▪ Abstract  The relationship between flow in the arteries, particularly the wall shear stresses, and the sites where atherosclerosis develops has motivated much of the research on arterial flow in recent decades. It is now well accepted that it is sites where shear stresses are low, or change rapidly in time or space, that are most vulnerable. These conditions are likely to prevail at places where the vessel is curved; bifurcates; has a junction, a side branch, or other sudden change in flow geometry; and when the flow is unsteady. These flows, often but not always involving flow separation or secondary motions, are also the most difficult ones in fluid mechanics to analyze or compute. In this article we review the modeling studies and experiments on steady and unsteady, two-and three-dimensional flows in arteries, and in arterial geometries most relevant in the context of atherosclerosis. These include studies of normal vessels—to identify, on the basis of the fluid mechanics, lesion foci—and stenotic vessels, to model and measure flow in vessels after the lesions have evolved into plaques sufficiently large to significantly modify the flow. We also discuss recent work that elucidates many of the pathways by which mechanical forces, primarily the wall shear stresses, are transduced to effect changes in the arterial wall at the cellular, subcellular, and genetic level.

BACKGROUND AND PURPOSE:Hemodynamics is often recognized as one of the major factors in aneurysm rupture. Flow impingement, greater pressure, and abnormal wall shear stress are all indications for aneurysm rupture. Characterizing wall shear stress for intracranial aneurysms at similar anatomic locations may help in understanding its role.

Magnetic Resonance Angiography (MRA) has become a routine imaging modality for the clinical evaluation of obstructive vascular disease. However, complex circulatory flow patterns, which redistribute the Magnetic Resonance (MR) signal in a complicated way, may generate flow artifacts and impair image quality. Numerical simulation of MRAs is a useful tool to study the mechanisms of artifactual signal production. The present study proposes a new approach to perform such simulations, applicable to complex anatomically realistic vascular geometries. Both the Navier-Stokes and the Bloch equations are solved on the same mesh to obtain the distribution of modulus and phase of the magnetization. The simulated angiography is subsequently constructed by a simple geometric procedure mapping the physical plane into the MRA image plane. Steady bidimensional numerical simulations of MRAs of an anatomically realistic severely stenotic carotid artery bifurcation are presented, for both time-of-flight and contrast-enhanced imaging modalities. These simulations are validated by qualitative comparison with flow phantom experiments performed under comparable conditions.

A novel computer simulation technique is presented that allows the calculation of images from Magnetic Resonance Angiography (MRA) studies of blood flow in realistic curving and branching two-dimensional vessel geometries. Fluid dynamic calculations provide flow streamlines through curved or branching vessels. MR simulations generate images for specific MR pulse sequence parameters. Simulations of steady flow in carotid bifurcation and carotid siphon geometries as imaged by a standard, flow-compensated, spoiled gradient echo sequence illustrate the major features seen in clinical time of flight MRA studies. The simulations provide insight into a number of artifacts encountered in MRA such as displacement artifacts, signal pile-up, truncation artifacts, and intravoxel phase dispersion.