Wavefront estimation in the human breast

We acquired conventional and harmonic channel r.f. ultrasound echo data using a 3x80 element, 8.5 MHz multirow array from the breasts of eight volunteers. The data acquisition was interleaved to allow direct comparison normal and harmonic echo wavefronts. Harmonic imaging data was acquired using the pulse inversion technique. Data was acquired from extended regions of interest (25 mm deep, 10 mm wide). Time shift estimates from pairs of elements were combined using a weighted least squares algorithm to obtain a wavefront arrival time error estimate. Low spatial frequencies dominated most of the wavefront estimates, and many had a curvature suggesting a gross sound speed error. Wavefront estimates were often stable over lateral translations of a few millimeters, although they often changed significantly with range, particularly at tissue boundaries observed in the B-mode image. Averaging wavefront estimates over range yielded phase aberration estimates that generally improved image quality. We measured relatively small wavefront arrival errors with both conventional (22.9 +/- 7.6 ns r.m.s.) and harmonic (22.8 +/- 7.8 ns r.m.s.) echoes. For any particular measurement, the difference between conventional and harmonic wavefront estimates was small (0 +/- 4.5 ns r.m.s). Our measurements suggest relatively mild phase aberrations in the breast, although they may be more significant for higher frequency transducers and deeper imaging depths.