Thresholds for rectified diffusion and acoustic microstreaming by bubbles in biological tissue

Results are presented of the calculation of the thresholds (in terms of peak acoustic pressure as a function of frequency of the incident ultrasonic wave), at which rectified diffusion may begin in environments represented by the liquid/cell structure of biological tissue, exposed to ultrasonic frequencies, 1–4 MHz, typical for clinical devices. Computations based on excitation by peak pressure amplitude values typical for continuous and pulse-echo diagnostic devices, suggests that rectified diffusion is unlikely to occur for the latter only. Acoustically induced shear stresses, caused by bubble pulsation produced microstreaming and affecting the integrity of cellular membranes, are evaluated and are found to lie above levels at which biological effects have been observed.