Abstract
Abnormal blood flow and wall shear stress (WSS) can cause and be caused by cardiovascular disease. To date, however, no standard method has been established for mapping WSS in vivo. Here we demonstrate wide-field assessment of WSS in the rabbit abdominal aorta using contrast-enhanced ultrasound image velocimetry (UIV). Flow and WSS measurements were made independent of beam angle, curvature or branching. Measurements were validated in an in silico model of the rabbit thoracic aorta with moving walls and pulsatile flow. Mean errors over a cardiac cycle for velocity and WSS were 0.34 and 1.69%, respectively. In vivo time average WSS in a straight segment of the suprarenal aorta correlated highly with simulations (PC = 0.99) with a mean deviation of 0.29 Pa or 5.16%. To assess fundamental plausibility of the measurement, UIV WSS was compared to an analytic approximation derived from the Poiseuille equation; the discrepancy was 17%. Mapping of WSS was also demonstrated in regions of arterial branching. High time average WSS (TAWSSxz = 3.4 Pa) and oscillatory flow (OSIxz = 0.3) were observed near the origin of conduit arteries. In conclusion, we have demonstrated that contrast-enhanced UIV is capable of measuring spatiotemporal variation in flow velocity, arterial wall location and hence WSS in vivo with high accuracy over a large field of view.
Highlights
Hemodynamic wall shear stress (WSS) is the frictional force per unit area exerted by the flow of blood on the inner surface of blood vessels
WSS is known to influence the normal physiology of the endothelial cells (EC) lining the wall and is thought to be critical in the development of atherosclerosis in arteries, where spatial variation in WSS may explain the patchy distribution of disease.[15,20,21,24,32]
Two-dimensional shear rate measurements have been validated in vitro against peak velocities measured by Doppler, ultrasound image velocimetry (UIV) deviating by a maximum of 6.6%
Summary
Hemodynamic wall shear stress (WSS) is the frictional force per unit area exerted by the flow of blood on the inner surface of blood vessels. It is the product of the near-wall velocity gradient and the viscosity of the blood. Blood flow in large arteries is pulsatile, arteries branch, curve, twist, taper and translate, and their walls are viscoelastic For vessels of this size, blood can be assumed a Newtonian fluid (i.e., its viscosity is independent of shear rate) but the WSS cannot be obtained analytically; it must be obtained numerically, which involves making assumptions about boundary conditions, or by measurement. The latter requires broadview, angle-independent velocity measurement and wall tracking with high spatiotemporal resolution
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