An investigation into the use of cavitation for the optimisation of high intensity focused ultrasound (HIFU) treatments

High intensity focused ultrasound (HIFU) is an emerging treatment for soft tissue tumours, but long treatment times (3-4 hours for tumour volumes ≤4 cm3) remain a significant problem. Cavitation can increase the local energy deposition in tissue, which may result in faster, more efficient treatments. There is, however, a lack of understanding of HIFU generated cavitation activity in tissue. This thesis investigates the role that cavitation could play in the optimisation of HIFU treatments, with a view to incorporation of the findings into a clinical HIFU system. An extensive study of cavitation activity in ex-vivo bovine liver tissue has been undertaken. Passive and active cavitation detection techniques have been combined to give a system for use in HIFU fields. This has been used to investigate cavitation activity over a range of intensities, and to determine appropriate methods for cavitation detection during clinical use. Cavitation activity in tissues exposed to HIFU falls into two categories: micron sized bubbles generated by the peak negative acoustic pressures (acoustic cavitation) and millimetre sized bubbles generated by thermal ex-solution of gas vapour (‘boiling’). Acoustic cavitation itself can be split into two types of behaviour, non-inertial (stable) cavitation and inertial (transient) cavitation. Thresholds for these different categories were investigated for clinically relevant exposure times (2-5 s) for degassed water, ex-vivo and perfused liver and ex-vivo kidney. A sonochemical study was also undertaken in water, to further elucidate cavitation behaviour. Two methods of using cavitation to increase HIFU energy deposition by “preconditioning” tissue prior to an ablative exposure, were investigated. Firstly, the ability to ‘seed’ acoustic cavitation in a controlled and repeatable way was studied. Secondly, the use of ‘boiling’ bubbles as reflectors post-focally was tested. Both techniques are implementable in the new clinical system under construction at the Institute of Cancer Research (ICR).