NONLINEAR PRESSURE FIELDS DUE TO FOCUSED CIRCULAR APERTURES

The use of high amplitude focused ultrasound fields is widespread in medical diagnosis and therapy but there has been relatively little work published that compares experimental measurements with appropriate theory for such systems. In this paper, comparisons are made between the measured continuous-wave pressure field of a focused circular aperture operating at 2.25 MHz and a numerical solution of the nonlinear parabolic wave equation. The measurements were made in water using a 38-mm diam plane circular transducer as the acoustic source with perspex lenses providing focusing, focal lengths of 440, 216, and 142 mm were examined. Results are presented for the amplitudes of the fundamental, second, and third harmonics along and across the acoustic axis of the source. In general, the agreement between experiment and theory is good.