Pulse distortion in nondiffracting beam pulse echo systems.

Abstract

Ultrasonic nondiffracting beams, generated by J0 radial weighing sources, have recently been introduced and suggested for medical imaging and nondestructive testing. Due to inherent dispersion, a pulse generated by such an aperture exhibits distortion as it propagates away from the source. Performance of a pulse echo system depends, in part, on pulse shape. A model for the point response of a pulse echo system with J0 aperture, derived by a short bandpass pulse, is presented here. The point response is defined as the received pulse by the aperture when the transmitted pulse is reflected back by a point target. The model describes the envelope of the point response by three terms: amplitude, delay, and spread. Oscillatory component of the point response is described by instantaneous frequency and a constant phase term. Computer simulation for the special case of a Gaussian pulse will be presented. The model provides computationally simple expressions for evaluating parameters of nondiffracting pulse echo systems, such as axial resolution, intensity, and echo displacement, in terms of aperture beamwidth, center frequency, and target-to-aperture distance.