A theoretical and experimental investigation of nonlinear ultrasound propagation through tissue mimicking fluids

A numerical model is used to investigate finite amplitude ultrasound propagation through multiple layers of tissue-like media. This model uses a finite difference method to solve the nonlinear parabolic KZK wave equation. The code is modified to include an arbitrary frequency dependence of absorption and transmission effects for wave propagation across a plane interface at normal incidence. Measurements are taken of the axial nonlinear pressure field generated from a circular focused, 2.25 MHz source, through single- and multiple-layered tissue mimicking fluids. Two tissue mimicking fluids are developed to show acoustic properties similar to amniotic fluid and a typical soft tissue. The measured values of the nonlinearity parameter sound velocity and frequency dependence of absorption for both fluids are presented. These acoustic parameters, in addition to the measurement of the source conditions, are input to the numerical model allowing the experimental conditions to be simulated. Comparisons are made between the model predictions and the axial pressure field measurements. Results are presented in the frequency domain showing the fundamental and three subsequent harmonic amplitudes on axis, as a function of axial distance. The results show that significant nonlinear distortion can occur through media with characteristics typical of tissue. Excellent agreement is found between theory and experiment indicating that the model can be used to predict nonlinear ultrasound propagation through multiple layers of tissue-like media.