Closed-loop adaptive feedback control of cavitation during high-intensity focussed ultrasound therapy.

Prior experimental evidence indicates that the occurrence of inertial cavitation at the HIFU focus enhances the local rate of heat deposition, due to increased absorption of the broadband emissions associated with collapsing bubbles. However, initiating and sustaining cavitation activity throughout HIFU exposure remain a major challenge. Previous open-loop attempts at maintaining cavitation activity by varying the HIFU pulse duration have necessitated the use of duty cycles on the order of 20%, which are too low to achieve ablative temperatures. The present work presents the design, validation, and testing of a real-time closed-loop adaptive controller for sustaining thermally relevant cavitation within the HIFU focal region at duty cycles in excess of 95%. During 1.067 MHz HIFU exposure of an agar tissue-mimicking material, cavitation activity is continuously monitored and quantified using a single-element broadband passive cavitation detector (PCD) positioned coaxially with the HIFU transducer. The HIFU amplitude is adaptively modulated in direct response to the PCD signal, making it possible to maintain cavitation activity and a temperature rise in excess of 30 °C for more than 30 s. Most importantly, the acoustic energy needed to achieve such focal heating is only a fraction of that required in the absence of cavitation control.