Cloud cavitation control for lithotripsy using high intensity focused ultrasound.

Cloud cavitation is potentially the most destructive form of cavitation. When the cloud cavitation is acoustically forced into a collapse, it has the potential to concentrate a very high pressure, more than 100 times the acoustic pressure, at its center. We experimentally investigate a method to control the collapse of high intensity focused ultrasound (HIFU)-induced cloud cavitation to fragment kidney stones. Our study examines a novel two-frequency wave designed to control the cloud cavitation (cavitation control [C-C] waveform); a high-frequency ultrasound pulse (1 to 4 MHz) to create the cloud cavitation and a low-frequency trailing pulse (545 kHz) following the high-frequency pulse to force the cloud into collapse. High-speed photography has revealed that a localized distribution of the cloud cavitation can be produced within 1 mm on the solid surface by the high-frequency pulse. The low-frequency ultrasound was irradiated to the high-frequency-induced cloud cavitation. A subsequent shock wave emitted from the cloud cavitation was observed both in the shadowgraph photography and the remote hydrophone measurement. Furthermore, in vitro erosion tests of model and natural stones were conducted. In the case of model stones, the erosion rate of the C-C waveform showed a distinct advantage with the combined high- and low-frequency waves over either wave alone. Natural stones were eroded and most of the resulting fragments were less than 1 mm in diameter. The results show that the control of the cloud cavitation has untapped potential for the lithotripsy applications upon further optimization of the ultrasound parameters and complementary in vivo studies.