memory means comprising a digital memory receiving said data for each beam M sets of N ultrasound return samples, each of the N ultrasound return samples in each set being taken in succession from the same ultrasound transmission at N different discrete sample sites and each of the M sets of such return samples being taken at the same beam location from M successive ultrasound transmissions, said memory means outputting said data corresponding to N sets of M ultrasound return samples, all of the M ultrasound return samples in each set being output in succession and corresponding to output returns taken from M successive ultrasound transmissions at the same discrete sample site, and each of the N sets of such return samples being taken at said N respective sample sites, whereby data are written into said memory means in the sequence of data words S₁,1 ;

S₁,2 ;

S₁,3 ;

. . . S₁,N ;

S₂,1 ;

S₂,2 . . . S_2N ;

. . . S_M1 ;

S_M,2 . . . S_M,N and read out from said memory means in the sequence of data words S₁,1 ;

S₂,1 ;

S₃,1 . . . S_M,1 ;

S₁,2 ;

S₂,2 ;

S₃,2 ;

. . . S_M,2 ;

. . . S₁,N ;

S₂,N ;

S₃,N ;

. . . S_M,N.

20. The ultrasound imaging system of claim 1 wherein said flow-processing means generates data indicative of the magnitude of each of a plurality of discrete frequency components in said delayed output signal, each of said discrete frequencies corresponding to a discrete blood flow velocity, said flow processor means further calculating said single blood flow velocity from said frequency component data receives successive sets of data, each corresponding to a plurality of ultrasound return samples taken from the same sample site, each of said ultrasound return samples having a frequency spectrum of f_d, f_d ±

f_r, f_d ±

2f_r, f_d ±

3f_r . . . f_d ±

Mf_r, where f_r is the repetition frequency of said ultrasound transmissions, and M is an integer greater than 1, and f_d is the center frequency of said ultrasound electric signal, said flow processor comprising N band-pass filters, each having respective pass bands effectively centered at f_o, f_o ±

f_r . . . f_o ±

2f_r . . . f_o ±

3f_r . . . f_o ±

Mf_r, where f_o =(f_r /N)P and P=0, 1, 2, . . . N-1, whereby the magnitude of the output of each band-pass filter is indicative of the velocity of moving sound scatterers at said sample site.

21. The ultrasound imaging system of claim 20, wherein said flow processor means receives successive sets of data, each corresponding to a plurality of ultrasound return samples taken from the same sample site, each of said ultrasound return samples having a frequency spectrum of f_d, f_d ±

f_r, f_d ±

2f_r, f_d ±

3f_r . . . f_d ±

Mf_r, where f_r is the repetition frequency of said ultrasound transmissions, and M is an integer greater than 1 and f_d is the center frequency of said ultrasound electric signal, said flow processor comprising N band-pass filters, each having respective pass bands effectively centered at f_o, f_o ±

f_r . . . f_o ±

2f_r . . . f_o ±

3f_r . . . f₀ =Mf_r, where f_o =(f_r /N)P and P=0, 1, 2, . . . N-1, whereby the magnitude of the output of each band-pass filter is indicative of the velocity of moving sound scatterers at said sample site.

22. The ultrasound imaging system of claim 21 wherein said band-pass filters comprise a digital comb filter.

23. The ultrasound imaging system of claim 21 wherein said band-pass filters comprise:

multiplying means generating the products of each of said ultrasound return samples and 2N coefficients corresponding to Cos2π

pf_o t and Sin 2π

f_o t where t=Y(f_r) and Y=0,1, 2, 3, 4 . . . N-1; and

a plurality of accumulator means for adding the products from said multiplying means for each value of f_o to all previous products for each value of f_o so that, for each sample site, said accumulator means outputs an accumulated product for each value of f_o, whereby accumulated products correspond to respective magnitudes of flow velocity of moving sound scatterers.

24. The ultrasound imaging system of claim 23 wherein N is equal to 16, whereby said coefficients have only 5 unique absolute values, thereby reducing the complexity of multiplication and coefficient storage.

25. The ultrasound imaging system of claim 23 wherein said accumulator means are implemented with arithmetic logic units so that the output of said multiplying means can be set to zero thereby eliminating the need for negative and zero valued coefficients to be applied to said multiplying means.

26. The ultrasound imaging system of claim 25 wherein said accumulator means are grouped such that a single coefficient is used for the computation of the in-phase or quadrature outputs of four of said filters, thereby minimizing the number of required coefficients.

27. The ultrasound imaging system of claim 25 wherein said accumulator means are implemented with a pair of storage registers connected in series such that the multiplying means and arithmetic logic units can be time shared in order to reduce the required number of multiplying means and arithmetic logic units.

28. The ultrasound imaging system of claim 20 wherein said flow processor means further includes centroid means comprising:

amplitude-determining means receiving said flow velocity data and determining the amplitude of each of said discrete Doppler frequencies;

peak amplitude-detecting means for determining which of said Doppler frequencies has the maximum amplitude and for providing an output indicative thereof;

first calculating means receiving said flow velocity data and the output of said peak amplitude detection means, said first calculating means calculating a first value from said digital flow velocity data, said first value corresponding to the sum of the magnitudes of at least one discrete Doppler frequency above and at least one discrete Doppler frequency below the discrete Doppler frequency having the maximum amplitude;

second calculating means receiving said flow velocity data and the output of said peak power detection means, said second calculating means calculating a second value from said digital flow velocity data, said value corresponding to the sum of the magnitudes of the discrete Doppler frequency having the maximum amplitude and the magnitudes of at least one discrete Doppler frequency above and at least one discrete Doppler frequency below the discrete Doppler frequency having the maximum amplitude, and

third calculating means calculating as said data indicative of a single blood flow velocity the ratio of said first value to said second value.

29. The ultrasound imaging system of claim 28, further including thresholding means substracting a fixed value from each spectral amplitude at said discrete Doppler frequencies and then outputting to said centroid means the magnitudes of all discrete Doppler frequencies having a magnitude greater than a predetermined value.

30. The ultrasound imaging system of claim 1, further including flow velocity correction means receiving said data indicative of a single blood flow velocity, said flow velocity correction means scaling the blood flow velocity corresponding to said flow velocity data by the cosine of an intercept angle approximating the angle between said ultrasound beam and the blood vessel being imaged, thereby providing a more accurate indication of the velocity of the blood in said vessel.

31. The ultrasound imaging system of claim 30, wherein said ultrasound beam passes through a coupling wedge positioned between said transducer and the skin of a patient, said coupling wedge positioning said transducer at a fixed angle with respect to the skin of said patient, and wherein said intercept angle is selected to correspond to said fixed coupling wedge angle.

32. The ultrasound imaging system of claim 30, further including means for selecting a specific intercept angle for each of a plurality of respective sample site ranges such that blood velocity may be accurately determined in a plurality of vessels positioned at different depths and having different angular orientations.

33. The ultrasound imaging system of claim 1 wherein said system measures blood flow velocity and tissue return average power at discrete physical locations along said transducer and at discrete sample sites, said system further displaying measured blood velocity or tissue power image components at discrete locations on said screen corresponding to said discrete physical locations and sample sites, said system further including interpolator means for displaying interpolated blood flow velocity image or tissue power components at discrete locations on said screen without measuring blood flow velocities or tissue power at physical locations and sample sites corresponding thereto, said interpolated image components being interspersed among said measured blood flow velocity or tissue power image components and being derived from the measured blood flow velocity or tissue power image components adjacent thereto.

34. The ultrasound imaging system of claim 1 wherein said system further provides tissue data indicative of the magnitude and position of ultrasound returns from non-moving tissue, and wherein said system further includes merge means receiving said data from said flow processor means and said tissue data, said merge means applying said data from said flow processor means to said image memory for beam locations and sample sites in which the magnitude of data from said flow processor is greater than a predetermined value and, if said flow processor data is not greater than said predetermined magnitude, said merge means applying said tissue data to said image memory for said beam locations and sample sites.

35. A crossbar switch adapted for use in an ultrasound imaging system for receiving a plurality of ultrasound beams through N adjacent transducer elements, said crossbar switch comprising:

transducer element selector means outputting a plurality of transducer element enable signals designating the transducer elements forming said beam; and

N switching circuits, each of which has a plurality of inputs connected to one of said transducer elements and all transducer elements spaced multiples of N transducer elements from said transducer element, respectively, each of said switching circuits connecting the transducer element designated by said transducer element selector means to an output port, whereby all of said transducer elements may be processed in N channels.

36. A system for processing signals corresponding to reflected ultrasound received through a plurality of adjacent ultrasound transducer elements, comprising multiplexer means for summing the outputs of transducer elements positioned equidistant from a transducer element at the approximate center of said plurality of adjacent transducer elements, whereby said multiplexer means reduces by approximately fifty percent the number of channels required to process said ultrasound signals from said transducer elements.

37. In a delay line comprising a plurality of inductors connected in series, a plurality of varactors connected between the junction between adjacent inductors, and a control input line connected to said varactors to control the capacitance of said varactors and hence the delay of said delay line, the improvement comprising impedance control means for continuously varying the input and output impedance of said delay line as a function of said control signal so that the input and output impedance varies in accordance with the impedance of said delay line as the impedance of said delay line varies with the capacitance of said varactors.

38. A delay line comprising a plurality of inductors connected in series, a plurality of varactors connected between the junction between adjacent inductors, and a control input line, said varactors being arranged in alternating polarities, with varactors connected in one polarity connected to a first control input line and varactors connected in the other polarity connected to a second control input line.

39. In an ultrasound imaging system generating blood flow velocity data from a beam of ultrasound returns taken from different sample site depths through an ultrasound transducer having a plurality of transducer elements, said beam being centered about at least one transducer element, a dynamic aperture system comprising:

a plurality of variable gain circuits receiving respective signals from said transducer elements, each of said variable gain circuits generating an output bearing a ratio to the input that is determined by a respective gain control signal; and

gain control means generating said gain control signals, said gain control means increasing the ratio between their output and input for signals from transducer elements further away from the center of each ultrasound beam as ultrasound returns from deeper sample sites are processed, whereby the aperture of said transducer becomes larger for sample sites of increasing depth.

40. The dynamic aperture system of claim 39 wherein said variable gain circuits are voltage-controlled attenuators.

41. In an ultrasound imaging system generating blood flow velocity data from a beam of ultrasound returns taken from different sample site depths, memory means for reordering said blood flow velocity data, comprising a digital memory receiving said blood flow velocity data for each beam of said ultrasound returns corresponding to M sets of N ultrasound return samples, each of the N ultrasound return samples in each set being taken in succession from the same ultrasound transmission at N different discrete sample site depths and each of the M sets of such return samples being taken at the same beam location from M successive ultrasound transmissions, said memory means outputting said data corresponding to N sets of M ultrasound return samples, all of the M ultrasound return samples in each set being output in succession and corresponding to ultrasound returns taken from said M successive ultrasound transmissions at the same discrete sample site depth, and each of the N sets of such return samples being taken at said N respective discrete sample byte depths, whereby data are written into said memory means in the sequence of data words S₁,1 ;

S₁,2 ;

S₁,3 ;

. . . S₁,N ;

S₂,1 ;

S₂,2 . . . S_2N ;

. . . S_M,2 . . . S_M,N and read out from said memory means in the sequence of data words S₁,1 ;

S₂,1 ;

S₃,1 ;

. . . S_M,1 ;

S₁,2 ;

S₂,2 ;

S₃,2 ;

. . . S_M,2 ;

. . . S₁,N ;

S₂,N ;

S₃,N ;

. . . S_M,N.

42. In an ultrasound imaging system generating successive sets of data, each corresponding to a plurality of ultrasound return samples taken from the same sample site, said return samples having a frequency spectrum of f_d, f_d ±

f_r, f_d ±

2f_r, f_d +3f_r . . . f_d ±

Mf_r, where f_r is the repetition frequency of said ultrasound transmissions and M is an integer greater than 1, a flow processor comprising N band-pass filters, each having respective pass bands centered at f_o, f_o ±

f_r, f_o ±

2f_r, f_o ±

3f_r . . . f_o Mf_r, where f_o =(f_r /N)P and P=0, 1, 3 . . . N-1, whereby the magnitude of the output of each band-pass filter is indicative of the velocity of moving sound scatterers at said sample site.

43. The flow processor of claim 42 wherein said band-pass filters comprise a digital comb filter.

44. The flow processor of claim 42 wherein said band-pass filters comprise:

multiplying means generating the products of each of said ultrasound return samples and 2N coefficients corresponding to Cos2π

f_o t and Sin 2π

f_o t, where t=Y/(f_r) and Y=0, 1, 2, 3, 4 . . . N-1; and

a plurality of accumulator means for adding the products from said multiplying means for each value of f_o to all previous products for each value of f_o to any previous products for each value of f_o so that, for each sample site, said accumulator means outputs an accumulated product for each value of f_o, whereby said accumulated products correspond to respective magnitudes of flow velocity of moving sound scatterers at said sample site.

45. The ultrasound imaging system of claim 44 wherein N is equal to 16, whereby said coefficients have only 5 unique absolute values, thereby reducing the complexity of multiplication and coefficient storage.

46. The ultrasound imaging system of claim 44 wherein said accumulator means are implemented with arithmetic logic units so that the output of said multiplying means can be set to zero, thereby eliminating the need for negative and zero valued coefficients to be applied to said multiplying means.

47. The ultrasound imaging system of claim 44 wherein said accumulator means are grouped such that a single coefficient is used for the computation of the in-phase or quadrature outputs of four said filters, thereby minimizing the number of required coefficients.

48. The ultrasound imaging system of claim 46 wherein said accumulator means are implemented with a pair of storage registers connected in series such that the multiplying means and arithmetic logic units can be time shared in order to reduce the required number of multiplying means and arithmetic logic units.

49. In an ultrasound imaging system generating flow velocity data indicative of the magnitude of a plurality of discrete ultrasound Doppler frequencies, velocity-determining means receiving said flow velocity data and generating a word indicative of a single blood flow velocity that approximates a range of said ultrasound frequencies, said velocity-determining means comprising:

power-determining means receiving said flow velocity data and determining the power of each of said discrete Doppler frequencies;

peak power-detecting means for determining which of said Doppler frequencies has the maximum power and for providing an output indicative thereof;

first calculating means receiving said flow velocity data and the output of said peak power detection means, said first calculating means calculating a first value from said digital flow velocity data, said first value corresponding to the sum of the magnitude of at least one discrete Doppler frequency above and at least one discrete Doppler frequency below the discrete Doppler frequency having the maximum power;

second calculating means receiving said flow velocity data and the output of said peak power detection means, said second calculating means calculating a second value from said digital flow velocity data, said second value corresponding to the sum of the magnitude of at least one discrete Doppler frequency above and at least one discrete Doppler frequency below the discrete Doppler frequency having the maximum power; and

third calculating means calculating as said data indicative of a single blood flow velocity the ratio of said first value to said second value.

50. In an ultrasound transducer adapted for use with an ultrasound imaging system transmitting and receiving a plurality of ultrasound signals from respective transducer elements arranged in a linear array, said ultrasound signals being coupled through a coupling wedge comprising a hollow, wedge-shaped body defining a cavity filled with a coupling fluid, said wedge-shaped body having a first wall in contact with said transducer elements, a second wall positioned at an angle with respect to said first wall and adapted to contact the skin of a patient in order to position said transducer elements at a fixed predetermined angle with respect to the skin of said patient, and a third wall extending between said first and second walls to form a triangle with said first and second walls, the improvement comprising a sound-absorbing material lining the inner surface of said third wall to absorb ultrasound signals reflected from said second wall back into said coupling fluid.

51. The transducer of claim 50 wherein said third wall and said sound-absorbing material are angled toward one of said side walls so that any ultrasound reflected from said sound-absorbing material is reflected toward one of said side walls rather than toward said first or second walls.

52. In an ultrasound transducer adapted for use with an ultrasound imaging system transmitting and receiving a plurality of ultrasound signals from respective transducer elements arranged in a linear array, said ultrasound signals being coupled through a coupling wedge comprising a hollow, wedge-shaped body defining a cavity filled with a coupling fluid, said wedge-shaped body having a first wall in contact with said transducer elements, a second wall positioned at an angle with respect to said first wall and adapted to contact the skin of a patient in order to position said transducer elements at a fixed predetermined angle with respect to the skin of said patient, a third wall extending between said first and second walls to form a triangle with said first and second walls, and two opposed, generally parallel, triangularly shaped side walls enclosing the volume between said first, second and third walls, the improvement comprising a configuration for said coupling wedge wherein said third wall is angled toward one of said side walls so that any ultrasound reflected from said third wall is reflected toward one of said side walls rather than toward said first or second walls.