Estimation and correction of ultrasonic wavefront distortion using pulse-echo data

Pulse-echo measurements from random scattering and from a point target were used to quantify the transmitter beam size effects, the isoplanatic patch size, and to evaluate the performance of different compensation techniques. Measurements were made using unperturbed transmit beams produced by a single-element transmitter with a diameter of 1/2", 1", or 2", each focused at 3". Scattered signals were received in a two-dimensional aperture after propagation through a specimen of abdominal wall, and b-scan images were formed with no compensation, with time-shift compensation in the receiving aperture, and with backpropagation followed by time-shift compensation. The investigation indicates that a tightly focused transmitter beam is necessary to create a scattered wavefront satisfactory for time-shift estimation, the isoplanatic patch is about twice longer in the range direction than in the azimuthal direction, and backpropagation followed by time-shift compensation provides better compensation of distortion than time-shift compensation alone.