Correlating high-speed optical imaging and passive acoustic mapping of cavitation dynamics

Abstract

The biological effects of acoustic cavitation are mediated by a range of phenomena associated with different types of bubble activity, e.g., micro-streaming, micro-jetting, and shockwave generation. The acoustic emissions generated by cavitation are also correlated to bubble dynamics. Hence, monitoring these emissions during ultrasound therapy is desirable, to maximise treatment safety and efficacy. The precise relationship between the spectral content of acoustic emissions and bubble dynamics is, however, less well understood. The aim of this study was to use simultaneous ultra-high-speed optical imaging (1–10 MHz) and passive acoustic mapping to characterise the behaviour of individual and clusters of microbubbles over a range of ultrasound exposures. As expected from the literature and numerical modelling, both the number of discrete harmonic components and amplitude of broadband content in the emissions increased with increasing amplitude of bubble oscillation. This suggests that passive acoustic mapping provides a useful indicator of spherical bubble behaviour. Frequently, however, complex bubble behaviour, such as fragmentation and coalescence was observed, which could produce substantially different effects in tissue compared with spherical bubble collapse. Future work will focus on determining whether these differences can be adequately captured in defining a future cavitation “dose” based on acoustic emissions for different applications.