A theoretical study of nonlinear effects with focused ultrasound in tissues: an "acoustic bragg peak".

This is a theoretical study of the absorbed power density that arises when a focused ultrasound beam is absorbed in a tissue-like medium. The specific application is selective tissue heating for cancer hyperthermia. Results are expressed in terms of an enhancement ratio which describes the fractional gain in absorbed power density when nonlinear effects are taken into account relative to the absorbed power density that results when only linear effects are considered. Starting from a standard configuration in which the transducer has a (Gaussian) diameter of 0.12 m, a radius of curvature of 0.16 m and an operating frequency of 1 MHz a numerical sensitivity analysis is performed in which transducer and tissue parameters are varied one at a time. Enhancement ratios are invariably greater than unity, often being in the range of 1.5-2 for regions near the focus of the beam. The prediction is that nonlinear effects will probably be useful in selectively increasing the temperature rise in the focal region of a focused applicator.