Global and local relationship between hemodynamics and intraluminal thrombus in abdominal aortic aneurysms in longitudinal data

Abdominal aortic aneurysms (AAAs) represent the 10° leading cause of death and accounted for nearly 16,000 deaths overall from the year 2000, according to the National Vital Statistics Report on Deaths. AAAs are characterized by a focal dilation of the aorta often with the presence of an intraluminal thrombus (ILT) that is unsafe for rupture, it may occlude blood flow through the aneurysm, and bits of it may break off and generate emboli that could interrupt the flow of blood on smaller vessels. The hemodynamics of the blood flow has an impact on the vessel and on the enlargement of the ILT, so recently some indicators have been proposed based on the Wall Shear Stress (WSS), a vectorial quantity that represents the tangential stress that acts on the vessel wall. In this work, WSS-based parameters were applied on 3D models of the aneurysmatic abdominal aorta of six patients who underwent CT scans at different time points. This thesis considers (1) the TAWSS (Time Averaged WSS), that quantifies the average magnitude of WSS over a cardiac cycle, (2) the OSI (Oscillatory Shear Index), that reflects the changes in directions of the WSS vector, (3) the Trans-WSS (Transverse WSS), that is the average over the cardiac cycle of WSS components perpendicular to the temporal mean WSS vector. The LNH (Local Normalized Helicity) was considered to show helical patterns in the bulk flow. All these indicators were put in relation to geometrical factors quantifying the progress of the disease, such as the thickness and growth of the ILT, the curvature of the vessel, its torsion and tortuosity, the cross-section areas, diameters, and volumes of both lumen and ILT. The final goal was to find relationships among geometrical factors of the lumen, hemodynamics, and features of the ILT, to find the existence of cause-effect relationships between some of these features. Correlations on a global scale were explored, through mean, maximum, and minimum values of the descriptors in the whole geometry. Then, the geometries were divided into patches: rectangular spaces parametrized longitudinally through the curvilinear abscissa computed over the centerlines and circumferentially, by the angular position of each point on the surface with respect to the centerlines, to obtain more local values of the hemodynamic and geometric descriptors. Finally, the sections delimiting the patches were split into angular sectors, to obtain more local data. All the correlations showed p-values = 0-0,5. The greatest R2 coefficients, used to quantify the quality of correlations, were referred to the TAWSS and aortic size relationships (50%-70% on two patients, 30%-40% on one patient). OSI was correlated with the lumen size with R2 = 25%-45% on two patients. Max diameters were related to the max curvature and tortuosity with R2 = 20%-30%. Pearson’s rho coefficients showed both positive and negative correlations with low values. It was observed that the lumen max cross-section areas and diameters increase as the max curvatures and tortuosity increase. Lumen max cross-section areas and diameters increase with the corresponding ILT diameters, areas, and volumes. The lumen geometry also influences the hemodynamics: the aortic size, curvature, and tortuosity increase as OSI and Trans-WSS increase, while TAWSS decreases. Hemodynamics affects the growth of ILT as its size increases with the increase of OSI and the decrease of TAWSS. Trans-WSS seems to play a less significant role.