Thermal-mechanical response modelling and thermal damage prediction of soft tissues during thermal ablation
Creators: name: Li, Xin
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400303
subject: Biomechanical engineering
classificationCode: 460207
subject: Modelling and simulation
classificationCode: 461306
subject: Numerical computation and mathematical software

description: During thermal ablation, target soft tissue responses both thermally and mechanically simultaneously. However, current thermal ablation treatment mainly relies on the quantitative temperature indication to evaluate tissue behaviours and control the delivered thermal energy, which is ineffective and inaccurate. Based on these, our research study focuses on: bioheat transfer theory, linear and nonlinear elasticity of soft tissues at varied temperatures, as well as thermal damage prediction theory, and the whole program was developed in Netbeans IDE 8.1. The main contributions of our research work lie in the following aspects:<br><br>Firstly, considering a situation where soft tissue’s mechanical deformation during thermal ablation is only caused by thermal loading, it is reasonable to assume that the generated strain value is within the linear range of stress-strain relationship characterisation which is also thermal stable (nearly temperature independent). Therefore, we propose our first model by integrating the heating process with thermally-induced mechanical deformations of soft tissues for simulation and analysis of the thermal ablation process. This method combines classical Fourier based bioheat transfer and constitutive elastic mechanics derived from the method of multiplicative decomposition of thermal mechanical deformation gradient, as well as non-rigid motion dynamics. The 3D governing equations are discretised spatially using finite difference scheme and temporally using implicit time integration scheme and the obtained linear system of equations are subsequently solved using a Gauss-Seidel iterative solver. Simulation implement based on proposed method can serve as a visible assistance for relevant surgeons on analysing soft tissue’s behaviours from both thermal and mechanical deformation fields rather than from just determined temperature distribution.<br><br>Secondly, we present a method to characterize soft tissue thermal damage by taking into account of thermal mechanical interactions during thermal ablation, concerning stored energy by both thermal and mechanical effects can affect the energy barrier for macromolecular transitions, leading to further or the reverse damage to treated biological tissues. To do this, traditional tissue damage model of Arrhenius integration is improved by including the thermally and mechanically induced strain energy term. Simulations and comparison analysis based on different types of soft tissues are also performed to study its influences. Our findings may provide more reliable guidelines for relevant surgeons to control the tissue damage zone during thermal ablation practice.<br><br>Thirdly, thermal relaxation time used to describe heating process in homogeneous substance is usually referred to as the characteristic time in non-homogeneous biological materials, which is needed to accumulate enough energy to transfer to the nearest point. Such non-Fourier thermal behaviour has also been experimentally observed in biological tissues. Our second model is presented by integrating non-Fourier bioheat transfer and constitutive elastic mechanics derived from the method of multiplicative decomposition of thermal mechanical deformation gradient, as well as non-rigid motion of dynamics to predict and analyse thermal distribution, thermal-induced mechanical deformation and tissue damage under purely thermal loads. The simulation performances are compared between two numerical methods: Finite Difference Method and Finite Element Method, from perspectives of accuracy and computing efficiency, and also against available existed experimental data and other commercialized analysis tools.<br><br>Finally, our research moves on to nonlinear range characterization of tissue deformation under combined thermal and mechanical loads. Basically, the contribution of our proposed nonlinear thermal mechanical model is by extending the finite strain framework of Neo-Hookean energy function to the heating process of soft tissues during thermal ablation. Meanwhile, our nonlinear thermal mechanical model also considers the effect of collagen fibre bundles as embedded in many biological tissues. Separating free energy density modelling into isotropic and anisotropic parts, it is assumed that the anisotropy is due to the collagen fibre bundles behaviour, while the ground substance, behaves in an isotropic manner can be modelled using selected nonlinear biomaterial model. The necessary ingredients for the finite element method implementation including: weak form and time integration are also included in this chapter.<br>Keywords: Thermal ablation, soft tissue, non-Fourier bioheat transfer, thermal mechanical deformation, anisotropic nonlinear, tissue damage.