Thermal-mechanical response modelling and thermal damage prediction of soft tissues during thermal ablation

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: 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. 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. 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. 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. Keywords: Thermal ablation, soft tissue, non-Fourier bioheat transfer, thermal mechanical deformation, anisotropic nonlinear, tissue damage

Ultrasonic Pulse Wave Imaging for in vivo Assessment of Vascular Wall Dynamics and Characterization of Arterial Pathologies

Arterial diseases such as hypertension, carotid stenosis, and abdominal aortic aneurysm (AAA) may progress silently without symptoms and contribute to acute cardiovascular events such as heart attack, stroke, and aneurysm rupture, which are consistently among the leading causes of death worldwide. The arterial pulse wave, regarded as one of the fundamental vital signs of clinical medicine, originates from the heart and propagates throughout the arterial tree as a pressure, flow velocity, and wall displacement wave, giving rise to the natural pulsation of the arteries. The dynamic properties of the pulse wave are intimately related to the physical state of the cardiovascular system. Thus, the assessment of the arterial wall dynamics driven by the pulse wave may provide valuable insights into vascular mechanical properties for the early detection and characterization of arterial pathologies. The focus of this dissertation was to develop and clinically implement Pulse Wave Imaging (PWI), an ultrasound elasticity imaging-based method for the visualization and spatio-temporal mapping of the pulse wave propagation at any accessible arterial location. Motion estimation algorithms based on cross-correlation of the ultrasound radio-frequency (RF) signals were used to track the arterial walls and capture the pulse wave-induced displacements over the cardiac cycle. PWI facilitates the image-guided measurement of clinically relevant pulse wave features such as propagation speed (pulse wave velocity, or PWV), uniformity, and morphology as well as derivation of the pulse pressure waveform. A parametric study investigating the performance of PWI in two canine aortas ex vivo and 10 normal, healthy human arteries in vivo established the optimal image acquisition and signal processing parameters for reliable measurement of the PWV and wave propagation uniformity. Using this framework, three separate clinical feasibility studies were conducted in patients diagnosed with hypertension, AAA, and carotid stenosis. In a pilot study comparing hypertensive and aneurysmal abdominal aortas with normal controls, the AAA group exhibited significantly higher PWV and lower wave propagation uniformity. A “teetering” motion upon pulse wave arrival was detected in the smaller aneurysms ( 5.5 cm in diameter). While no significant difference in PWV or propagation uniformity was observed between normal and hypertensive aortas, qualitative differences in the pulse wave morphology along the imaged aortic segment may be an indicator of increased wave reflection caused by elevated blood pressure and/or arterial stiffness. Pulse Wave Ultrasound Manometry (PWUM) was introduced as an extension of the PWI method for the derivation of the pulse pressure (PP) waveform in large central arteries. A feasibility study in 5 normotensive, 9 pre-hypertensive, and 5 hypertensive subjects indicated that a significantly higher PP in the hypertensive group was detected in the abdominal aorta by PWUM but not in the peripheral arteries by alternative devices (i.e. a radial applanation tonometer and the brachial sphygmomanometer cuff). A relatively strong positive correlation between aortic PP and both radial and brachial PP was observed in the hypertensive group but not in the normal and pre-hypertensive groups, confirming the notion that PP variation throughout the arterial tree may not be uniform in relatively compliant arteries. The application of PWI in 10 stenotic carotid arteries identified phenomenon such as wave convergence, elevated PWV, and decreased cumulative displacement around and/or within regions of atherosclerotic plaque. Intra-plaque mapping of the PWV and cumulative strain demonstrated the potential to quantitatively differentiate stable (i.e. calcified) and vulnerable (i.e. lipid) plaque components. The lack of correlation between quantitative measurements (PWV, modulus, displacement, and strain) and expected plaque stiffness illuminates to need to consider several physiological and imaging-related factors such as turbulent flow, wave reflection, imaging location, and the applicability of established theoretical models in vivo. PWI presents a highly translational method for visualization of the arterial pulse wave and the image-guided measurement of several clinically relevant pulse wave features. The aforementioned findings collectively demonstrated the potential of PWI to detect, diagnose, and characterize vascular disease based on qualitative and quantitative information about arterial wall dynamics under pathological conditions