ESTIMATION AND CORRECTION OF ULTRASONIC WAVE-FRONT DISTORTION USING PULSE-ECHO DATA RECEIVED IN A 2-DIMENSIONAL APERTURE

Pulse-echo measurements from random scattering and from a point target have been used to quantify transmitter beam size effects and isoplana tic patch size as well as to evaluate the performance of different abe rration compensation techniques. Measurements were made using a single -element transmitter with a diameter of 1/2 in., 1 in., or 2 in., each focused at 3 in. A tissue-mimicking scattering phantom or a point tar get was used to produce echoes that were received in a two-dimensional aperture synthesized by scanning a linear array. A specimen of abdomi nal wall was placed in the reception path to produce aberration. B-sca n images were formed with no compensation, with time-shift compensatio n in the receiving aperture, and with backpropagation followed by time -shift compensation. The isoplanatic patch size was estimated by compe nsating the focus of a test point target with the parameters estimated for an original point target position, and observing the deterioratio n of compensation effects with increasing distance between the test an d the original point targets. The results of the measurements using di fferent transmitter diameters quantify the improvement of time-delay e stimation with the increase in wavefront coherence that accompanies de creased transmitter beam size. For seven specimens, the average isopla natic patch size determined from a 10% increase in the -10 dB effectiv e diameter was 16.7 mm in the azimuthal direction and 39.0 mm in the r ange direction. These sizes increased after backpropagation to 19.0 mm and 41.4 mm, respectively. For the 1/2 in., 1 in., and 2 in. diameter transmitters, the average contrast ratio improvement was 2.0 dB, 2.1 dB, and 2.8 dB, respectively, with time-shift, compensation, and 2.3 d B, 2.7 dB, and 3.5 dB, respectively, with backpropagation of 20 mm fol lowed by time-delay estimation and compensation. The investigation ind icates that a tightly focused transmitter beam is necessary to create a scattered wavefront satisfactory for time-shift estimation, the isop lanatic patch is about twice as long in the range direction as in the azimuthal direction, and backpropagation followed by time-shift compen sation provides better compensation of distortion than time-shift comp ensation alone.