Comparison of the (linear) 2f field transmitted with a fully realized two-dimensional effective apodization and the (nonlinear) 2f harmonic field

Abstract

Results presented at previous IEEE Ultrasonics Symposia demonstrated that, in a non-aberrating medium, the field obtained by transmitting (linearly) at 2f with an applied only in the azimuthal direction with a one- dimensional transmitting array produces a field in the azimuthal plane that is nearly identical to the field obtained at 2f by propagating a finite amplitude signal originally transmitted at 1f. However, this one-dimensional approximation to the two- dimensional effective apodization is not adequate in the presence of significant aberration, presumably because the aberrators compromise the independence of the azimuthal and elevational planes. Current instrumentation does not permit the implementation of a two-dimensional effective apodization with purely electronic means. Therefore, the goal of the present work is to design and construct mechanical apodization plates to permit a fully realized two-dimensional effective apodization and to investigate the resulting ultrasonic fields. For lossy polyurethane plates of approximately 1 cm in thickness, through transmission hydrophone measurements demonstrate the effectiveness of this approach for achieving the desired two- dimensional apodization using plates of realistic dimensions.