The present invention relates to method and apparatus which perform aberration correction on signals output by an ultrasound transducer.
In calculating delays used to form and detect acoustic beams in ultrasound imaging, the velocity of sound is usually assumed constant. This greatly simplifies the calculations and generally provides a usable image. However, the velocity of ultrasound waves in body tissues is affected by the types of tissues through which the ultrasound waves pass. Passing ultrasound waves through a variety of tissue type causes sound velocity inhomogeneities, which in turn cause wavefront distortion, disrupting diffraction patterns and producing image artifacts.
Several approaches have been proposed to correct the effects of sound velocity inhomogeneities. One approach is to assume that the effect of the various tissues is similar to the effect of a simple phase screen at or near the face of the transducer (the so-called near field thin phase screen model). Using this assumption, sound velocity inhomogeneities can be modeled as time-of-flight errors (i.e., phase aberrations) and the received signal in one channel can be approximated by a time-delayed replica of the signal received by another channel. Thus, phase aberrations can be estimated (1) by determining the peak position in the cross-correlation of signals received by two adjacent channels or subarrays (S. W. Flax and M. O""Donnell, xe2x80x9cPhase aberration correction using signals from point reflectors and diffuse scatterers: basic principles,xe2x80x9d IEEE Trans. Ultrason., Ferroelect. Freq. Contr., vol. 35, no. 6, pp. 758-767, 1988), or (2) by maximizing speckle brightness via time delay adjustment (L. F. Nock, G. E. Trahey, and S. W. Smith, xe2x80x9cPhase aberration correction in medical ultrasound using speckle brightness as a quality factor,xe2x80x9d J. Acoust. Soc. Am., vol. 85, no. 5, pp. 1819-1833). 
As with most assumptions, the near field thin phase screen model does not exactly capture the totality of distortions introduced by tissues in a body. See, for example: D. L. Liu and R. C. Waag, xe2x80x9cCorrection of ultrasonic wavefront distortion using backpropagation and a reference waveform method for time shift compensation,xe2x80x9d J. Acoust. Soc, Am., vol. 96, no. 2, pp. 649-660, 1994. It has been found that by modeling the thin phase screen at a distance away from the transducer, some additional waveform distortions can be somewhat corrected, however such models introduce distributed aberrations. Various methods have been proposed to correct for distributed aberrations (or displaced phase screens). They include a back propagation method (Liu, et al., supra), a total least squares (TLS) based approach called PARCA (S. Krishnan, P. C. Li, and M. O""Donnell, xe2x80x9cAdaptive compensation of phase and magnitude aberrations,xe2x80x9d IEEE Trans. Ultrason., Ferroelect. Freq. Contr., vol. 43, no. 1, pp. 44-55, 1996), and a time reversal focusing technique (M. Fink, xe2x80x9cTime reversal focusing in ultrasound: basic principles,xe2x80x9d IEEE Trans. Ultrason., Ferroelect. Freq. Contr., vol. 39, no. 5,
Recently, other alternative approaches have been developed to correct distributed aberrations, including a phase conjugation approach (G. C. Ng, P. D. Freiburger, W. F. Walker, and G. E. Trahey, xe2x80x9cA speckle target adaptive imaging technique in the presence of distributed aberrations,xe2x80x9d IEEE Trans. Ultrason., Ferroelect. Freq. Contr., vol. 44, no. 1, pp. 140-151, 1997), which independently corrects for time delay errors for each frequency component, and an inverse filtering approach (Q. Zhu and B. Steinberg, xe2x80x9cDeaberration of incoherent wavefront distortion: an approach toward inverse filtering,xe2x80x9d IEEE Trans. Ultrason., Ferroelect. Freq. Contr., vol. 44, no. 3, pp. 575-589, 1997), which compensates for both phase and amplitude distortion in the frequency domain.
Wright U.S. Pat. No. 5,570,691 discloses an aberration correction value estimation system in which ultrasonic energy from a single firing or transmit event is used both in the formation of the ultrasonic image and in the calculation of aberration correction values. In this way, the need for separate aberration correction lines or frames can be eliminated.
Langdon et al. U.S. Pat. No. 6,023,977 discloses an ultrasonic imaging aberration correction system and method using a harmonic component of the fundamental transmitted frequency for aberration correction. The system selects the frequency bands of filters used in the image signal path and in the aberration correction path so that aberration correction values may be calculated concurrently with image formation.
The above referenced publications are representative of the work being done to provide real/time aberration correction. Unfortunately, while there have been many attempts at providing real-time aberration correction, none have resulted in a commercially viable system. In fact, when implemented, some of the above referenced methods produce images that are worse than un-corrected images. The most promising algorithms require tremendous computing power, placing systems out of reach of most buyers. For example, a recent GENERAL ELECTRIC system is referenced in an article entitled Real-Time Correction of Beamforming Time Delay Errors in Abdominal Ultrasound Imaging by K. W. Rigby published in Medical Imaging 2000: Ultrasonic Imaging and Signal Processing (Proceedings of SPIE Vol. 3982 (2000)). The system uses a small multi-row array (6xc3x9796 elements, not steerable in the elevation direction) linked to a specialized multi-processor array of 56 MOTOROLA processors to perform aberration correction and is estimated to cost upwards of $500,000.00. The cost to correct aberrations in a full size (56xc3x9756 elements), fully steerable 2-D array would be astronomical. Furthermore, the multi-row array is not steerable in the elevation direction.
The present inventors have recognized a need for an affordable aberration correction system that produces images better than pre-correction images. The present inventors have discovered methods and systems that enable the use of affordable processors to provide meaningful aberration correction by modifying known methods.
An ultrasound system and method for aberration correction processing in conjunction with finely pitched transducer elements and/or aberration correction processing in conjunction with a hierarchical control scheme. Results obtained from known aberration correction algorithms may be improved with the use of finely pitched transducer elements wherein the pitch of the elements (or subgroup of elements) is less than or equal to the wavelength of an ultrasound signal at the fundamental frequency. The fundamental frequency is approximately the center frequency of the transducer""s wide-band response. Elements of a transducer may be grouped into subgroups with aberration correction algorithms being applied to the output of each subgroup.
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