Correction of phasefront aberrations and pulse reverberations in medical ultrasound imaging

• US 6,485,423 B2
• Filed: 01/31/2001
• Issued: 11/26/2002
• Est. Priority Date: 01/31/2000
• Status: Active Grant

• Current Assignee: Bjorn A.J. Angelsen, Johansen, Tonni F.
• Original Assignee: Bjorn A.J. Angelsen, Johansen, Tonni F.
• Inventors: Angelsen, Bjorn A. J., Johansen, Tonni F.
• Primary Examiner(s): Lateef, Marvin M.
• Assistant Examiner(s): Jain, Ruby
• Application Number:
  US09/773,335
• Publication Number:
  US 20020002333A1
• Time in Patent Office: 664 Days
• Field of Search: 600/458, 600/443, 736/27, 736/02, 367/87
• US Class Current: 600/458
• CPC Class Codes: A61B 8/00 Diagnosis using ultrasonic, sonic or infrasonic waves imaging of objects using sonar G01S15/00
• CPC Class Codes: A61B 8/0833 involving detecting or locating foreign bodies or organic structures
• CPC Class Codes: A61B 8/481 involving the use of contrast agent, e.g. microbubbles introduced into the bloodstream analysis of echo signal exploiting properties of a contrast enhancer in devices for short-range imaging with acoustic waves G01S7/52039
• CPC Class Codes: G01S 15/8925 the array being a two-dimensional transducer configuration, i.e. matrix or orthogonal linear arrays
• CPC Class Codes: G01S 7/52038 involving non-linear properties of the propagation medium or of the reflective target
• CPC Class Codes: G01S 7/52049 using correction of medium-induced phase aberration
• CPC Class Codes: G01S 7/5205 Means for monitoring or calibrating

Abstract

A method of correcting for phasefront aberrations in ultrasound imaging uses highly spaced apart point scatterers artificially placed in the tissue being imaged. The point scatterers reflect the transmitted sound and are individually differentiated to provide singular reference points for correction of signals reflected from the surrounding tissue. The differentiation is performed by comparison of the third or fourth harmonic frequencies of the reflected signals. To ensure the necessary high dispersal of the point scatterers, high amplitude pulses of the transmitted signal destroy point scatterers in selected image regions. In an alternate embodiment, correction is performed by stochastic analysis of signals reflected from the highly dispersed point scatterers. A reference signal is compared to the second harmonic of the reflected signal to reduce noise.

Claims

1. A method for corrections of phasefront aberrations and pulse reverberations in medical ultrasound imaging, where

ultrasound beams on transmit and/or receive are formed with an ultrasound transducer array that has a two-dimensional distribution of elements, a maximal linear dimension of the elements being smaller than a spatial correlation length of the phase amplitude seen across the array surface,

a individual array element signals are filtered in a correction filter before standard beam forming is done, both for the transmit and/or the receive beams,

the correction filters are estimated by

introducing into a soft tissue image field one or more artificial ultrasound point scatterers,

receiving backscatterd signal from the point scatterers with the transducer array,

the point scatterers being spaced apart that the signals from the individual scatterers can clearly be discriminated from each other,

the signals from the point scatterers having characteristics in amplitude or frequency content, or both, so that they are clearly separable from the tissue signal,

the signals from the individual point scatterers being used to derive correction filters to correct for phase aberrations and pulse reverberations both in the receive and the transmit beams.

2. A method according to claim 1, where the frequency responses of the element signal correction filters for focusing the transmit and/or the receive beams onto the location of a point scatterer, are obtained by

Fourier transforming along time coordinate the received element signals from the point scatterers and utilizing the frequency components where amplitude of the Fourier transform is larger than the noise level,

correcting phase of the Fourier transforms of the element signals by a spherical approximation of a propagation delay from the point scatterer to the array elements, which is in essence a division of the Fourier transform with the Green'"'"'s function of the Helmholtz equation,

the complex conjugate of the corrected Fourier transforms of the elements signals being used as transfer functions of element signal correction filters for the receive and/or the transmit beams.

3. A method according to claim 1, where the impulse responses of the element signal correction filters for focusing the transmit and/or the receiver beams onto the location of a point scatterer, are obtained by

recording the back scattered element signals from the point scatterer and delay correcting these element signals by a spherical approximation of propagation delay from the point scatterer to the individual array elements,

using time inverted version of the delay corrected element signals as impulse responses for the element signal correction filters for the receive and/or the transmit beams.

4. A method according to claim 2, where the correction filters are approximated by delay and amplitude corrections.

5. A method according to claim 4, where for focusing the beam onto the location of a point scatterer

the amplitude corrections for each element signal are obtained as the amplitude of the received element point scatterer signals, and

the time delay corrections are obtained by comparing arrival times on each element signal channel of the pulses from the point scatterer.

6. A method according to claim 1, where a 2^nd harmonic band of the backscattered element signals are used to reduce the pulse reverberations in the backscattered signal for estimation of the correction filters or approximate amplitude and delay corrections.

7. A method according to claim 1, where the artificially introduced point scatterers are attached to an intervention tool that is inserted into the body.

8. A method according to claim 7, where the point scatterers are made as indentations in a smooth tool surface, or scatterers attached to a smooth tool surface, or scatterers buried in indentations in the smooth tool surface.

9. A method according to claim 1, where the artificially introduced point scatterers are obtained with a dilute concentration of contrast agent bubbles in the blood or other body fluids.

10. A method according to claim 9, where adequate distance between the contrast agent bubbles are obtained by first destroying the contrast agent bubbles in a selected imaging region, and performing the correction estimation in a subsequent time interval where adequate inflow of new contrast agent bubbles to the region has occurred.

11. A method according to claim 9, where steadily new point scatterers with varying locations and adequate distances are obtained by using high amplitude of the transmitted pulses that continuous destruction of some contrast agent bubbles occurs, while new contrast agent bubbles enters the region with the blood or other body fluids.

12. A method according to claim 7 where the point scatterers on the intervention tool are made by contrast agent bubbles.

13. A method according to claim 9, where the signals from the point scatterers are discriminated from the tissue signal by using frequency components in a sub, 2^nd, 3^rd, or 4^th harmonic band of the fundamental frequency band of the transmitted pulse.

14. A method according to claim 9, where a coded sequence is transmitted at the contrast agent bubbles, and pulse compression is used in the receiver to enhance the signal to noise ratio of the received signal from the contrast agent bubble, to improve the detection of the contrast agent bubble.

15. A method for corrections of phasefront aberrations and pulse reverberations in medical ultrasound imaging, where

the ultrasound beams on transmit and/or receive are formed with an ultrasound transducer array that has a two-dimensional distribution of elements, a maximal linear dimension of the elements being smaller than a spatial correlation length of the phase amplitude seen across the array surface,

individual array element signals being filtered in correction filters before standard beam forming is done, both for the transmit and/or the receive beams,

the correction filters being estimated from the received element signals from the array elements back scattered from a distribution of scatterers with short correlation length,

using an estimation algorithm for the correction filters that reduces the effect of pulse reverberations in the backscattered signal in a final estimate of correction filters.

16. A method according to claim 15, where a 2^nd harmonic band of the element signals is used to suppress the pulse reverberations in the element signals.

17. A method according to claim 15, where multiple transmit pulses with the same beam direction and focus is used, and the back scattered element signals for the multiple transmit pulses from moving or time varying scatterers are used for improved estimation of the correction filter responses.

18. A method according to claim 17, where the moving scatterers are scatterers in the blood or other body fluids, the myocardium, or a vessel wall.

19. A method according to claim 18, where the scattering is enhanced by contrast agent bubbles in the moving fluids or tissue.

20. A method according to claim 17, where ultrasound contrast agent bubbles are used for time varying scatterers where transmitted pulse amplitudes are high that destruction of at least some bubbles occur, and new bubbles enter the tissue through the blood or other body fluids.

21. A method according to claim 19, where the signal from the contrast agent bubbles is enhanced over that from the tissue signal and pulse reverberations, by using sub, 2^nd, 3^rd, or 4^th harmonic components of the backscattered signal from the contrast agent bubbles.

22. A method according to claim 21, where the signal from the contrast agent bubbles is further enhanced by transmitting coded pulse sequences and using pulse compression in the receiver to enhance a signal to noise ratio of the received signal from the contrast agent.

23. A method according to claim 17, where the effect of pulse reverberations in the estimate is suppressed by high pass filtering the received element signals along the pulse number coordinate for each depth sample.

24. A method according to claim 17, where temporal averaging of correction filter parameters along a pulse number coordinate is used to reduce a estimation variance of the parameters.

25. A method according to claim 15, where errors introduced in the estimates of the correction filters by strong, off axis scatterers is reduced by lowpass filtering the received element signals along an element position coordinate along the array surface.

26. A method according to claim 15, where the correction filter frequency responses are estimated from the temporal Fourier transform of the received element signals, where the amplitude of the frequency response is estimated as the average of a amplitude of this Fourier transform for each frequency, within bands where the amplitude is larger than the noise power, and slowly set to zero outside these bands.

27. A method according to claim 26, where phases of the correction filters for each frequency are estimated based on correlation methods.

28. A method according to claim 27, where the phases of the correction filters for each frequency are estimated by integration of the phases of the correlation functions between neighboring element signals.

29. A method according to claim 28, where a plane skew in the phases of the correction filters over the array is removed either after integration, or through removing the DC component of the neighbor correlations before the integration.

30. A method according to claim 28, where the integration of the neighbor phase correlation is broken into several hierarchical levels, where in a first level the element signals are grouped in subgroups of limited number of neighboring elements, where the neighbor phase integration is carried through in each subgroup, and the element signals within each subgroup are corrected and summed together to form a new set of element signals on a higher level, where neighbors on the new level are grouped together in subgroups, and the phase estimates between the neighbors on the new level are integrated in the new subgroups, and the element signals in the new subgroups are corrected and summed to form a new, second level of corrected element signals, continuing this leveling process, by grouping, phase gradient estimation and integration, and signal correction and summing for neighboring elements in the same manner for each new level until all original element signals are combined into a single, corrected beam signal, the final original element correction filters being obtained as a product of all participating correction transfer functions on all levels for the path of that particular element signal.

31. A method according to claim 26, where the phases of the correction filters for each frequency are estimated according to a parameter estimation scheme, for example a maximum likelihood scheme.

32. A method according to claim 15, where the correction filters are approximated by delay and amplitude corrections, where the amplitude corrections for an element signal are determined by averaging the element signal amplitudes over depth and over time between transmit pulses for moving or time varying signals.

33. A method according to claim 32, where the delay corrections are estimated by correlation methods.

34. A method according to claim 33, where the delay corrections are estimated by integration of the delays between neighboring element signals obtained through correlation analysis.

35. A method according to claim 34, where a plane skew in the delays of the correction filters over the array is removed either after the integration, or through removing the DC component of the neighbor delays before the integration.

36. A method according to claim 34, where the integration of the neighbor delays is broken into several hierarchical levels, where in a first level the element signals are grouped in subgroups of limited number of neighboring elements, where the neighbor delay integration is carried through in each subgroup, and the element signals within each subgroup are corrected and summed together to form a new set of element signals on a higher level, where neighbors on the new level are grouped together in subgroups, and the delay estimates between the neighbors on the new level are integrated in the new subgroups, and the element signals in the new subgroups are corrected and summed to form a second level of corrected element signals, where the grouping, delay gradient estimation and integration, and signal correction and summing are done for neighboring elements in the same manner for each level until all original element signals are combined into a single, corrected beam signal, the final original delays being obtained as a sum of the delays on all levels for the path of that particular element signal.

37. A method according to claim 33, where the delay corrections are estimated by correlation between element signals and Hilbert transforms of element signals.

38. A method according to claim 32, where the correction delays are estimated through a parameter estimation scheme, for example a maximum likelihood scheme.

39. A method according to claim 15, where the number of element signals used for estimation of the correction filters is reduced by combining the element signals from neighboring, small array elements into groups where a total dimension of the combined array elements in each group being smaller than the correlation length of the phase aberrations and pulse reverberations.

40. A method according to claim 15, where a minimal parameter representation of the phase or delay corrections are represented by a truncated generalized Fourier series over an element position coordinate, the number of elements in the series being chosen so that the correlation properties of the phase or delay corrections are represented, coefficients in the series forming the parameters to be estimated.

41. A method according to claim 15, where the phase or delay corrections are estimated by correlation analysis of the element signals with a reference signal obtained by combinations of element signals or a parts of the element signals.

42. A method according to claim 41, where only the reference signal is filtered in any harmonic bands, Hilbert transformed, or pulse to pulse high-pass filtered to remove the stationary reverberation components, before the correlation analysis.

43. A method according to claim 15, where estimates of the correction filters are used to correct new transmit beams producing new backscattered element signals to be used for new estimations of the correction filters, in an iterative manner.