Commentary on the detection of bubble activity generated in ex-vivo tissue by high intensity focused ultrasound (HIFU) with respect to the generation of therapeutic lesions in tissue for the treatment of cancer

Abstract

Cancer treatment by extracorporeal high intensity focused ultrasound (HIFU) is constrained by the time needed to ablate relevant tumour volumes clinically. Controlled cavitation might be used to optimise HIFU treatments, but such control requires a greater understanding of its role in lesion formation, and the provision of appropriate techniques to monitor cavitation in tissue. During HIFU exposure various forms of cavitation can occur: acoustic cavitation (both non-inertial and inertial), and bubble formation due to two thermally-driven effects (the vaporisation of liquid into vapour, and the exsolution of formerly dissolved permanent gas out of the liquid and into gas spaces). Different forms of cavitation gives rise to characteristic signals that can be monitored during HIFU. Furthermore, the character of the signal can change depending on the stage of the cavitation in question (nucleation, established cavitation, population effects etc.). Prior to undertaking experiments using tissue, studies were performed by exposing degassed water. The aim of these experiments was to test a detection system in a minimally attenuating medium known to cavitate, in order to provide data for comparison with ex-vivo tissue results.This report is written in support of the journal paper “A Study of Bubble Activity Generated in Ex-Vivo Tissue by High Intensity Focused Ultrasound (HIFU)” by the same authors (McLaughlan et al. 2010). It outlines supporting material for the discussion and conclusions contained in that paper, a study involving monitoring clinically relevant HIFU exposures in degassed water and ex-vivo bovine liver. Monitoring is accomplished using a suite of cavitation detection techniques (exploiting passive and active acoustics, audible and ultrasonic emissions and electrical drive power fluctuations). The aim of the paper is to allow informed progress towards a monitoring system specifically tailored for use during clinical HIFU treatment