Ultrasound imaging system with intrinsic doppler capability

• US 6,520,915 B1
• Filed: 11/24/2000
• Issued: 02/18/2003
• Est. Priority Date: 01/28/2000
• Status: Expired due to Term

• Current Assignee: U Systems Inc (General Electric Company)
• Original Assignee: U Systems Inc (General Electric Company)
• Inventors: Phung, Hue, Lin, Shengtz
• Primary Examiner(s): Jaworski, Francis J.
• Assistant Examiner(s): PATEL, MAULIN M
• Application Number:
  US09/721,888
• Time in Patent Office: 816 Days
• Field of Search: 600/454, 600/443, 600/441, 600/447, 600/456, 600/449, 600/455, 600/453, 128/916
• US Class Current: 600/453
• 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 8/13 Tomography A61B8/10, A61B8/12 take precedence; tomography for radiation diagnosis A61B6/02
• CPC Class Codes: G01S 15/8979 Combined Doppler and pulse-echo imaging systems
• 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 7/52047 for elimination of side lobes or of grating lobes; for increasing resolving power

Abstract

An ultrasound imaging system capable of generating a flow image from B-mode echo frames. For each location in a target region, phase information is derived from the B-mode echo frames, and frame-to-frame changes in the phase information are processed to detect the presence of fluid flow at that location. A demodulator receives the B-mode echo frames and demodulates them into baseband signal components from which the phase information is derived. Preferably, the demodulator is highly accurate and robust, such that frame-to-frame changes in the phase information are reliably measured. From the reliably measured phase information, a phase shift metric is then computed and thresholded by a user-adjustable threshold value. Flow is present if the phase shift metric is greater than the threshold value and is absent otherwise. The user may adjust the threshold value in real time through an input device, such as a knob or keyboard, to increase or decrease the flow sensitivity as desired and/or to eliminate the slow-moving clutter. Together with a B-mode image, the flow image is provided to an ultrasound display. For further clutter removal, the flow image may be suppressed for those locations having B-mode intensities below a predetermined lower threshold or above a predetermined upper threshold. The provided system provides a real-time flow image having the same high frame rate, spatial resolution, and coverage area as the B-mode image.

Claims

1. A method for imaging fluid flow in a target region, comprising the steps of:

transmitting a sequence of B-mode interrogating frames into the target region;

receiving from the target region a sequence of B-mode echo frames corresponding to the sequence of B-mode interrogating frames;

processing each B-mode echo frame to derive phase information for each location in the target region; and

for each location in the target region, detecting changes in the phase information across multiple B-mode echo frames for detecting the presence of fluid flow at that location.

2. The method of claim 1, further comprising the steps of:

deriving a B-mode output image for each B-mode echo frame, the B-mode output image comprising, at each location of the target region, an intensity value corresponding to an echo amplitude at that location;

deriving a flow image for each B-mode echo frame, the flow image comprising, at each location of the target region, a flow image value corresponding to fluid flow at that location; and

providing said flow image and said B-mode output image to an ultrasound display device;

wherein said flow image and said B-mode output image have a substantially identical frame rate and a substantially identical spatial resolution.

3. The method of claim 2, said step of deriving a flow image comprising the steps of:

computing a phase shift metric from the detected changes in phase information across multiple B-mode echo frames;

comparing the phase shift metric with a first threshold value; and

if the phase shift metric is less than the first threshold value, setting the flow image value to a null value.

4. The method of claim 3, said step of deriving a flow image further comprising the step of, if the phase shift metric is greater than the first threshold value, setting the flow image value to constant non-null value for indicating the presence of flow at that location.

5. The method of claim 3, said step of deriving a flow image further comprising the step of, if the phase shift metric is greater than the first threshold value, setting the flow image value to color-coded identifier that corresponds to a B-mode echo amplitude at that location.

6. The method of claim 3, further comprising the step of selectively suppressing the flow image value at each location according to said B-mode echo amplitude by performing the steps of:

comparing said B-mode echo amplitude to a predetermined lower threshold;

if said B-mode echo amplitude is lower than said lower threshold, resetting said flow image value to said null value;

comparing said B-mode echo amplitude to a predetermined upper threshold;

if said B-mode echo amplitude is higher than said predetermined upper threshold, resetting said flow image value to said null value.

7. The method of claim 3, said step of processing each B-mode echo frame to derive phase information yielding, for each location in the target region, a temporal sequence of phase values, and wherein, for that location, said step of computing a phase shift metric comprises the steps of:

computing a difference sequence from the temporal sequence of phase values, the difference sequence corresponding to the difference between adjacent elements of the temporal sequence of phase values;

computing an average difference sequence from the difference sequence by averaging each element therein with at least one neighboring element;

replacing each element in the average difference sequence with its absolute value;

filtering the average difference sequence with a contrast-preserving temporal filter; and

setting the phase shift metric equal to an output of the contrast-preserving temporal filter.

8. The method of claim 7, wherein said contrast-preserving temporal filter comprises a time-varying, first order, infinite impulse response filter designed to have a fast attack time during a systolic cycle period and a slow decay time during a diastolic cycle period.

9. The method of claim 3, wherein a user may dynamically adjust the first threshold value to optimize the resulting flow image.

10. The method of claim 8, wherein for each location in the target region, said step of comparing the phase shift metric with a first threshold value comprises the steps of:

spatially averaging the phase shift metric with that of neighboring locations in the target region; and

comparing the spatial average to the first threshold.

11. An apparatus for processing ultrasound signals, comprising:

a demodulator for receiving a sequence of frames of pulse-echo ultrasound data corresponding to pulse-echo ultrasound reflections received from a target region, said demodulator being adapted to generate, for each location in the target region and for each frame, a plurality of component baseband signals from which amplitude and phase information may be derived;

an amplitude detector coupled to receive said component baseband signals from said demodulator, said amplitude detector for computing, for each location in the target region and for each frame, an amplitude signal from said component baseband signals;

a phase detector coupled to receive said component baseband signals from said demodulator, said phase detector for computing, for each location in the target region and for each frame, a phase signal from said component baseband signals;

a phase discriminator coupled to receive said phase signals from said phase detector, said phase discriminator being adapted to generate, for each location in the target region, a phase difference metric corresponding to differences in said phase signals across a plurality of frames;

a motion discriminator coupled to receive said phase difference metrics from said phase discriminator, said motion discriminator being adapted to threshold said phase difference metrics by a predetermined threshold to determine the presence or absence of fluid flow at each location in the target region, said motion discriminator generating an output for each location in the target region equal to a null value in the absence of fluid flow and equal to a non-null value in the presence of fluid flow; and

an output device for providing said amplitude signal and said motion discriminator output to an ultrasound display device, whereby a user may perceive a pulse-echo amplitude display having a fluid flow display superimposed thereon.

12. The apparatus of claim 11, wherein said pulse-echo ultrasound data corresponds to B-mode signals, wherein said amplitude signals are B-mode intensity signals, wherein said pulse-echo amplitude display is a B-mode display, and wherein said fluid flow display has a spatial resolution similar to a spatial resolution of said B-mode display.

13. The apparatus of claim 12, wherein said component baseband signals comprise an in-phase component and a quadrature component.

14. The apparatus of claim 13, further comprising an input device for allowing the user to dynamically adjust said predetermined threshold value for optimizing discrimination between the presence and absence of fluid flow.

15. The apparatus of claim 14, wherein said non-null value is set equal to a constant value, whereby said fluid flow display is a binary display indicating the presence or absence of fluid flow at each location in the target region.

16. The apparatus of claim 15, wherein said phase discriminator comprises:

a difference sequence generator for generating, for each location in the target region, a difference sequence corresponding to the difference between phase signals of temporally adjacent frames at that location;

a temporal averaging device for computing an average difference sequence from said difference sequence;

an absolute value generator for computing the absolute value of said average difference sequence;

a digital filter for filtering said absolute value of said average difference sequence, said digital filter being a contrast-preserving temporal filter; and

a spatial averaging device for spatially averaging an output of said digital filter with corresponding outputs for nearby locations in the target region.

17. The apparatus of claim 16, wherein said contrast-preserving temporal filter comprises a time-varying, first order, infinite impulse response filter designed to have a fast attack time during a systolic cycle period and a slow decay time during a diastolic cycle period.

18. The apparatus of claim 13, the sequence of frames of pulse-echo ultrasound data comprising digital samples, the digital samples being taken at a sampling frequency, wherein said demodulator comprises:

a quadrature mixing device, said quadrature mixing device having a mixing frequency of one quarter of said sampling frequency, said quadrature mixing device comprising a half-band filter having a cutoff frequency of one-quarter of said sampling frequency, said quadrature mixing device for performing mirror-cancellation on said digital samples to produce intermediate signals; and

a complex mixer for rotating said intermediate signals to the baseband frequency to produce said in-phase component and said quadrature component.

19. The apparatus of claim 18, said sequence of frames of pulse-echo ultrasound data being associated with reflections from an ultrasound transducer generating acoustic bursts at a carrier frequency, wherein said complex mixer has a mixing frequency equal to one quarter of said sampling frequency minus said carrier frequency.

20. The apparatus of claim 19, said sequence of frames of pulse-echo ultrasound data being associated with reflections from an ultrasound transducer generating acoustic bursts at a carrier frequency, wherein said complex mixer has a mixing frequency equal to one quarter of said sampling frequency minus said a first frequency function, said first frequency function being a swept frequency function between a minimum frequency value and a maximum frequency value.

21. An apparatus for processing a sequence of B-mode echo frames received from a target region, comprising:

a demodulator for demodulating the B-mode echo frames into component baseband signals for each location in the target region;

a phase detector for deriving phase information from said component baseband signals for each location in the target region for each frame; and

a flow detector for deriving a flow image value at each location in the target region by detecting changes in said phase information across multiple B-mode frames for that location;

whereby a flow image may be derived from said flow image values, said flow image having a frame rate and spatial resolution similar to that of standard B-mode images that may be derived from the sequence of B-mode echo frames.

22. The apparatus of claim 21, wherein each of said flow image values is proportional to a fluid flow velocity at its respective location, whereby said flow image forms a non-aliased color Doppler ultrasound display for those locations having a flow velocity less than a B-mode Nyquist velocity or an aliasing display above the Nyquist velocity.

23. The apparatus of claim 22, wherein said B-mode Nyquist velocity is proportional to (i) a B-mode frame rate times the speed of sound in the target region, divided by (ii) a B-mode carrier frequency times the cosine of a Doppler angle for that location.

24. The apparatus of claim 21, the sequence of B-mode echo frames comprising digital samples taken at a sampling frequency, the component baseband signals comprising an in-phase component and a quadrature component, wherein said demodulator comprises:

a quadrature mixing device, said quadrature mixing device having a mixing frequency of one quarter of said sampling frequency, said quadrature mixing device comprising a half-band filter having a cutoff frequency of one-quarter of said sampling frequency, said quadrature mixing device for performing mirror-cancellation on said digital samples to produce intermediate signals; and

a complex mixer for rotating said intermediate signals to the baseband frequency to produce said in-phase component and said quadrature phase component.

25. The apparatus of claim 24, said flow detector being adapted and configured to perform the steps of:

computing a temporal difference sequence at each location corresponding to changes in said phase information at that location across multiple B-mode echo frames;

computing a phase shift metric from said temporal difference sequence;

comparing said phase shift metric with a first threshold value; and

setting said flow image value to a null value if said phase shift metric is less than said threshold value for indicating the absence of flow at that location.

26. The apparatus of claim 25, further. comprising an input device for allowing a user to dynamically adjust said first threshold value for optimizing said flow image.

27. The apparatus of claim 26, said flow detector being further adapted and configured to set said flow image value to non-null value if said phase shift metric is greater than said first threshold value for indicating the presence of flow at that location.

28. The apparatus of claim 27, said flow detector being further adapted and configured to setting said flow image value to a color-coded identifier corresponding to a magnitude of said phase shift metric if said phase shift metric is greater than said first threshold value for indicating both the presence of fluid flow and the phase shift metric magnitude corresponding thereto.

29. The apparatus of claim 28, said flow detector being adapted and configured to compute said phase shift metric from said temporal difference sequence by performing the steps of:

computing an average difference sequence from said temporal difference sequence by averaging each element therein with at least one neighboring element;

replacing each element in said average difference sequence with its absolute value;

filtering said average difference sequence with a contrast-preserving temporal filter; and

setting said phase shift metric equal to an output of said contrast-preserving temporal filter.

30. The apparatus of claim 29, wherein said contrast-preserving temporal filter comprises a time-varying, first order, infinite impulse response filter designed to have a fast attack time during a systolic cycle period and a slow decay time during a diastolic cycle period.

31. The apparatus of claim 25, further comprising:

an amplitude detector for deriving a B-mode image value from said component baseband signals for each location in the target region for each frame; and

a selective flow image suppressor coupled to receive said flow image value and said B-mode image value and to selectively suppress said flow image value based upon a comparison of said B-mode image value with a predetermined upper threshold and a predetermined lower threshold, wherein said flow image value is reset to null if said B-mode image value is greater than said upper threshold value, and wherein flow image value is reset to null if said B-mode image value is less than said lower threshold value.