Challenges in classifying cavitation: correlating high-speed optical imaging and passive acoustic mapping of cavitation dynamics

Both the biological effects and acoustic emissions generated by cavitation are functions of bubble dynamics. Monitoring of acoustic emissions is therefore desirable to improve treatment safety and efficacy. The relationship between the emission spectra and bubble dynamics is, however, complex. The aim of this study was to characterise this relationship for single microbubbles using simultaneous ultra-high-speed optical imaging and passive acoustic mapping of cavitation emissions. As expected, both the number of discrete harmonics and broadband content in the emissions increased with increasing amplitude of bubble oscillation, but the spectral content was also dependent upon other variables, including the frequency of bubble collapse and receiving transducer characteristics. Moreover, phenomena such as fragmentation and microjetting could not be distinguished from spherical oscillations when using the full duration acoustic waveform to calculate the emission spectra. There was also no correlation between the detection of broadband noise and widely used thresholds for distinguishing bubble dynamics. It is therefore concluded that binary categorisations such as stable and inertial cavitation should be avoided, and different types of bubble behaviour should not be inferred on the basis of frequency content alone. Treatment monitoring criteria should instead be defined according to the relevant bioeffect(s) for a particular application.