EFFECTS OF INTRAVOXEL VELOCITY DISTRIBUTIONS ON THE ACCURACY OF THE PHASE-MAPPING METHOD IN PHASE-CONTRAST MR-ANGIOGRAPHY

The phase-mapping method of phase-contrast magnetic resonance angiogra phy is shown to be based on an implicit assumption that the intravoxel velocity distribution is symmetric about its mean velocity. The effec t of asymmetric distributions on the accuracy of quantitative average velocity measurements is determined analytically and verified experime ntally. An explicit formulation is developed for the estimated average velocity in a voxel as a function of the true average velocity and th e asymmetry of the distribution about the true average velocity. Worst -case distributions are determined for undirectional and bidirectional flow, and the special case of laminar flow is also investigated. Comp uter simulations and phantom imaging experiments demonstrate the accur acy of the analysis. For voxels with undirectional flow, the phase-map ping method produces accurate estimates of average velocity, while res ults for bidirectional flow indicate possible large errors unless the aliasing velocity is increased, which decreases the signal-to-noise ra tio in the resultant velocity map image.