Forces acting in the direction of propagation in pulsed ultrasound fields.

This paper considers some non-thermal effects resulting from absorption of acoustic energy from an ultrasound beam. An experimental investigation of the location of the 'source pump', responsible for the generation of streaming in high amplitude diagnostic fields in water, is reported. Acoustically transparent membranes were inserted in the ultrasound field in order to restrict the streaming volume. It is shown that the major contribution to an acoustic stream is generated in the region near to the focus of a transducer where the intensity in the beam and the degree of non-linear distortion are both high. In the second part of the paper a simple model of non-linear propagation is used to predict the magnitude of the maximum pressure gradient induced in a medium by the absorption of acoustic energy from a beam. Propagation in water, in tissue and in amniotic fluid are considered. Within the limitations of this model it is shown that the pressure gradients induced in pulsed acoustic fields do not result in the ultimate shear stress of tissue being exceeded.