Bubbles behaving badly—Dose versus threshold in cavitation classification

The acoustic emissions generated by oscillating gas bubbles (cavitation) provide an extremely useful means of treatment monitoring in ultrasound-mediated therapy. Metrics such as the relative proportions of harmonics and broadband noise in the acoustic emissions frequency spectra are widely used to categorize cavitation, e.g., as “inertial” or “stable.” It is unclear, however, whether these simple categories can adequately describe the wide range of bubble dynamics that may occur or the effects that these may produce in tissue. We have previously shown through simultaneous capture of high-speed video footage and acoustic radiation that even for single bubbles there is no simple correlation between different types of bubble behavior and the frequency content of the acoustic emissions. This suggests that measures such as the onset of broadband noise, or the appearance of sub- or ultra-harmonics are not appropriate for use as universal thresholds in therapeutic ultrasound. Binary categories such as stable and inertial cavitation, or the use of spectral characteristics to infer these, should similarly be avoided. In this talk, the implications of these findings for defining a cavitation dose and applying it to different types of therapy will be discussed.