Nonlinearity Parameter Tomography — A Novel Imaging Method in Tissue Characterization

Abstract

The acoustic nonlinearity parameter B/A is a basic parameter in nonlinear acoustics and is defined as the ratio of the coefficients of quadratic term to linear term in the Taylor expansion of the state equation: B/A=2ρp₀c₀(∂c/∂p)_o,s, where ρ₀ and c₀ are the density and velocity, p is static pressure and s is entropy. This parameter can describe the nonlinear ultrasonic phenomena in the range of biomédical frequencies and intensities, for example, wave distortion, harmonic generation, sound saturation and extra-attenuation etc. In recent years, many research works indicated that nonlinearity parameter B/A can provide more important information, such as structural features and pathological state of tissues than that linear parameters and may be a new parameter in biological tissue characterization. Several efforts have been made in nonlinear parameter imaging by T.Sato¹ and Y.Nakagawa using phase shift method and finite amplitude method, respectively. In our previous works successful computer simulation of acoustic nonlinearity parameter tomography from calculated second harmonic data using conventional CT technique and some preliminary experimental results were reported^3,4. In this paper we will present our recent experimental investigation of B/A tomography for normal and several pathological liver tissues.