Cavitation induced by asymmetric distorted pulses of ultrasound: theoretical predictions

Prediction of the response of gas-filled microbubbles to ultrasound waves is complicated by the finite-amplitude distortion associated with large-amplitude acoustic fields. Typical finite-amplitude pulses in medical applications consist of a sharp positive spike followed by a smaller slowly varying negative pressure. A suitable model for pressure waveforms that have these temporal characteristics is described, and the response of microbubbles to such sound fields is subsequently computed. A selection of descriptive parameters is considered to determine the characteristic features of the distorted pulse which are most important in generating bubble response. The results show that (a) the peak-positive pressure is a very poor predictor of bubble response; (b) the peak-negative pressure typically underestimates the bubble response; and (c) the best tested predictor of bubble response is the pressure amplitude of the fundamental frequency in a Fourier series expansion of the distorted pulse. Theoretical thresholds for transient cavitation induced by a distorted pulse are presented for a range of frequencies and initial sizes of microbubbles.<
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