Method and apparatus for adjustable frequency scanning in ultrasound imaging

Claims

We claim:

1. A method for scanning a field of view with a plurality of ultrasonic transmit beams, at least a first and a second of which traverse different paths through said field of view, comprising the steps of:

modulating baseband waveforms for at least said first and said second of said transmit beams to ultrasonic frequencies, wherein an ultrasonic frequency to which said first transmit beam is modulated is higher than an ultrasonic frequency to which said second transmit beam is modulated; and

for at least said first and second of said transmit beams, exciting each respective transducer in an array of ultrasonic transducers with a respective excitation signal which is responsive to a respective one of said modulated baseband waveforms to produce the transmit beam.

2. A method according to claim 1, wherein said field of view has a central region and an outer region extending laterally from said central region in said field of view,

and wherein said first beam traverses said central region and said second beam traverses said outer region.

3. A method according to claim 2, wherein the frequencies for all of the beams in said plurality of ultrasonic beams which traverse said central region are equal to the frequency for said first beam, and wherein the frequencies for all of the beams in said plurality of ultrasonic beams which traverse one of said outer regions are equal to the frequency of said second beam.

4. A method according to claim 1, wherein said field of view has two laterally opposite edges, a central region, and a pair of outer regions each extending laterally from said central region to a respective one of said edges of said field of view,

and wherein said first beam traverses said central region and said second beam traverses one of said outer regions.

5. A method according to claim 4, wherein the frequencies for all of the beams in said plurality of ultrasonic beams which traverse said central region are equal to the frequency for said first beam, and wherein the frequencies for all of the beams in said plurality of ultrasonic beams which traverse one of said outer regions are equal to the frequency of said second beam.

6. A method according to claim 1, wherein each of said beams in said plurality of ultrasonic beams has a different apparent steering angle relative to a normal to a surface of said array.

7. A method according to claim 6, wherein the frequencies for the beams in said plurality of ultrasonic beams step down in more than two frequency steps from a beam with the smallest absolute apparent steering angle to a beam with the largest absolute apparent steering angle, inclusive.

8. A method according to claim 6, wherein the frequencies for the beams in said plurality of ultrasonic beams step down in at least one frequency step from a beam with the smallest absolute apparent steering angle to a beam with the largest absolute steering angle, inclusive.

9. A method according to claim 8, wherein the maximum steering angle .theta..sub.S,max any of the beams in said plurality which are in each of said frequency steps, is related to the frequency f for the beams in such frequency step according to the relationship

sin (.theta..sub.S,max).ltoreq.{.kappa./(2.multidot.f.sub.#)}{-b+{b.sup.2 -b+(.lambda./d.multidot.f.sub.#).sup.2 }.sup.1/2 }

where

.lambda.=c/f;

c is a constant approximating the speed of sound in the human body;

b=(1-{.lambda./(2d)}.sup.2);

f.sub.# is the f-number defined by the focal length r divided by the active aperture width D;

d is the transducer element spacing in said array; and

.kappa. is a desired value which is constant over all of said frequency steps, 0<.kappa.<1.

10. A method according to claim 8, wherein the frequency f.sub.i for the i'th one of the beams in said plurality of beams is approximately:

f.sub.i =(c/d) {sin (.vertline..theta..sub.S,i .vertline.)+k(.theta..sub.S,max)}.sup.-1,

where

c is a constant which approximates the speed of sound in the human body;

d is the transducer element spacing in said array;

.theta..sub.S,i is the steering angle for said i'th beam; and

k(.theta..sub.S,max) is a constant which depends upon the maximum steering angle .theta..sub.S,max.

11. A method according to claim 6, wherein the frequency f.sub.i for the i'th one of the beams in said plurality of beams is approximately:

f.sub.i =(c/d) {sin (.vertline..theta..sub.S,i .vertline.)+k(.theta..sub.S,max)}.sup.-1,

where

c is a constant which approximates the speed of sound in the human body;

d is the transducer element spacing in said array;

.theta..sub.S,i the steering angle for said i'th beam; and

k(.theta..sub.S,max) is a constant which depends upon the maximum steering angle .theta..sub.S,max.

12. A method according to claim 11, wherein .theta..sub.S,max .apprxeq.45.degree..

13. A method according to claim 11, wherein k(.theta..sub.S,max).apprxeq.1.29.

14. A method according to claim 1, wherein each of said beams in said plurality of ultrasonic transmit beams has a different apparent point of intersection with a surface of said array.

15. A method according to claim 14, wherein each of said beams in said plurality of ultrasonic transmit beams also has a different apparent steering angle relative to a normal to said surface of said array.

16. A method according to claim 14, wherein the frequency for the beams in said plurality of ultrasonic transmit beams step down in more than two steps from a beam whose point of intersection is nearest the center of said surface of said array.

17. A method according to claim 1, further comprising the step of repeating the steps of modulating and exciting, to produce a plurality of scans of said field of view separated in time, the ultrasonic frequency for each respective beam in said plurality of beams being the same in all of said scans.

18. A method according to claim 1, further comprising the steps of;

forming at least one ultrasonic receive beam corresponding to each of the beams in said plurality of ultrasonic transmit beams, each of said receive beams being represented with a plurality of input waveforms received from corresponding ones of said transducers and containing information for a plurality of ranges along the receive beam, at least a first and a second one of said receive beams traversing different paths through said field of view; and

demodulating the input waveforms of each of said receive beams with a respective demodulation frequency for the receive beam, the demodulation frequency for said first receive beam being higher than the demodulation frequency for said second receive beam for information at the same range.

19. A method according to claim 1, further comprising the steps of:

forming at least one ultrasonic receive beam corresponding to each of the beams in said plurality of ultrasonic transmit beams, each of said receive beams being represented with a plurality of input waveforms received from corresponding ones of said transducers and containing information for a plurality of ranges along the receive beam, at least a first and a second one of said receive beams traversing different paths through said field of view;

demodulating and combining the demodulated input waveforms of each of said receive beams to produce a beam output waveform for each of said receive beams; and

filtering across the beam output waveforms for different ones of said receive beams.

20. A method according to claim 1, further comprising the step of exciting said array of ultrasonic transducers during said scan to produce an additional ultrasonic beam which is not in said plurality of ultrasonic beams.

21. A method according to claim 1, wherein said baseband waveforms have frequency spectrums centered at approximately 0 Hz.

22. A method according to claim 1, wherein said baseband waveforms have frequency spectrums centered at frequencies which are as low as possible on a given apparatus.

23. A method according to claim 1, wherein said baseband waveforms are the same for all of said beams.

24. A method according to claim 1, wherein said ultrasonic beams in said plurality of ultrasonic beams are all separated in time.

25. A method according to claim 1, wherein said step of exciting each respective transducer in an array of ultrasonic transducers with a respective excitation signal comprises the steps of:

exciting each respective transducer in said array with a respective excitation signal which is responsive to a respective one of said modulated baseband waveforms for one of said transmit beams; and

simultaneously exciting each respective transducer in said array with a respective excitation signal which is responsive to a respective one of said modulated baseband waveforms for another of said transmit beams.

26. A method according to claim 1, wherein a surface of said array of transducers is planar.

27. A method according to claim 1, wherein a surface of said array of transducers is curved.

28. The method for scanning a field as in claim 1 wherein a baseband waveform from which said first beam is formed is identical to a baseband waveform from which said second beam is formed.

29. The method for scanning a field as in claim 1 in which said baseband waveforms are comprised of initial waveform samples.

30. The method according to claim 1 wherein each of said baseband waveforms is a baseband waveform envelope.

31. A method for scanning a field of view with a plurality of ultrasonic receive beams to form an image, each of said receive beams being represented with a plurality of input waveforms received from corresponding transducers in an array of transducers and containing ensonified object responses for a plurality of ranges, at least a first and a second one of said receive beams traversing different paths through said field of view comprising the steps of:

forming said receive beams; and

demodulating the input waveforms to form baseband waveforms for each of said receive beams with at least one respective demodulation frequency, a demodulation frequency for said first receive beam at a given range being higher than a demodulation frequency for said second receive beam at said given range.

32. A method according to claim 31, wherein the demodulation frequency for each particular one of said receive beams is constant for all ranges of said particular receive beam.

33. A method according to claim 31, wherein the demodulation frequency for each of said receive beams varies as a function of range.

34. A method according to claim 31, wherein said field of view is two-dimensional.

35. A method according to claim 31, wherein the step of forming said receive beams comprises, for each particular one of said receive beams, the step of combining waveforms responsive to the input waveforms of the particular receive beam to produce a beam waveform for the particular receive beam,

further comprising the step of phase-aligning the beam waveforms to achieve phase coherence for said first and second receive beams at said given range.

36. A method according to claim 35, wherein said step of phase-aligning comprises the step of remodulating the beam waveform for at least one of said first and second beams at said given range with respective remodulation frequencies .omega..sub.r.sup.1 and .omega..sub.r.sup.2 such that

.omega..sub.d.sup.1 +.omega..sub.r.sup.1 =.omega..sub.d.sup.2 +.omega..sub.r.sup.2

where

.omega..sub.d.sup.1 is the demodulation frequency for said first receive beam at said given range, and

.omega..sub.d.sup.2 is the demodulation frequency for said second receive beam at said given range.

37. A method according to claim 36, wherein said demodulation frequencies .omega..sub.d.sup.1 and .omega..sub.d.sup.2 and said remodulation frequencies .omega..sub.r.sup.1 and .omega..sub.r.sup.2 vary with range.

38. A method according to claim 36, wherein said ensonified object responses are represented in complex for