Analysis of bulk flow as a future direction in image-based modeling of arterial hemodynamics: assumptions and applications

Atherosclerosis is the commonest cause of death in the Western world. The frequent occurrence of intimal thickening/growth and atherosclerotic plaques in well-recognized arterial districts, together with the focal distribution of these phenomena in regions of curvature, bifurcation, and branching of the vessels, have suggested that fluid dynamics and vessel geometry play a key role in modulating and localizing the pathogenesis of the disease. Since local flow patterns are known to influence the interaction between flowing blood and vessel's wall, differences in the flow patterns may indicate differences in atherosclerotic potential, and hence a thorough understanding of hemodynamics in human vessels becomes of great interest. Consequently, new diagnostic indicators are being sought to improve risk assessment of vessel wall pathologies. The objective of this work is to complement, integrate and extend with a quantitative characterization of the bulk flow the description currently adopted to classify altered hemodynamics, which is based on wall shear stress (WSS). The definition of new bulk flow metrics may lead to the development of an instrument to relate the bulk flow to vascular pathophysiological events, involving not only fluid-related forces but also transport phenomena within blood. In this work specific descriptors of the bulk flow are defined to give measure of the highly intricate hemodynamics within numeric models of healthy carotid artery bifurcations. In detail, those descriptors quantitatively describe i) the helical content in the streaming blood, and ii) vorticity dynamics. The sensitivity to assumptions required in computational modeling of blood flow dynamics within carotid bifurcations are extended also to the descriptors of bulk flow structures, and not only to the WSS-based parameters. Afterwards, helicity-based bulk flow metrics are calculated in 50 normal carotid bifurcation models and a direct relationship between those descriptors and exposure to disturbed shear is investigated. The second objective of the thesis is to confidently model thoracic aorta hemodynamics, exploring the effect of assumptions about boundary conditions on hemodynamic wall parameters and on bulk flow. The clinical motivation of using bulk flow descriptors is to predict clinical consequences of complex blood flows, since it has been observed that stable rotating blood flows may play a beneficial role in vascular hemodynamics, inducing stability and reducing turbulence in the arterial tree, and helical flow could be a signature of a local fluid dynamics which is altered, with respect to the physiological one. In conclusion, the helicity-based hemodynamic descriptors defined in this work are demonstrated to be powerful indicators of atherogenesis risk related to local hemodynamics. Therefore, since helicity-based descriptors can be evaluated in vivo from velocity fields measured by medical imaging techniques (e.g., phase contrast magnetic resonance imaging), we foresee a practical way to large-scale in vivo studies of local risk factors in atherosclerosi