In vitro and in vivo comparison of three MR measurement methods for calculating vascular shear stress in the internal carotid artery

BACKGROUND AND PURPOSE: Vascular abnormalities, such as atherosclerosis and the growth and rupture of cerebral aneurysms, result from a derangement in tissue metabolism and injury that are, in part, regulated by hemodynamic s tress. The purpose of this study was to establish the feasibility and accur acy of determining wall shear rate in the internal carotid artery from phas e-contrast MR data. METHODS: Three algorithms were used to generate shear rate estimates from b oth ungated and cardiac-gated 2D phase-contrast data. These algorithms were linear extrapolation (LE), linear estimation with correction for wall posi tion (LE*), and quadratic extrapolation (QE). In vitro experiments were con ducted by using a phantom under conditions of both nonpulsatile and pulsati le how. The findings from five healthy volunteers were also studied. MR ima ging-derived shear rates were compared with values calculated by solving th e fluid flow equations. RESULTS: Findings of in vitro constant-flow experiments indicated that at o ne or two excitations, QE has the advantage of good accuracy and low varian ce. Results of in vitro pulsatile flow experiments showed that neither LE* nor QE differed significantly from the predicted value of wall shear stress , despite errors of 17% and 22%, respectively. In vivo data showed that QE did not differ significantly from the predicted value, whereas LE and LE* d id. The percentages of errors for QE, LE, and LE* in vivo measurements were 98.5%, 28.5%, and 36.1%, respectively. The average residual of QE was low because the residuals were both above and below baseline whereas, on averag e, LE* tended to be a more biased overestimator of the shear rate in volunt eers. The average and peak wall shear force in five volunteers was approxim ately 8.10 dyne/m(2) and 13.2 dyne/cm(2), respectively. CONCLUSION: Our findings show that LE consistently underestimates the shear rate. Although LE* and QE may be used to estimate shear rate, errors of up to 36% should be expected because of variance above and below the true val ue for individual measurements.