Purpose:To assess the distribution and regional differences of flow and vessel wall parameters such as wall shear stress (WSS) and oscillatory shear index (OSI) in the entire thoracic aorta.
Materials and Methods:Thirty-one healthy volunteers (mean age ϭ 23.7 Ϯ 3.3 years) were examined by flow-sensitive four-dimensional (4D)-MRI at 3T. For eight retrospectively positioned 2D analysis planes distributed along the thoracic aorta, flow parameters and vectorial WSS and OSI were assessed in 12 segments along the vascular circumference.
Conclusion:The normal distribution of vectorial WSS and OSI in the entire thoracic aorta derived from flow-sensitive 4D-MRI data provides a reference constituting an important perquisite for the examination of patients with aortic disease. Marked regional differences in absolute WSS and OSI may help explaining why atherosclerotic lesions predominantly develop and progress at specific locations in the aorta. COMPLEX VASCULAR GEOMETRY AND PULSATILE FLOW in the human arterial system lead to regionally different flow characteristics and thus spatial and temporal changes in shear forces acting on the vessel wall. These forces can be characterized by wall shear stress (WSS) or oscillatory shear index (OSI) that play an important role in flow-mediated atherogenesis and arterial remodeling (1-3). While WSS values reported in the literature typically reflect the time-averaged shear forces acting on the vessel wall, OSI describes the existence and magnitude of WSS changes over the cardiac cycle. Recent reports stressed the importance of WSS and OSI with respect to the formation and stability of atherosclerotic plaques (4). A number of studies have shown that low WSS and high OSI represent sensitive markers for formation of plaques in the aorta, carotid, or coronary arteries (5,6). Particularly, the assessment of both WSS and OSI can help to determine the complexity of the lesions. A recent study with animal models and deliberately altered flow characteristics in the carotid arteries demonstrated the close correlation of low WSS with the development of vulnerable high-risk plaques whereas high OSI induce stable lesions (4). In addition, the effects of selected pathologies on regionally-varying WSS and OSI values have been reported (7,8).Among other methods, MRI is a feasible and extensively validated technique to derive quantitative flow information from arterial vessels (9 -12). Due to its intrinsic sensitivity to flow and the possibility to acquire true time-resolved three-dimensional (3D) data, in vivo analyses of blood-flow and derived vessel wall parameters are promising. However, earlier reports on MRbased analysis of aortic hemodynamics were either based on incomplete vascular coverage and separately acquired 2D slices (13-17), a combination of MR mea-