Acoustic streaming in a confined medium from high intensity focused ultrasound

High intensity focused ultrasound (HIFU) is a non-invasive therapeutic modality for the removal of tumors deep inside the body. The radio-frequency transducer is placed on the skin surface with the geometrically focused beam targeting a millimetric spot within the undesired mass. When a fluid medium is in the acoustic path, it can be set into flowing motion. Streaming has been utilized to characterize the acoustic output from transducers by measuring the flow velocity. Such implementation has been predominantly in unbounded medium settings under continuous beam application; and a typical streaming pattern exhibits steady forward flow along the acoustic axis, and backward entrainment flow off-axis. This investigation aims to probe the effect of fluid confinement by nearby boundaries on the streaming behavior. The motivation is therapeutic HIFU scenarios involving small liquid-filled cavities such as in the kidney or in the cardiac chambers. In the current experimental setup, two thin-walled rectangular cylinders measuring 5 or 20 mm were filled with water and placed concentrically with the HIFU focal spot in a water bath for acoustic coupling. Tracer nano particles were added to the water for PIV velocity measurements. The streaming velocity under different acoustic and experimental parameters is presented. A discussion of the dramatic effects of confinement on streaming is presented.