Localization and Interpretation of Bubble Activity during HIFU Exposure

Bubble activity has been found to play a key role in many HIFU applications. ranging from thermal ablation to histotripsy. A combined active-passive cavitation detection system is presented, which enables localization of bubble activity with respect to the HIFU focus during HIFU exposure. The system consists of a high-frequency detector positioned coaxially with the therapy transducer, either driven in pulse-echo mode or used passively. Spectral analysis of the received signal makes it possible to distinguish between inertial cavitation and thermally induced bubble activity, and to track the evolution of either bubble population during HIFU exposure. Inertial cavitation is found to play a key role in the early stages of HIFU exposure, beyond which it gradually shuts down under the effect of heating. Thermal bubbles appear in the later stages of HIFU exposure, are most readily tracked by the active detection scheme, and gradually migrate towards the pre-focal region. This dual active-passive cavitation system could provide an invaluable tool for ensuring both treatment safety and efficacy during clinical HIFU application.