Cavitation and microbubble modeling

Abstract

Bubbles in tissue may either help or hinder biomedical ultrasound. Exogenously introduced ultrasound contrast agents may comprise suspensions of micrometer-sized gas bubbles of sulfur hexafluoride, perfluorocarbon, or other low-solubility gas that are stabilized against dissolution by shells of lipids, proteins or synthetic, or alternatively, quasi-stable micro-droplets of certain liquids that nucleate gas bubbles in response to an impinging acoustic beam. However, tiny vapor bubbles may form spontaneously in tissues and act as potentially damaging cavitation nuclei under the influence of an acoustic wave. Microbubbles respond to the acoustic field by contracting and expanding to a much greater extent than does the surrounding tissue. This ‘acoustic cavitation’ may be violent, producing highly reactive chemical species, emission of ultraviolet and soft x-ray photons, and radiated shock waves, or less powerful, inducing, e.g., continuous heat production, radiation forces on neighboring particles, and microstreaming. Much early work assumed cavitation bubbles were surrounded by simple liquids, but biological tissues are viscoelastic, which can significantly affect bubble responses and thus must be considered. Mathematical models for these and related processes will be presented and compared, and details of their various physical mechanisms and the effects they may produce in biological tissue will be discussed.