The effect of displacement distribution asymmetry on the accuracy of phase-shift velocimetry in porous media

Phase-shift velocimetry provided by pulsed magnetic field gradient experiments is the fastest magnetic resonance technique for measuring velocity. However, critical issues have been reported when studying flow through porous media. These lead to inaccurate velocimetry results, with no clear explanation emerging. In this work we investigate the effect of asymmetries in the distribution of intra-voxel displacements on the accuracy of phase-shift velocimetry. This is particularly relevant to flow through porous media, where the presence of stagnant pores or differential flow is known to result in asymmetric displacement distributions. Here, tailored intra-voxel distributions are achieved using a simple phantom combined with a single large voxel. Combining experimental results with simulations, we demonstrate that the presence of asymmetries in the intra-voxel displacement distributions, common in studies of porous materials, can lead to important velocimetry errors.