Evaluation of backpropagation methods for transmit focus compensation

A method for compensation of ultrasonic wavefront distortion produced by a thin time-shift screen distant from the aperture is extended to transmit focusing. In this technique, time shifts are estimated after numerical backpropagation of pulse-echo data acquired with a two-dimensional array, and the results are used to specify compensated transmit pulses. The backpropagation method was less effective for transmit focusing than anticipated from receive focusing experiments. The results indicate that transmit focus characteristics below the -20-dB level are highly sensitive to differences between the propagation medium and the assumed aberration model.