Med. Eng. Lab., Toshiba Corp., Med. Eng. Ctr., 1385 Shimoishigami Otawara-shi, Tochigi-ken 329-26, Japan
Dong-Lai Liu Robert C. Waag
Univ. of Rochester, Rochester, NY 14627
Time-shift compensation of wave-front distortion that degrades images in medical ultrasound has been studied using computations and measurements. In the computations, wave-front distortion was assumed to arise from a phase screen placed at various distances from the receiving aperture. Intensity patterns were determined from uncompensated data and from data compensated with time-shift estimates made in the aperture. In the experiments, waveforms scattered by a wire target and randomly distributed particles were collected for a water path and for a tissue path. Images were reconstructed from uncompensated data, from data compensated with time-shift estimates made in the receiving aperture, and from data compensated with time-shift estimates made after a backpropagation step in which a waveform similarity factor was maximized. The results show the limitations of time-shift estimation in the aperture for compensation of wave-front distortion produced by a phase screen that is not close to the receiving aperture. The results also show that the addition of a backpropagation step before time-shift compensation can improve the characteristics of the focus.