A USAE frequency shift method for monitoring stiffness variations in tissues

Abstract

In solid mechanics, the resonance frequency of a material is typically linked to its mechanical properties. A frequency shift therefore typically denotes a change in stiffness. It has already been shown that temperature variation causes stiffness changes in tissues. The amplitude of the ultrasound-stimulated acoustic emission (USAE) signal has also been shown to vary with temperature as well as indicate temperature elevation. Its amplitude, however, is sensitive to both acoustical and mechanical parameters, which at most frequencies have opposite effects resulting from temperature changes. In this paper, the measurement of a frequency shift of the USAE resonant peaks is used for monitoring of the tissue stiffness variation with temperature. In a numerical simulation, the variation of the frequency shift (ranging between −150 and 500 Hz) at different temperatures is shown. Then, in a series of experiments involving a gel phantom and porcine muscle tissue, the frequency shift variation (ranging between −200 and 200 Hz) is shown to follow the known stiffness changes due to temperature. Both simulation and experimental results indicate that the USAE frequency shift method can dissociate the mechanical from the acoustical parameter dependence and detect tissue coagulation. [Work supported by NIH Grant CA82275.]