The Paradoxical Flow Hypothesis of the Carotid Artery: Supporting Evidence from Phase-contrast Magnetic Resonance Imaging

Abstract

Narrowing of the poststenotic internal carotid artery (ICA) has been found to be associated with reduced risk of ipsilateral stroke. A paradoxical mechanism has been hypothesized to explain this finding: narrowing of the distal-normal (reference) ICA is associated with low blood flow rates (Q) in the stenotic ICA, and lower Q causes lower risk of ipsilateral stroke, perhaps by an associated reduction in mechanical stress on the atherosclerotic plaque. The purpose of this study was to confirm that the reference ICA diameter (RICAD) is indeed predictive of Q, a finding that would indirectly support the hypothesis of a relationship between lower Q and lower risk of ipsilateral stroke. Magnetic resonance imaging from 38 patients was included in the study. The study included 17 stenotic carotid arteries and 59 normal carotid arteries. All patients underwent contrast-enhanced magnetic resonance angiography from which measurements were obtained of the RICAD and the internal-common carotid diameter ratio. Patients underwent cardiac-gated, velocity-encoded phase-contrast magnetic resonance imaging for measurement of Q. Mean flow rates differed between stenotic (4.3 +/- 1.7 mL/s) and normal (5.4 +/- 1.7 mL/s) arteries (P = .02). RICAD was found to be a predictor of Q for stenotic arteries (P = .009) and for all arteries (P = .025) but not for the group of normal arteries (P = .162). Right-left differences in RICAD were highly predictive of right-left differences in Q in the subgroup of individuals with normal arteries (P < .001) and in the group of all participants (P < .001). Internal-common carotid diameter ratio was not found to be a statistically significant predictor of Q in the subgroup of stenotic arteries (P = .156). This study demonstrated that, as hypothesized, RICAD is correlated with Q.