In this numerical study areas of the carotid bifurcation and of a distal
stenosis in the internal carotid artery are closely observed to evaluate the
patient's current risks of ischemic stroke. An indicator for the vessel wall
defects is the stress the blood is exerting on the surrounding vessel tissue,
expressed standardly by the amplitude of the wall shear stress vector (WSS) and
its oscillatory shear index. In contrast, our orientation-based shear
evaluation detects negative shear stresses corresponding with reversal flow
appearing in low shear areas. In our investigations of longitudinal component
of the wall shear vector, tangential vectors aligned longitudinally with the
vessel are necessary. However, as a result of stenosed regions and imaging
segmentation techniques from patients' CTA scans, the geometry model's mesh is
non-smooth on its surface areas and the automatically generated tangential
vector field is discontinuous and multi-directional, making an interpretation
of the orientation-based risk indicators unreliable. We improve the evaluation
of longitudinal shear stress by applying the projection of the vessel's
center-line to the surface to construct smooth tangetial field aligned
longitudinaly with the vessel. We validate our approach for the longitudinal
WSS component and the corresponding oscillatory index by comparing them to
results obtained using automatically generated tangents in both rigid and
elastic vessel modeling as well as to amplitude based indicators. The major
benefit of our WSS evaluation based on its longitudinal component for the
cardiovascular risk assessment is the detection of negative WSS indicating
persitent reversal flow. This is impossible in the case of the amplitude-based
WSS