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.