Interpreting passive cavitation detection signals during high-intensity focused ultrasound (HIFU)

Inertial cavitation is known to be associated with significantly enhanced rates of heating during cancer treatment by high-intensity focused ultrasound (HIFU). However, during prolonged exposure, the cavitation region gradually shifts towards the HIFU transducer, causing shielding of the original focus. It is hoped that controlling the HIFU field to constrain cavitation to the focal region will lead to sustained enhanced focal heating, thus reducing treatment times and improving treatment resolution. Monitoring of cavitation can be achieved during insonation by using a focussed passive cavitation detector (PCD) to listen for broadband noise emissions resulting from microbubble collapse. The ability to effectively control the cavitation field relies on accurate interpretation of the PCD data. During HIFU exposure of a tissue-mimicking material, it was noted that the frequency content of the PCD signal shifted downwards with increasing phantom propagation path length to the PCD, as a result of depth-dependent attenuation. This has significant implications for the effective sensing volume of the PCD. In addition, it was noted that separate analysis of the peak and rms time-domain PCD values provides an incomplete description of inertial cavitation activity, but that combining these two parameters enables a better understanding of bubble cloud dynamics.