Quantitative phase-velocity MR imaging of in-plane laminar flow: effect of fluid velocity, vessel diameter, and slice thickness.

Quantitative MR phase imaging is frequently used to measure spin velocities. A potential difficulty may arise, however, when in-plane phase images are acquired of a vessel carrying laminar flow, for which the fluid velocity profile is parabolic. In that case, depending on the flow velocity (v), the vessel diameter (D), and the chosen MR slice thickness (ST), a spin velocity gradient will be present to some extent within each intraluminal voxel. The resulting intravoxel phase dispersion may be expected to affect the net pixel phase value, and hence compromise the assumed linear correlation between phase shift and velocity. In this study, the effects of alterations of v, D, and ST on the apparent image phase are investigated for the case of laminar flow directed parallel to the sequence read gradient. A theoretical model is developed and the conclusions experimentally tested using a flow phantom. The data demonstrate that when quantitating inplane phase-flow images, significant velocity underestimations may occur when the net flow-induced phase shifts are small and the MR slice thickness is an appreciable fraction of the vessel diameter.