Low-noise ultrasound method and beamformer system for doppler processing

• US 20060079784A1
• Filed: 10/04/2005
• Published: 04/13/2006
• Est. Priority Date: 10/08/2004
• Status: Active Grant

• Current Assignee: Microchip Technology Incorporated
• Original Assignee: Supertex Incorporated (Microchip Technology Incorporated)
• Inventors: Shifrin, Lazar A.
• Granted Patent: US 7,513,873 B2
• Time in Patent Office: Days
• US Class Current: 600/457
• 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: G01S 15/8979 Combined Doppler and pulse-echo imaging systems

Abstract

An ultrasonic low-noise analog beamformer for Doppler acquisition achieves high sensitivity by translating the frequency of the ultrasound echoes to an intermediate frequency, which is well above of the 1/f corner. This is accomplished by beamforming the downconverted RF signals instead of using their baseband representation. The baseband conversion, succeeding the beamformation, also incorporates the steps of clutter filtering and anti-aliasing. The invention is particularly suitable for low-voltage process technologies that support broadband applications.

Claims

1. A method for receive beamforming in a CW Doppler ultrasound imaging system, comprising the steps of:

receiving a plurality of RF ultrasound signals from a respective plurality of N transducer elements incorporated in an array, said RF signals are characterized by a frequency, f_RF;

translating the frequency of said RF signals to an intermediate frequency, f_IF, to produce a respective plurality of IF signals, said intermediate frequency is above the flicker noise corner frequency;

aligning the plurality of IF signals by a predetermined phasing;

summing said plurality of IF signals to produce a beamformed IF output; and

downconverting the beamformed IF output to a complex Doppler signal.

2. A method according to claim 1, wherein said step of translating the frequency comprises the step of mixing the RF signals with a local oscillator (LO) clock signal (f_LO) and IF filtering.

3. A method according to claim 2, wherein the step of IF filtering is performed after the step of mixing, and wherein the step of downconverting is performed after the step of summing.

4. A method according to claim 2, wherein the step of IF filtering is performed after the step of summing, and wherein the step of downconverting is performed after the step of IF filtering.

5. A method according to claim 1, wherein said step of aligning comprises the steps of:

calculating a desired delay profile across said plurality of transducer elements;

computing the per-element IF phase shift in conformity with said delay profile;

providing a predetermined set of LO clocks having their phases equally spaced in a range of 0°

to 360°

; and

selecting an adequate LO clock signal, f_LO, for the IF phase shift using the best-fit criteria.

6. A method according to claim 5, wherein all LO clocks are displaced up in frequency by f_IF with respect to a nominal transmit/receive frequency f_RF so that f_LO=f_IF+f_RF.

7. A low-noise ultrasonic CW Doppler beamformer for processing a plurality of RF signals produced by a transducer array, comprising:

a plurality of Doppler channels, each Doppler channel operative to receive an RF signal from a respective transducer element and to produce a phase-rotated signal at an intermediate frequency IF responsive to said RF signal and to a local oscillator clock;

an N-input summer coupled to said plurality of Doppler channels for summing all of said phase-rotated signals and to produce a beamformed IF signal;

a downstream processor coupled to said N-input summer operative to downconvert said beamformed IF signal and to provide an approximately clutter-free complex signal, said processor having an input signal terminal;

a multi-phase clock oscillator coupled to said plurality of Doppler channels for providing a plurality of k phase-shifted LO clocks having their phases evenly spaced within a 360°

range.

8. A low-noise ultrasonic CW Doppler beamformer according to claim 7, wherein said multi-phase clock oscillator is a k-phase (k/2-stage) twisted ring counter supplied by an external clock at the rate of 2k·

f_LO.

9. A low-noise ultrasonic CW Doppler beamformer according to claim 7, further comprising at least one low-pass filter (LPF) having an input port and an output port, said LPF is operative to filter the IF signals and to cancel the high-frequency products above a cut-off frequency.

10. A low-noise ultrasonic CW Doppler beamformer according to claim 7, wherein each of said plurality of Doppler channels comprises:

a buffer amplifier;

a mixer coupled to said buffer amplifier configured to mix a buffered RF signal with a LO clock, said mixer having a signal port, a local oscillator (LO) port, and an output; and

a phase-selecting means operative to select a LO clock from said plurality of k phase-shifted clocks.

11. A low-noise ultrasonic CW Doppler beamformer according to claim 10, wherein said phase-selecting means comprises:

a divide-by-2 counter connected to the LO port of the mixer; and

a multiplexer coupled to said divide-by-2 counter, said multiplexer comprising an output terminal providing a selected clock to said counter, a control port responsive for the clock selection, and k inputs.

12. A low-noise ultrasonic CW Doppler beamformer according to claim 10, wherein each of said plurality of Doppler channels further comprises an LPF, and wherein:

the input port of said per-channel LPF connected to the mixer output;

the output port of said per-channel LPF connected to a respective input of the summer; and

the output IF node connected to the input terminal of said downstream processor.

13. A CW Doppler beamformer according to claim 9, wherein each of the Doppler channels arranged to apply the mixer output to a respective input of the summer directly, and wherein:

the output IF node connected to the input port of a LPF; and

the output port of said LPF connected to the input terminal of said downstream processor.

14. A CW Doppler beamformer according to claim 7, wherein said downstream processor further comprises:

a first and a second clock terminals operative to provide a first and a second reference clocks, a first and a second demodulators, each having a signal port, a reference port, and an output, and wherein;

the first and second demodulators operative to convert the beamformed IF signal to in-phase and quadrature-phase components, respectively;

the signal ports of the first and second demodulators are coupled to said input signal terminal; and

the reference ports of the first and second demodulators are coupled to the first and second clock terminals, respectively.

15. A CW Doppler beamformer according to claim 14, wherein said downstream processor further comprises:

a first and a second filtering means connected to the outputs of the first and second demodulators, respectively, wherein each of said filtering means comprises a high-pass filter (HPF) and an anti-aliasing filter connected sequentially.

16. Downstream processor of claim 14, wherein said first and second reference clocks are in quadrature and running at the f_IF rate.

17. A method for receive beamforming in a CW Doppler ultrasound imaging system, comprising the steps of:

receiving a plurality of RF ultrasound signals from a respective plurality of N transducer elements incorporated in an array;

translating the plurality of RF ultrasound signals to a plurality of intermediate frequency (IF) signals, each of said plurality of IF signals being above the flicker noise corner frequency;

aligning the plurality of IF signals by a predetermined phasing;

summing said plurality of IF signals to produce a beamformed IF output; and

downconverting the beamformed IF output to a complex Doppler signal.