Tissue atomization by high intensity focused ultrasound

Simon, J.; Sapozhnikov, O.; Khokhlova, V.; Y-N, W.; Crum, L.; Bailey, M.

Авторы: Simon J., Sapozhnikov O., Khokhlova V., Y-N Wang, Crum L., Bailey M.
Сборник: 2012 IEEE International Ultrasonics Symposium Proceedings
Год издания: 2012
Место издания: Dresden, Germany
Первая страница: 1003
Последняя страница: 1006
Аннотация: Liquid atomization and fountain formation by focused ultrasound was first published by Wood and Loomis (1927). 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-sized 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 pressures up to 65 MPa positive, and 16 MPa negative (linear intensity = 24,000 W/cm2) was focused at the interface between a liquid or tissue and air. A thin liquid or tissue layer (thickness 2-15 mm) was held in a container with an acoustically transparent, thin plastic film bottom. The container was submerged in a water tank so that the bottom of the container was positioned just below the water surface. A high-speed camera was used to monitor atomization and fountain formation. The atomization threshold, time to atomize, and velocity of the ejected fragments were compared between 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. At the maximum intensity, water atomized in 2 µs, while liver atomized in 20 µs. Jet velocities at the maximum acoustic intensity were similar for all materials except ethanol and ranged from 10-13 m/s; however, in ethanol jet velocities were approximately double at 25 m/s. The tissue erosion rate of liver approached saturation at around 300 10-ms pulses and had an average erosion rate of 0.15 ± 0.7 mm3/10-ms pulse. While tissue atomization and fountain formation only slightly mimics what is observed in liquids, atomization provides a plausible explanation of how tissue is fractionated in millisecond boiling and possibly even pulsed-cavitation histotrispy therapies. Interestingly, tissue preparation methods, specifically whether or not the tissue is maintained in PBS, substantially affects the threshold of atomization. While this study were conducted in ex vivo tissues, these observations may be useful in predicting which in vivo tissues can be successfully fractionated in boiling histotripsy. Work supported by NIH DK43881, DK092197, EB007643 and NSBRI through NASA NCC 9-58.
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