Subharmonics are not the full story—Searching for acoustic signatures of cavitation and other fantastic beasts

Cavitation induced bio-effects are being exploiting in a wide range of applications from physiotherapy to brain surgery. The acoustic emissions generated by bubble activity are extremely useful in enabling real time treatment monitoring. The relationship between the spectral characteristics of these emissions and bubble dynamics is, however, complex. Here we report data from experiments with simultaneous ultra-high-speed optical imaging and passive acoustic mapping of individual microbubbles exposed to 50 cycles of ultrasound at 0.5 MHz with varying peak negative pressures. The spectral content of the acoustic emissions from individual microbubble was compared to the bubble dynamics observed by the imaging. As expected from prior work, both the number of discrete harmonics and broadband content in the emissions increased with increasing amplitude of bubble oscillation. There was no clear correlation, however, between the presence of ultra and sub-harmonic components and bubble behaviour. Indeed, these components were frequently absent. Moreover, phenomena such as microjetting, fragmentation and coalescence, that could produce very different effects in tissue, were indistinguishable acoustically. The results thus indicate that the definition of cavitation thresholds or doses should be very carefully considered depending on the therapeutic effect (or avoidance of unwanted bioeffects) required for a particular application.