Method, apparatus and applications for combining transmit wave functions to obtain synthetic waveform in ultrasonic imaging system

• US 5,891,038 A
• Filed: 12/30/1996
• Issued: 04/06/1999
• Est. Priority Date: 12/30/1996
• Status: Expired due to Term

• Current Assignee: General Electric Company
• Original Assignee: General Electric Company
• Inventors: Chim, Stanley Siu-Chor, Mo, Larry Y. L., Seyed-Bolorforosh, Mir Said
• Primary Examiner(s): Jaworski, Francis J.
• Application Number:
  US08/778,202
• Time in Patent Office: 827 Days
• Field of Search: 600/441, 600/443, 600/447, 73/599, 73/602, 367/11, 367/103, 367/105, 367/138
• US Class Current: 600/447
• CPC Class Codes: G01S 15/107 using frequency agility of carrier wave
• CPC Class Codes: G01S 15/108 using more than one pulse per sonar period
• CPC Class Codes: G01S 15/8952 using discrete, multiple frequencies
• CPC Class Codes: G01S 15/8977 using special techniques for image reconstruction, e.g. FFT, geometrical transformations, spatial deconvolution, time deconvolution
• CPC Class Codes: G01S 7/52028 using digital techniques
• CPC Class Codes: G01S 7/52033 Gain control of receivers for seismic signals G01V1/245
• CPC Class Codes: G03B 42/06 using ultrasonic, sonic or infrasonic waves

Abstract

A signal processing technique for improving the axial resolution and/or the sensitivity of an ultrasonic imaging system. This technique also improves the lateral resolution and the depth of field. The technique is based on combining two or more transmit wave functions in an optimal manner to achieve a synthetic waveform which has greater bandwidth and/or energy than the individual wave functions. This scheme operates on the imaging data before the data reaches the envelope detector, while the phase information is still maintained within the signal. Using a synthetic transmit wave design approach, the effective emitted pressure waveform would have a bandwidth which would be wider than the transducer bandwidth with high sensitivity. The synthetic transmit waveform design scheme consists of firing two or more relatively long transmit waveforms for each single A-line in a given focal zone. The frequency spectrum for each of these transmit pulses is centered at a slightly different frequency. The received signals from all of these transmit waveforms are added, while maintaining their phase information, to produce a synthetic waveform having an wider bandwidth than that of an impulse excitation.

Claims

1. A method of operating an ultrasound imaging system having an array of transducer elements, comprising the steps of:

transmitting a first transmit ultrasound waveform having a first frequency spectrum which is centered at a first frequency by exciting selected transducer elements during a first time interval, said first transmit ultrasound waveform being focused at a focal point;

transmitting a second transmit ultrasound waveform having a second frequency spectrum which is centered at a second frequency by exciting selected transducer elements during a second time interval, said second transmit ultrasound waveform being focused at said focal point, said second frequency being different than said first frequency, and said second time interval being immediately subsequent to said first time interval;

forming a first complex signal pair of a first receive ultrasound waveform derived from the portion of said first transmit ultrasound waveform reflected back to selected receiving transducer elements by scatterers in a focal zone encompassing said focal point;

forming a second complex signal pair of a second receive ultrasound waveform derived from the portion of said second transmit ultrasound waveform reflected back to selected receiving transducer elements by scatterers in said focal zone;

filtering said first and second complex signal pairs using the same filter transfer function for both waveforms;

adding said filtered first and second complex signal pairs to form a third complex signal pair having components which are the sum of the respective components of said first and second complex signal pairs;

forming the envelope of said third complex signal pair; and

displaying an image vector which is a function of said envelope of said third complex signal pair.

2. The method as defined in claim 1, wherein said first frequency is greater than a resonance frequency of said transducer array and said second frequency is less than said resonance frequency.

3. The method as defined in claim 1, wherein the peak of the frequency domain sidelobes of the first waveform are at the same frequencies as the nulls of the second waveform.

4. The method as defined in claim 1, wherein the receiver for each of the two narrow bandwidth excitation waveforms comprise filters having narrow frequency bandwidth response to improve the signal-to-noise ratio of the imaging system.

5. The method as defined in claim 1, wherein the receiver filters corresponding to each of the two excitation waveforms have different impulse responses.

6. The method as defined in claim 1, wherein said first transmit ultrasound waveform has at least two cycles and said second transmit ultrasound waveform has at least two cycles.

7. The method as defined in claim 1, wherein said first transmit ultrasound waveform has a first number of cycles and said second transmit ultrasound waveform has a second number of cycles less than said first number of cycles.

8. The method as defined in claim 1, further comprising the step of using the first frequency signal for Doppler, color or power Doppler imaging, and when the first and second waveforms are combined, using the resultant waveform for gray scale (B-mode) imaging for improved resolution and depth of field.

9. The method as defined in claim 1, wherein the first and second transmit waveforms are only separated in time by the interval required to allow the acoustic signal from the first firing to decay to a typical level of approximately -40 dB.

10. The method as defined in claim 1, wherein the waveform having the higher frequency is transmitted first.

11. The method as defined in claim 1, wherein the phase of the first and second transmit waveforms is adjusted to minimize the point spread function.

12. The method as defined in claim 1, wherein any motion in the image between the first and second firings is detected and compensated before the first and second complex waveforms are added.

13. The method as defined in claim 1, wherein said first transmit ultrasound waveform is transmitted by exciting selected elements in a first aperture of said transducer array, and said second transmit ultrasound waveform is transmitted by exciting selected elements in a second aperture of said transducer array, said first aperture being wider than said second aperture.

14. The method as defined in claim 13, wherein said first and second apertures are selected to maintain a substantially constant lateral point spread function.

15. The method as defined in claim 1, further comprising the steps of:

weighting said first receive ultrasound waveform using a first weighting coefficient; and

weighting said second receive ultrasound waveform using a second weighting coefficient different than said first weighting coefficient,

wherein said first and second weighting coefficients are different at all depths for said first and second waveforms, said first and second weighting coefficients being selected to compensate for frequency-dependent tissue attenuation in said focal zone.

16. A method of operating an ultrasound imaging system having an array of transducer elements, comprising the steps of:

transmitting a first transmit ultrasound waveform having a first frequency spectrum which is centered at a first frequency by exciting selected transducer elements during a first time interval, said first transmit ultrasound waveform being focused at a first focal point in a focal zone;

transmitting a second transmit ultrasound waveform having a second frequency spectrum which is centered at a second frequency by exciting selected transducer elements during a second time interval, said second transmit ultrasound waveform being focused at a second focal point in said focal zone, said first and second focal points being separated by a distance less than a depth of said focal zone, said second frequency being different than said first frequency, and said second time interval being immediately subsequent to said first time interval;

forming a first complex signal pair of a first receive ultrasound waveform derived from the portion of said first transmit ultrasound waveform reflected back to selected receiving transducer elements by scatterers in said focal zone;

forming a second complex signal pair of a second receive ultrasound waveform derived from the portion of said second transmit ultrasound waveform reflected back to selected receiving transducer elements by scatterers in said focal zone;

filtering said first and second complex signal pairs;

adding said filtered first and second complex signal pairs to form a third complex signal pair having components which are the sum of the respective components of said first and second complex signal pairs;

forming the envelope of said third complex signal pair; and

displaying an image vector which is a function of said envelope of said third complex signal pair.

17. The method as defined in claim 16, wherein said first frequency is greater than a resonance frequency of said transducer array and said second frequency is less than said resonance frequency.

18. The method as defined in claim 16, wherein said first transmit ultrasound waveform has at least two cycles and said second transmit ultrasound waveform has at least two cycles.

19. The method as defined in claim 16, wherein said first transmit ultrasound waveform has a first number of cycles and said second transmit ultrasound waveform has a second number of cycles less than said first number of cycles.

20. The method as defined in claim 16, wherein said first transmit ultrasound waveform is transmitted by exciting selected elements in a first aperture of said transducer array, and said second transmit ultrasound waveform is transmitted by exciting selected elements in a second aperture of said transducer array, said first aperture being wider than said second aperture.

21. The method as defined in claim 20, wherein said first and second apertures are selected to maintain a substantially constant lateral point spread function.

22. The method as defined in claim 16, further comprising the steps of:

weighting said first receive ultrasound waveform using a first weighting coefficient; and

weighting said second receive ultrasound waveform using a second weighting coefficient different than said first weighting coefficient,

wherein said first and second weighting coefficients are selected to compensate for frequency-dependent tissue attenuation in said focal zone.

23. The method as defined in claim 16, further comprising the step of using the first frequency signal for Doppler, color or power Doppler imaging, and when the first and second waveforms are combined, using the resultant waveform for gray scale (B-mode) imaging for improved resolution and depth of field.

24. The method as defined in claim 16, wherein the first and second transmit waveforms are only separated in time by the interval required to allow the acoustic signal from the first firing to decay to a typical level of approximately -40 dB.

25. The method as defined in claim 16, wherein the waveform having the higher frequency is transmitted first.

26. The method as defined in claim 16, wherein the phase of the first and second transmit waveforms is adjusted to minimize the point spread function.

27. The method as defined in claim 16, wherein any motion in the image between the first and second firings is detected and compensated before the first and second complex waveforms are added.

28. A ultrasound imaging system comprising:

an array of ultrasound transducer elements;

means for transmitting a first transmit ultrasound waveform having a first frequency spectrum which is centered at a first frequency by exciting selected transducer elements during a first time interval, said first transmit ultrasound waveform being focused at a focal point;

means for transmitting a second transmit ultrasound waveform having a second frequency spectrum which is centered at a second frequency by exciting selected transducer elements during a second time interval, said second transmit ultrasound waveform being focused at said focal point, said second frequency being different than said first frequency, and said second time interval being immediately subsequent to said first time interval;

beamforming means for generating first and second complex signal pairs of first and second receive ultrasound waveforms respectively derived from the respective portions of said first and second transmit ultrasound waveforms reflected back to selected receiving transducer elements by scatterers in a focal zone encompassing said focal point;

means for filtering said first and second complex signal pairs;

means for adding said filtered first and second complex signal pairs to form a third complex signal pair having components which are the sum of the respective components of said first and second complex signal pairs;

an envelope detector for forming the envelope of said third complex signal pair; and

means for displaying an image vector which is a function of said envelope of said third complex signal pair.

29. The system as defined in claim 28, wherein said first frequency is greater than a resonance frequency of said transducer array and said second frequency is less than said resonance frequency.

30. The system as defined in claim 28, further comprising:

means for weighting said first receive ultrasound waveform using a first weighting coefficient; and

means for weighting said second receive ultrasound waveform using a second weighting coefficient different than said first weighting coefficient,

wherein said first and second weighting coefficients are selected to compensate for frequency-dependent tissue attenuation in said focal zone.

31. The system as defined in claim 28, wherein said first and second transmit ultrasound waveforms are transmitted from different sub-apertures of said array of ultrasound transducer elements.