Aperture size effect on ultrasonic wavefront distortion correction.

The influences of aperture size on wavefront distortion correction are investigated both theoretically and numerically. A multilayer, phase-screen model is assumed to be the underlying, distorting medium. Numerical simulations were performed using three wavefront distortion correction methods: time-shift compensation (TSC), backpropagation followed by time-shift compensation (BP+TSC), and the previously proposed, multilayer, phase-screen compensation (MPSC) method. The distorted wavefronts were generated by propagating a planar wavefront through a multilayer, phase-screen model constructed with a two-dimensional (2-D) scanned map of a real abdominal slice. Performances were evaluated by L2 errors between the corrected wavefronts and the undistorted planar wavefront. Point spread functions also were calculated to evaluate the relative image quality. Theoretical analysis shows L2 error will decrease as aperture size grows when exact phase compensation (EPC) is applied, although finite errors will always exist along the edges of the corrected wavefront. Three different aperture sizes, 14.24 mm (64 elements), 28.48 mm (128 elements), and 56.96 mm (256 elements) are considered in this study. Numerical results show that the quality of wavefront with EPC is essentially limited by the aperture size, and the correction methods considered are relatively robust against the aperture size. It also shows that, for low aberration, results with MPSC and EPC are comparable. However, for high aberration, MPSC significantly outperforms EPC in suppression of L2 error and sidelobes. This study suggests that, for most medical ultrasound imaging systems, the exact structure of the distorting medium may not be necessary to be known a priori for optimal distortion correction because of the limitation imposed by finite aperture size.