Characterization of cavitation based on autocorrelation of acoustic emissions

Cavitation is a phenomenon that causes different bioeffects depending on its stability and strength (that is, inertial character). In current thinking, stable cavitation is associated with acoustic emissions at the harmonics, subharmonics, and ultraharmonics of the driving insonation frequency while inertial cavitation is associated with broadband emissions. To assess the validity of these assumptions, cavitation emissions from microbubbles excited at 1 MHz in 3% agar gel were simulated as well as experimentally recorded. The results show that cavitation may occur “stably” (repetitively) without sub-, ultra-, or integer harmonic emissions, highlighting the need for more precise measures of stability that cannot be captured by frequency-based methods, namely, the variances of emission amplitude and phase. The results also show that inertial cavitation need not cause broadband emissions. In contrast to these ambiguities, the autocorrelation of cavitation emissions allows the estimation of amplitude and phase stability, while the first zero-crossing of the auto-correlation is shown to be closely related to the widely-used measure Rmax/R0 of cavitation strength. Therefore, autocorrelation-based cavitation characterization promises to be a more accurate method of inferring cavitation bioeffects than its frequency-based counterpart.