Use of passive arrays for characterization and mapping of cavitation activity during HIFU exposure

Inertial cavitation occurring during HIFU exposure can act both as a promoter and as a marker of heat deposition, because bubbles re-radiate part of the incident field as broadband noise emissions that are detectable over a frequency range far removed from the main HIFU excitation frequency. In the present work, a commercially available ultrasound scanning system (with the transmit signal turned off) and linear array are used to detect the emissions emanating from cavitating bubbles passively on 64 independent channels during HIFU exposure. A novel algorithm is presented that enables localization and mapping of cavitation activity, both for the case of a single bubble and in the context of contiguous, disjoint cavitating regions. By contrast to B-mode hyperechogenicity imaging that is widely used in ultrasound-guided HIFU systems but can only operate whilst the HIFU is off, the passive imaging technique presented here can be implemented during HIFU exposure, thus providing a potential means of real-time treatment monitoring during ablation.