Effect of transmit focus characteristics on estimates of aberration

The effect of multirow array transmit focusing characteristics on estimation and compensation of time-shift distortion is demonstrated experimentally. Multirow ultrasonic transducer arrays were emulated by combining adjacent elements of a 3.0 MHz, 0.6 mm pitch two-dimensional array to define larger virtual elements. Random scattering data were acquired at f/2.0 through a tissue-mimicking distributed aberrator (65 ns rms arrival time fluctuation with 8.2 mm FHWM correlation length) and time-shift maps estimated from the data were used for transmit focus compensation. Compensated beams formed by 2-D and 1.75-D arrays with fine row pitches were similar, but focus restoration was significantly less effective for a 1.75-D array with coarse row pitch. For example, the mean -20 dB lateral beamwidths measured using transmit focus compensation were 5.2 mm for arrays with 0.6 and 1.8 mm row pitches and 9.5 mm for an array with 5.4 mm pitch. When random scattering data were acquired using the compensated beams, time-shift maps produced with the 5.4 mm pitch array included large discontinuities and had higher root mean square values. The results indicate that multirow arrays designed for use with aberration correction should have element dimensions much less than 67% of the correlation length of the aberration.