Abstract

In this work, a new numerical framework is proposed and implemented to simulate acoustic wave propagation in 3D viscoelastic heterogeneous media. The framework is based on the elastodynamic wave equation in which a 3D second-order time-domain perfectly matched layer (PML) formulation is developed to model unbounded media. The numerical framework is discretized by a finite difference formulation and its stability analysis is discussed. The proposed numerical method is capable of simulating 3D shear and longitudinal acoustic waves for arbitrary source geometries and excitations, together with arbitrary initial and boundary conditions. After validation of the framework, it was used to simulate the propagation of ultrasound shear wave in high intensity focused ultrasound (HIFU) induced thermal lesions located within soft tissue. The parameters in these simulations were obtained from standard double-indentation measurements of the viscoelastic parameters of normal and thermally coagulated chicken breast tissue samples. A HIFU system was used to induce thermal lesions in tissue. In this study, a new elastography procedure was also introduced to differentiate between the normal and HIFU induced thermal lesions. This method is based on time-frequency analysis of shear wave propagation within the tissue. In the proposed method, the Wigner-Ville distribution has been used as a time-frequency analytical technique to detect the location of shear wave propagating within the tissue, and to estimate the shear speed of the wave as well as its center frequency and attenuation coefficient. This method was applied to the acoustic wave propagation simulation results of the HIFU thermal lesion. It was finally used to estimate the local viscoelastic parameters of the medium. It was demonstrated that the proposed method is capable of differentiating the thermal lesions from the normal tissue based on their viscoelastic parameters.

Highlights

  • In this chapter, the motivations of this thesis study are first introduced

  • After validation of the numerical simulation framework, two simulation examples of acoustic wave propagation in the field of biomedical ultrasound were presented in which thermally-induced high intensity focused ultrasound (HIFU) lesions placed in the middle of normal soft tissue to demonstrate the capabilities of this simulation framework in biomedical elastography applications

  • The parameters used in these simulations were based on double-indentation mechanical measurements of the viscoelastic parameters of ex vivo chicken breast samples with and without high intensity focused ultrasound induced thermal lesions

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Summary

Introduction

The motivations of this thesis study are first introduced. the goals and the specific aims are listed and briefly explained. The contributions of this research are described and an overview of the dissertation is presented. First a brief review of the biomedical applications of ultrasound in diagnosis and treatment is provided. The elastography as an imaging modality and the high intensity focused ultrasound (HIFU) as a treatment application of ultrasound are reviewed in more details. This chapter is continued by reviewing the acoustic wave propagation formulations and their numerical simulation techniques. This provides the background review of the ultrasound researches related to this thesis study. The well-known time-frequency methods are described and the WVD method is explained in more details. We provide the basis for the time-frequency analytical method used in this research

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