HIFU-induced cloud cavitation control for kidney stone treatment

Abstract

The shock wave lithotripter uses an order of microsecond pulse durations and up to a 100 MPa pressure spike triggered at approximately 0.5—2 Hz to fragment kidney stones through mechanical mechanisms. One important mechanism is cavitation. We proposed an alternative type of lithotripsy method that maximizes cavitation activity to disintegrate kidney stones using high-intensity focused ultrasound (HIFU). We designed a two-frequency wave (cavitation control (C-C) waveform); a high-frequency ultrasound pulse to create a cavitation cloud, and a low-frequency pulse following the high-frequency pulse to force the cloud into collapse. High-speed photography showed cavitation collapse on a kidney stone and shock wave emission from the cloud. We also conducted in-vitro erosion tests of model and natural stones. For the 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 small fragments were small enough to pass through the urethra. The results demonstrate that, with the precise control of cavitation activity, focused ultrasound has the potential to be used to develop a less invasive and more controllable lithotripsy system.