Non-Invasive Monitoring and Control of Inertial Cavitation Dynamics during HIFU Exposure In Vitro

Inertial cavitation has been shown to play a significant role in enhancing HIFU ablation, especially when the focal temperatures do not exceed 100 degrees. However, once cavitation is instigated, the bubble cloud grows unstably towards the HIFU transducer, shielding the original focus. In this study, a passive cavitation detection (PCD) technique was used to monitor inertial cavitation activity during 1.1-MHz CW exposure of an agar-graphite tissue mimicking material for peak-negative focal pressures in the range 1.84-4.98 MPa. In all cases, the RMS time trace of the PCD signal exhibited a plateau region of some 3 seconds, followed by an extremely sharp peak, beyond which a rapid decay in broadband noise emissions was observed. No significant increase in broadband noise was induced for HIFU amplitudes in excess of 3.42 MPa, which suggests that the heating enhancement provided by cavitation saturates beyond that peak negative pressure amplitude. Based on these results, the use of pulsed excitation was investigated as a means of controlling cavitation activity following a brief period of CW exposure. Cavitation activity was readily sustained in the focal region for periods in excess of 60 s in all cases. However, the level of sustained activity was found to be highest for 20% duty cycle, and lower for 10% and 40 % duty cycle. It is concluded that pulsed HIFU excitation can be used to sustain and confine cavitation activity to the focal region, and that RMS analysis of PCD signals provides a very robust descriptor of bubble cloud dynamics.