Normal patterns of thoracic aortic wall shear stress measured using 4D-flow MRI in a large population.

Wall shear stress (WSS) plays a governing role in vascular remodelling and a pathogenic role in vessel wall diseases. Little however, is known of the normal WSS patterns in the aorta as there is currently no practical means to routinely measure WSS, and no normal ranges derived from population data exist. WSS measurements were made on the aorta of 224 subjects with normal anatomy using 4-dimensional flow MRI (4D-flow) with multiple encoding velocities and an optimized post-processing routine. The spatial and temporal variation in WSS and oscillatory shear index was analyzed using a flat map representation of the unfolded aorta. The influence of aortic shape and velocity on WSS was evaluated using regression analysis. WSS in the thoracic aorta is dominated by axial flow. Average peak systolic WSS was 1.79±0.71Pa in the aortic arch and was significantly higher at 2.23±1.04Pa in the descending aorta, with a strong negative correlation with advancing age. The spatial distribution of WSS is highly heterogeneous, with a localized region of elevated WSS along the length of the anterior wall seen across all individuals. Our data demonstrate that accurate 4D-flow derived WSS measurement is feasible, and we further provide a standardized parametric approach for presentation and analysis. We present a normal range for WSS across the lifespan, demonstrating a decrease in WSS with advancing age as well as illustrating the high degree of spatial and temporal variation.