Analysis of Nonlinear Effects in Ultrasound Diagnostics

The model study of nonlinear effects in the ultrasound diagnostics is focused on developing and experimental verification of new computer algorithms for numerical modeling of the ultrasound field including nonlinear effects accompanying the ultrasound propagation in biological tissues. A theoretical analysis of nonlinearities in ultrasound propagation and their contribution to thermal and mechanical effects of diagnostic ultrasound applications was conducted. A numeric model of ultrasound propagating through a tissue model formed of a set of homogenous layers with defined physical parameters was developed in Matlab environment. The computation of ultrasound field spatial distribution is based on the angular spectrum method. For simulating of nonlinearities Burgers nonlinear wave equation is used. The resulting spatial distribution of the ultrasound field is described by the amplitudes of selected number of harmonic components of the acoustic pressure, by the mechanical index, by the harmonic distortion of the signal and by the spatial peak temporal average intensity of all considered components. The numerical model was experimentally verified using an unique ultrasound measurement system – the ultrasound measurement basin. The verified numeric model was used for assessing the veracity of mechanical index and tissue heating prediction during ultrasound applications in cases when the nonlinear effects are neglected in computations. The reliability of the mechanical index indicated by the diagnostic ultrasound instruments is also evaluated using the designed numeric model. The aim was to compare the indicated value usually obtained by derating the measurements in a water medium to the actual mechanical index present in the insonated tissue