Acoustic propagation effects in therapeutic ultrasound

Accurate understanding of ultrasound-tissue interaction is important for realization of clinically useful therapeutic ultrasound methods and devices. Linear acoustic propagation in homogeneous media, including diffraction and absorption effects, provides a useful first approximation but fails to accurately model many problems of interest. Depending on the therapy regime, other important effects can include cavitation and other gas activity, inhomogeneous tissue structure, finite-amplitude propagation, temperature-dependent tissue properties, and irreversible tissue modification. For ablation of soft tissue using ultrasound, prediction of therapeutic effects requires accurate knowledge of space- and time-dependent heat deposition from acoustic absorption. In addition to perfusion losses, acoustically inhomogeneous tissue structure, even in nominally homogeneous organs such as the liver, can modify heating patterns enough to change treatment outcomes. Gas activity due to boiling and tissue property changes due to local ablation, both of which markedly affect treatment, can be approximated by appropriate modification of the initial heat deposition pattern. These issues are illustrated by simulations of ultrasound therapy and comparison with in vivo and in vitro ultrasound ablation experiments.