Limits to flow detection in phase contrast MRI

•Slow- and fast-flow regimes are defined.•Optimal q-space sampling is presented for the slow-flow regime.•Minimum detectable velocity is derived.•Theories are experimentally validated.•Phase contrast MRI may be sensitive to glymphatic clearance. [Display omitted] Pulsed gradient spin echo (PGSE) complex signal behavior becomes dominated by attenuation rather than oscillation when displacements due to flow are similar or less than diffusive displacements. In this “slow-flow” regime, the optimal displacement encoding parameter q for phase contrast velocimetry depends on the diffusive length scale qslow=1/lD=1/2DΔ rather than the velocity encoding parameter venc=π/(qΔ). The minimum detectable mean velocity using the difference between the phase at +qslow and −qslow is 〈vmin〉=1/SNRD/Δ. These theories are then validated and applied to MRI by performing PGSE echo planar imaging experiments on water flowing through a column with a bulk region and a beadpack region at controlled flow rates. Velocities as slow as 6 μm/s are detected with velocimetry. Theories, MRI experimental protocols, and validation on a controlled phantom help to bridge the gap between porous media NMR and pre-clinical phase contrast and diffusion MRI.