Page 29 - Volume 8, Issue 4 - Winter 2012

Fig. 1. Diagram of HIFU surgery. The transducer emits a focused ultrasound beam through the overlying tissue layers to create a high intensity region within the organ. The focus can be translated to ablate the entire volume of interest. (Adapted from part of a figure in reference 6)
tion at the shocks can induce boiling in tissue in millisec- onds, much faster than typical HIFU exposures. Extracorporeal shockwave lithotripsy uses such focused
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This ultrasound-induced tissue disintegration opens a new direction in development of HIFU medical technology. The technique has been dubbed ‘histotripsy’ (the prefix histo- translated from Greek to mean tissue), as an analog to lithotripsy. Two separate techniques to perform histotripsy with ultrasound shock waves have been demonstrated thus far: one using a cloud of cavitation bubbles and another that uses boiling bubbles. The cavitation-based approach has been developed over the last 11 years at the University of
shock waves and cavitation to break up kidney stones. Recent HIFU studies have shown that the presence of shocks and cavities, induced by HIFU either as a cavitation cloud or millisecond boiling, can be also used to break up or mechan- ically fractionate soft tissues to tiny debris—an outcome sim- ilar to disintegration in a “remote” blender.
Fig. 2. Bioeffects of focused ultrasound at different focal intensity levels. At lower intensities, heating through acoustic absorption is the dominant mechanism, denaturing proteins within the tissue, leaving a blanched appearance. Boiling cavities form in the lesion when the temperature reaches 100oC. At higher intensities, heating combined with microbubble cavitation can cause mechanical trauma to the tissue structure. At very high intensities, shockwaves form at the focus and the wave itself can impart sig- nificant mechanical damage such as comminution of kidney stones (lithotripsy) or fractionation of soft tissues (histotripsy).
Disintegration of Tissue Using HIFU 25