Principles of High-Intensity Focused Ultrasound

Although its use for therapeutic purposes predates diagnostic applications by several decades, ultrasound is most widely known for its imaging capabilities. The passage of ultrasound (US) through tissue can lead to biological changes that may be reversible or irreversible. The biological significance of these effects depends to a large extent on the energy in the ultrasound beam and the goal of the exposure. At diagnostic levels, any changes are largely believed to be biologically insignificant. For therapeutic ultrasound, beneficial cellular or functional effects are deliberately sought, whether these are at the cell membrane level (e.g., transient changes in permeability to facilitate drug delivery) or less subtle effects such as the localised temperatures rises that are required to achieve immediate thermal necrosis in high intensity focused ultrasound (HIFU; this technique is sometimes also referred to as FUS).