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

Characterising fluid flow within porous media is of great importance for a wide range of research fields ranging from chemical engineering to geology. Accurate velocimetry can be crucial in understanding transport processes and developing models. The ability to probe flow properties in opaque systems makes MRI velocimetry based on the use of pulsed magnetic field gradients (PFG) a precious tool for the characterisation of flow in porous media. There are two main methods of PFG velocimetry, propagator velocimetry and phase-shift velocimetry. Propagator velocimetry requires several gradient encoding steps to resolve the probability distribution of displacements for each voxel. Phase-shift velocimetry is faster, requiring only two gradient encoding steps to measure the average voxel velocity.