Measurement of fluid-shear rate by Fourier-encoded velocity imaging.

A new technique for estimating the blood fluid shear rate at the vessel wall is presented. The technique uses Fourier-encoded velocity imaging to determine the velocity distribution within a spatial element (voxel) that straddles the blood-vessel wall interface. By appropriate processing, the velocity distribution (1) can determine the location of the wall-blood interface within the voxel and (2) estimate the velocity profile across the spatial extent of the voxel. From this information, accurate estimates of fluid shear rate may be obtained. Simulations are presented to illustrate this technique and to show the effects of various error sources, including differences in proton densities between blood and wall tissues and flow-related signal changes. Experimental evidence obtained for steady flow in straight tubes is also presented in support of the technique. The mean error in the experimental shear rate estimates found using the proposed technique was -15%. This represents a significant improvement over estimates obtained by extrapolation of the velocity profile over multiple voxels (mean error of -73%).