Tissue Atomization by High Intensity Focused Ultrasound.

Liquid atomization and fountain formation by focused ultrasound was first published by Wood and Loomis [1]. Since then, the cavitation-wave hypothesis emerged to explain atomization in a fountain, which states atomization arises from a combination of surface capillary waves and the collapse of cavitation bubbles. More recently, high intensity focused ultrasound (HIFU) has been shown to fractionate tissue through either pulsed-cavitation or millisecond boiling histotripsy therapies; however it is unclear how millimeter-size boiling bubbles or cavitation bubble clouds fractionate tissue into submicron-size fragments. The objective of this work is to test the hypothesis experimentally that atomization and fountain formation occurs similarly in liquids and tissues and results in tissue erosion. A 2-MHz HIFU transducer operating at peak in situ pressures of 50 MPa and -11 MPa (linear intensity = 14,000 W/cm²) was focused at the interface between a liquid or tissue and air. A high-speed camera was used to monitor atomization and fountain formation in water, ethanol, glycerol, bovine liver, and porcine blood clots. The in situ linear intensity threshold for consistent atomization in one 10-ms pulse increased in the order: ethanol (180 W/cm²) < blood clot (250 W/cm²) < water (350 W/cm²) < liver (6200 W/cm²); glycerol did not atomize. Average jet velocities for the initial spray at the maximum acoustic intensity were similar for all materials and on the order of 20 m/s. The tissue erosion rate of liver approached saturation at around 300 10-ms pulses repeated at 1 Hz, which had an average erosion volume of 25.7±10.9 mm³. While tissue atomization and fountain formation does not completely mimic what is observed in liquids, atomization provides a plausible explanation of how tissue is fractionated in millisecond boiling and possibly even cavitation cloud histotrispy therapies.