Effect of mode conversion on ultrasonic heating at tissue interfaces.

A number of investigators have observed localized heating by ultrasound beams near impedance discontinuities within tissues. It has been suggested that mode conversion to shear waves at impedance discontinuities and subsequent absorption of these waves in a very small distance was the explanation for this heating. A mathematical model for mode conversion at a plane interface between two viscoelastic media is presented. Longitudinal and shear properties are used to calculate the amount of mode conversion that occurs at muscle-air and muscle-bone interfaces. Shear waves in bone are found to be an important source of heating, but shear waves in the muscle provide a negligible effect on heating at the interface.