Controlling a high intensity focused ultrasound induced cavitation field via duty cycle

Abstract

Cavitation has been implicated in the lack of control over the shape of thermal lesions generated by high-intensity focused ultrasound (HIFU). A coincident effect the decline in the acoustic emissions from cavitation at the focus suggests that the HIFU energy is shielded from the focal region, possibly by prefocal bubble activity. Most clinical techniques employ continuous-wave (CW) ultrasound, which can exacerbate the problem depending on the acoustic intensities employed. This talk presents a series of experiments investigating techniques to control HIFU energy delivered to, and cavitation activity within, a tissue phantom. A passive cavitation detector (PCD) is employed as a sensor of cavitation activity. For 1.1-MHz CW ultrasound at focal pressures above 3 MPa, bubble shielding was inferred from a steady decline in the PCD signal over time. By lowering the duty cycle the PCD output remained constant over time. Finally, driving the HIFU source initially with a CW signal and then switching to a pulsed signal resulted in shielding, recovery, and a stable PCD signal, thus demonstrating our ability to control cavitation activity during HIFU exposure. [Work supported by the U.S. Army and the Center for Subsurface Sensing and Imaging Systems via NSF ERC Award No. EEC-9986821.]