Mechanical ablation using high-intensity focused ultrasound

High-Intensity Focused Ultrasound (HIFU) is a non-invasive ultrasound technique which has been used to thermally necrose solid tumours without disruption to surrounding tissue. In recent years, an alternative HIFU technique to thermal ablation has been developed. This is known as mechanical tissue fractionation or histotripsy. Acoustic peak positive and negative pressures at the HIFU focus used in histotripsy are comparable to those in the shockwaves used in lithotripsy for kidney stone fragmentation. In histotripsy, there are two different methods of creating pure mechanical damage of soft tissue by (a) pulsed ultrasound cavitation (i.e., Cavitation cloud based histotripsy) or (b) shock wave heating and millisecond boiling (i.e., Boiling histotripsy). A well-defined lesion in the form of a cavity can be produced by histotripsy without any significant thermal damage at the periphery of the cavity. Recent in vivo studies have shown that a lesion produced by histotripsy contains complete fragmentation of tissue and is sharply demarcated between treated and untreated regions. Subcellular debris remaining inside a mechanically fractionated lesion can be absorbed as part of the physiologic healing mechanism. This Chapter reviews the principle mechanisms behind and the potential clinical applications of histotripsy.