Quantifying acoustic emissions from cavitation events by passive detection

Relative measures of the energy radiated as broadband noise emissions by inertially cavitating bubbles, such as the mean square voltage received using a passive cavitation detector (PCD), have been shown to be directly relatable to cavitation-mediated bioeffects. Though numerous techniques exist to detect inertial cavitation qualitatively, there is presently a lack of a unified approach that makes it possible to quantify and compare levels of cavitation activity across different experimental setups and conditions. Using a substitution technique with a reference hydrophone, and a point scatterer embedded in a gel with low acoustic attenuation, the sensitivity on receive of a spherically focused PCD transducer can be obtained. This makes it possible to relate the PCD signal received during inertial cavitation activity to the acoustic power radiated from the focus of the detecting transducer. The technique is validated against PCD measurements of single-bubble inertial cavitation activity in a tissue-mimicking material and compared to theoretical predictions using the Keller-Miksis model. Calibrated measurements of acoustic power radiated as broadband noise emissions could thus provide a quantitative measure of cavitation dose that can be directly related to resulting bioeffects, such as increased sonoporation or enhanced heating.