| Most breast tumors are detected by palpation, when the size is about a centimeter, and 95% are already invasive. It is also about the size at which today's best ultrasound scanners, which have about 2 mm lateral resolution, can detect and begin to identify them. Increasing the lateral resolution by nearly ten-fold, while preserving the excellent tissue contrast of ultrasound, will improve the probability of patient survival. In principle such an improvement is available by using transducers as large as the breast in the imaging system. The reason this has not been done is that the breast is complicated acoustically. The speed of sound within it varies as much as 10%, resulting in severe distortion of the propagating wavefront. We have studied wavefront distortion, as viewed by a large transducer, to assess the potential applicability of adaptive beamforming (ABF) algorithms to compensate such distortion. Data were collected from mammography volunteers at the Hospital of the University of Pennsylvania.; In vivo data analyses and phantom studies show that refraction rather than weak scattering dominates the wavefront distortion. As a result, severe amplitude distortion of the wavefront, in addition to the phase, is observed. Theoretical and experimental results show that sidelobe levels in the image, after phasefront compensation, rise proportionally to the wavefront amplitude distortion and are too high to be useful. Stronger algorithms are needed to correct wavefront distortion in a refractive medium as severe as the female breast.; Refraction introduces another problem. The isoplanatic patch, which is measured by the correlation distance of the distorted wavefront or of a image set obtained from a set of wavefronts, can be very small. We found that this distance is only a few mm at 3MHz in 44 breasts. Therefore even after we have learned how to make stronger ABF algorithms, a single wavefront correction will not be sufficient. Instead the system must compute a new weight vector for every correlation distance both laterally and longitudinally, and will have to coherently combine the subimages from them. |
