Demodulating wide-band ultrasound signals

• US 6,248,071 B1
• Filed: 01/28/2000
• Issued: 06/19/2001
• Est. Priority Date: 01/28/2000
• Status: Expired due to Fees

• Current Assignee: U-System Inc. (General Electric Company)
• Original Assignee: U-System Inc. (General Electric Company)
• Inventors: Lin, Shengtz
• Primary Examiner(s): Lateef, Marvin M.
• Assistant Examiner(s): PATEL, MAULIN M
• Application Number:
  US09/493,969
• Time in Patent Office: 508 Days
• Field of Search: 600/455, 600/456, 600/457, 600/443, 600/447, 600/448, 600/449, 600/444
• US Class Current: 600/443
• 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
• CPC Class Codes: G01S 7/52023 Details of receivers
• CPC Class Codes: G01S 7/52036 using analysis of echo signal for target characterisation
• CPC Class Codes: G01S 7/52046 Techniques for image enhancement involving transmitter or receiver image enhancement by image data processing G06T5/00

Abstract

A demodulator for use in an ultrasound information processing system is described, where in a preferred embodiment the demodulator uses a half-band filter to perform mirror-cancellation during the demodulation process. The demodulator comprises a mirror canceling quadrature mixer that mixes the input signal with quarter-sampling-frequency sinusoids and then low-pass-filters the mixed signals with half-band filters having a cutoff frequency of one-fourth the sampling frequency. In an ultrasound demodulator in accordance with a preferred embodiment, a significantly sharper mirror cancellation on the wideband input signal is achieved. Additionally, the number and complexity of mirror canceling filter operations significantly reduced, allowing for easier hardware implementation using less expensive, off-the-shelf components. Furthermore, simplified hardware for digitally providing harmonic imaging applications is achieved.

Claims

1. A demodulator for use in an ultrasound information processing system, said demodulator for receiving an input signal comprising digital samples, the digital samples being taken at a sampling frequency from an ultrasound transducer output, said demodulator for generating a demodulated output signal from said input signal, said demodulator comprising:

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 a baseband frequency to produce an in-phase component and a quadrature phase component of said demodulated output signal;

whereby an amplitude of said demodulated output signal corresponds to the vector magnitude of said in-phase component and said quadrature component.

2. The demodulator of claim 1, said ultrasound transducer generating acoustic bursts at a carrier frequency, wherein said complex mixer has a mixing frequency equal to one quarter of the sampling frequency minus a first frequency function, whereby said demodulated output signal contains frequency components substantially near a frequency equal to said carrier frequency minus said first frequency function.

3. The demodulator of claim 2, said half-band filter of said quadrature mixing device being a first half-band filter, said quadrature mixing device comprising:

a first mixer for multiplying said digital samples with a first sinusoid, said first mixer having an output coupled to an input of said first half-band filter;

a second mixer for multiplying said digital samples with a second sinusoid in quadrature phase with said first sinusoid; and

a second half-band filter, said second half-band filter having an input coupled to an output of said second mixer;

wherein said intermediate signals comprise an in-phase intermediate signal at the output of said first half-band filter and a quadrature-phase intermediate signal at the output of said second half-band filter.

4. The demodulator of claim 3, wherein said first mixer, said second mixer, said first half-band filter, and said second half-band filter are implemented on a single hardware chip, and wherein said first and second half-band filters are implemented in said hardware chip by finite impulse response filters having at least 40 taps.

5. The demodulator of claim 4, wherein said sampling frequency is greater than 40 MHz.

6. The demodulator of claim 2 wherein, an a Doppler processing mode, said first frequency function is fixed at said carrier frequency.

7. The demodulator of claim 6 wherein, in a B-mode processing mode, said first frequency function is a swept frequency function between a minimum frequency value and a maximum frequency value.

8. A demodulator for use in an ultrasound information processing system, said ultrasound system comprising an ultrasound transducer generating acoustic bursts at a carrier frequency, the ultrasound transducer receiving acoustic echo signals and generating an input signal therefrom, the ultrasound information processing system digitizing the input signal at a sampling frequency to generate digital samples, said demodulator comprising:

a quadrature mixer adapted to multiply said digital samples with quadrature-phase sinusoids at one quarter of said sampling frequency and filter the results with half-band filters each having a cutoff frequency of one-quarter of said sampling frequency to produce mirror-canceled signals; and

a complex mixer for rotating said mirror-canceled signals to baseband to produce complex mixer output signals, whereby amplitude and phase information of a demodulated signal can be derived from said complex mixer output signals.

9. The demodulator of claim 8, further comprising:

a secondary local oscillator for producing mixing signals that are mixed with the mirror-canceled signals by the complex mixer, said mixing signals being sinusoids at a frequency equal to the carrier frequency minus a first frequency function; and

for each complex mixer output signal, a programmable low-pass filter;

wherein, in a harmonic imaging mode, said first frequency function is set substantially near a frequency equal to twice the carrier frequency;

and wherein, in said harmonic image mode, said programmable low-pass filter is adapted to have a notch at the carrier frequency for producing a programmable low-pass filter output having suppressed first harmonic information and passed second harmonic information from said input signal.

10. The demodulator of claim 9, further comprising, for each programmable low-pass filter, an integrate-and-dump matched filter coupled to the output thereof for enhancing signal to noise performance of said demodulator.

11. The demodulator of claim 10, said demodulator also having a Doppler processing mode, wherein in said Doppler processing mode said first frequency function of said secondary local oscillator is fixed at said carrier frequency.

12. The demodulator of claim 11, said demodulator also having a B-mode processing mode, wherein in said B-mode processing mode said first frequency function is a swept frequency function between a minimum frequency value and a maximum frequency value.

13. The demodulator of claim 12, wherein said quadrature mixer is implemented on a first hardware chip having an at-least 40 tap finite impulse response (FIR) half-band filter, and wherein said programmable low-pass filter is implemented on a second hardware chip having substantially fewer taps than said first hardware chip, whereby said demodulator has reduced implementation cost by using fixed half-band filters for mirror canceling while using programmable digital filters in locations that do require a high number of FIR filter taps.

14. The demodulator of claim 13, wherein said sampling frequency is greater than 40 MHz.

15. A method for demodulating an ultrasound signal, said ultrasound signal having a carrier frequency, comprising the steps of:

sampling said ultrasound signal at a sampling frequency to produce digital samples;

mixing said digital samples with quadrature-phase signals at frequency equal to one-quarter of the sampling frequency;

filtering results of said mixing step with half-band filters each having a cutoff frequency at one-quarter of the sampling frequency to produce mirror-canceled signals; and

rotating said mirror-canceled signals to baseband using a complex mixer.

16. The method of claim 15, wherein said sampling frequency is at least four times greater than said carrier frequency.

17. The method of claim 16, further comprising the step of, prior to the step of sampling, filtering said ultrasound signal with an anti-aliasing filter having a cutoff frequency not greater than half the sampling frequency.

18. The method of claim 17, said step of rotating comprising the step of mixing said mirror-canceled signals with quadrature-phase sinusoids having a frequency equal to one-fourth of said carrier frequency minus a first frequency function, said first frequency function being equal to said carrier frequency in a Doppler mode, and said first frequency function being a swept frequency function a B-mode.

19. The method of claim 18, said first frequency function being substantially near twice the carrier frequency in a harmonic imaging mode.

20. The method of claim 19, further comprising the step of, in said harmonic imaging mode, low-pass-filtering at least one of said rotated signals with a low-pass filter having a notch at the carrier frequency for producing an output having suppressed first harmonic information and passed second harmonic information from said input signal.

21. An imaging method comprising:

sending ultrasound energy into a body, receiving from the body ultrasound energy related to interactions between the body and the ultrasound energy sent therein, and deriving an input signal related to the received ultrasound energy;

converting the input signal into digital samples related to a sampling frequency;

processing the digital samples through a digital quadrature mixer using first mixing signals at a frequency of approximating a quarter of the sampling frequency to produce quadrature mixer outputs;

filtering the quadrature mixer outputs through at least one digital filter having a passband approximately half the sampling frequency to produce intermediate digital signals;

processing the intermediate digital signals through a complex mixer to produce complex mixer outputs;

processing the complex mixer outputs to derive estimates of information related to at least one of an envelope of the input signal and a phase of the input signal; and

using the estimates to generate and display an image related thereto.

22. A method as in claim 21 in which the filtering comprises filtering the mixer outputs through at least one digital half-band filter.

23. A method as in claim 22 in which the processing of the complex mixer outputs comprises processing the complex mixer output through at least one low pass filter to produce low pass filter outputs.

24. A method as in claim 22 including processing the low pass filter outputs through at least one integrate-and-dump matched filter.

25. A method as in claim 21 in which the processing of the intermediate signals through the complex mixer comprises mixing the intermediate signals with second mixing signals at a frequency approximating the algebraic sum of quarter the sampling frequency and a frequency function.

26. A method as in claim 25 in which the imaging comprises a Doppler mode, and the frequency function is fixed.

27. A method as in claim 25 in which the imaging comprises a B-mode and the frequency function varies in a manner related to changes in an instantaneous frequency of the input signal.

28. An ultrasound imaging system comprising:

a transducer sending ultrasound energy into a body and receiving from the body ultrasound energy related to interactions between the body and the ultrasound energy sent therein, said transducer producing an input signal related to the ultrasound energy received thereby;

a converter receiving the input signal produced by the transducer and deriving therefrom digital samples related to a sampling frequency;

a source of first mixing signals at a frequency approximating a quarter of the sampling frequency;

a digital quadrature mixer receiving the first mixing signals and the digital samples and in response producing quadrature mixer outputs related thereto;

at least one digital filter receiving and filtering the quadrature mixer outputs to provide a passband approximately half the sampling frequency and thereby produce intermediate digital signals;

a source of second mixing signals;

a digital complex mixer receiving the second mixing signals and the quadrature mixing outputs to produce complex mixer outputs;

a detector receiving and processing the complex mixer outputs to derive estimates of information related to at least one of an envelope of the input signal and a phase of the input signal; and

a display displaying information related to the estimates of at least one of the envelope and the phase.

29. A system as in claim 28 in which the digital filter comprises at least one digital half-band filter.

30. A system as in claim 28 in which the complex mixer comprises at least one low pass filter produce low pass filter outputs.

31. A system as in claim 30 including at least one integrate-and-dump matched filter processing the low pass filter outputs.

32. A system as in claim 28 in which the source of the second mixing signals comprises a source of signals at a frequency approximating the algebraic sum of quarter the sampling frequency and a frequency function.

33. A system as in claim 32 in which the source of the second mixing signals produces signals at fixed frequency for system operation in a Doppler mode but at a frequency varying in a manner related to changes in an instantaneous frequency of the input signal for system operation in a B-mode.