Acoustic Streaming in Confined Liquid Subject to High Intensity Focused Ultrasound (HIFU) Exposure*

HIFU is an ex-corporeal therapeutic modality for the non-invasive treatment of tumors. A HIFU transducer emits spherically focused ultrasonic energy unto the target tissue deep under the skin, which is consequently absorbed and turned into heat, causing thermal ablation. Non-thermal effects can be present too, including cavitation bubble formation, radiation force, and acoustic streaming. The liquid streaming patterns were elucidated in previous HIFU investigations in an infinite medium. The aim of this study is to probe the effect of fluid confinement size on acoustic streaming. Two cubic water cuvettes of 5 mm, and 20 mm confinements were placed in an acoustic-coupling water bath at the geometric focus of the HIFU transducer (1.6 MHz, $f${\#} 1, 60 mm). A single 30 ms HIFU pulse exposure was used. PIV was implemented to measure the centerline streaming field (radial and axial components) at the end of the acoustic emission (avg. of 100 repeats). Significant differences were found in the flow patterns between the small (5 mm) and large (20 mm) confinements. The streaming velocity levels were also dramatically different. In conclusion, fluid confinement has a very important effect on the streaming flow from HIFU.