Therapeutic Bubbles: Basic Principles of Cavitation in Therapeutic Ultrasound

Abstract

At therapeutic intensities, the application of ultrasound is often accompanied by bubble activity. Bubbles concentrate (by orders of magnitude) acoustic energy and convert that acoustic energy into mechanical, thermal, chemical or frequency‐shifted acoustic energy. Depending on the specific clinical context, bubble and cavitation activity may be welcome or unwelcome. Whether desirable or not, certain basic phenomena may be observed, and certain basic principles deduced which can guide researchers in their efforts either to avoid or to exploit cavitation. Towards that end this paper will introduce the fundamentals of bubble dynamics. A bestiary of bubble phenomena and related concepts (such as the Blake threshold, inertial cavitation, shape instabilities, rectified diffusion) will be discussed. The dependence of bubble dynamics on control parameters (acoustic frequency and pressure, pulse parameters) and material properties (shear viscosity, surface tension, temperature, elasticity) will be considered. Experimental techniques and diagnostics for bubble detection and monitoring will be briefly mentioned. Tools for modeling bubble dynamics and their effects will be introduced. Some consequences of bubbles and cavitation activity in vivo (both physical and biological) will be presented, drawing on the example of bubbles in high‐intensity focused ultrasound (HIFU) applications. Finally, recommendations of techniques for either avoiding or enhancing (and maybe controlling) cavitation will be proffered.