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ASSESSING THE ROLE OF BIOMECHANICAL FLUID–STRUCTURE INTERACTIONS IN CEREBRAL ANEURYSM PROGRESSION VIA PATIENT-SPECIFIC COMPUTATIONAL MODELS
Three key challenges in developing advanced image-based computational models of cerebral aneurysms are: (i) disentangling the effect of biomechanics and confounding clinical risk factors on aneurysmal progression, (ii) accounting for arterial wall mechanics, and (iii) incorporating the effect of surrounding tissue support on vessel motion and deformation. This thesis addresses these knowledge gaps by developing fluid-structure interaction (FSI) models of subject-specific geometries of cerebral aneurysms to elucidate the effect of coupled hemodynamics and biomechanics. A consistent methodology for obtaining physiologically realistic computational FSI models from standard-of-care imaging data is developed. In this process, a novel technique to estimate heterogeneous arterial wall thickness in the absence of subject-specific arterial wall imaging data is proposed. To address a limitation in the mesh generation workflow of the state-of-the-art cardiovascular flow modeling tool SimVascular, generation of meshes with boundary-layer mesh refinement near the blood-vessel wall interface is proposed for computational geometries with nonuniform wall thickness. Computational murine models of thoracic aortic aneurysms were developed using the proposed methodology. These models were used to inform external tissue support boundary conditions for human cerebral aneurysm subjects via a scaling analysis. Then, the methodology was applied to subjects with multiple unruptured cerebral aneurysms. A comparative computational FSI analysis of aneurysmal biomechanics was performed for each subject-specific pair of computational models for the stable and growing aneurysms, which act as self-controls for confounding clinical risk factors. A higher percentage of area exposed to low shear and high median-peak-systolic arterial wall deformation, each by factors of 1.5 to 2, was observed in growing aneurysms, compared to stable ones. Furthermore, a novel metric – the oscillatory stress index (OStI) – was defined and proposed to indicate locations of oscillating arterial wall stresses. Growing aneurysms demonstrated significant areas with a combination of low wall shear and low OStI, which were hypothesized to be associated with regions of collagen degradation and remodeling. On the other hand, such regions were either absent (or were a small percentage of the total aneurysmal area) in the stable cases. This thesis, therefore, provides a groundwork for future studies, with larger patient cohorts, which will evaluate the role of these biomechanical parameters in cerebral aneurysm growth.