Abstract
In this study, a technique for high-frame-rate ultrasound imaging velocimetry (UIV) is extended first to provide more robust quantitative flow velocity mapping using ensemble correlation of images without coherent compounding, and second to generate spatio-temporal wall shear stress (WSS) distribution. A simulation model, which couples the ultrasound simulator with analytical flow solution, was implemented to evaluate its accuracy. It is shown that the proposed approach can reduce errors in velocity estimation by up to 10-fold in comparison with the coherent correlation approach. Mean errors (ME) of 3.2% and 8.6% were estimated under a steady flow condition, while 3.0% and 10.6% were found under a pulsatile condition for the velocity and wall shear rate (WSR) measurement, respectively. Appropriate filter parameters were selected to constrain the velocity profiles before WSR estimations and the effects of incorrect wall tracking were quantified under a controlled environment. Although accurate wall tracking is found to be critical in WSR measurement (as a 200 µm deviation from the wall may yield up to a 60% error), this can be mitigated by HFR imaging (of up to 10 kHz) with contrast agents, which allow for improved differentiation of the wall-fluid boundaries. In vitro investigations on two carotid bifurcation phantoms, normal and diseased, were conducted, and their relative differences in terms of the flow patterns and WSR distribution were demonstrated. It is shown that high-frame-rate UIV technique can be a non-invasive tool to measure quantitatively the spatio-temporal velocity and WSS distribution.
Highlights
Studies have shown that hemodynamic shear stresses have a strong influence on the initiation and development of various vascular diseases
The accuracy of such expression is dependent on the validity of the following assumptions: (i) The flow is steady and of a parabolic in nature; (ii) the blood behaves as a Newtonian fluid with constant viscosity; (iii) the vessel is axisymmetric; Spatio-temporal flow and wall shear stress mapping ● C
Error quantification The purpose of this study is to evaluate the performance of the developed technique, and two error metrics— the mean error (ME) and the normalized root mean square error (NRMSE)—were considered, as described in eqns (12) and (13)
Summary
Studies have shown that hemodynamic shear stresses have a strong influence on the initiation and development of various vascular diseases. Another technique to estimate the WSS is based on the determination of the wall shear rate (WSR), which is the flow velocity gradient near the vessel wall (Cheng et al 2002; Poelma et al 2012) This produces a more reliable WSR measurement with no assumption of the geometry and flow required; the accuracy of such a technique is dependent on the temporal and spatial resolution of the modality used to obtain the flow velocity profile and track the wall position, and the interpolation or extrapolation algorithm. Because of such constraints, in vivo WSS measurement is limited and relies primarily on computational fluid dynamics (CFD). The accuracy of the simulation can be affected by the underlying assumptions on the geometry, wall properties, fluid properties and most importantly the initial and boundary conditions
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