Velocity detection with single pulsed gradient spin echo (PGSE) NMR is known to be limited by diffusion smearing the phase coherence and obscuring dispersive attenuation. We present an equation for the lowest detectable velocity with phase contrast velocimetry which incorporates the signal-to-noise ratio. We show how to best sample q-space when approaching this limit. We confirm that the apparent diffusion/dispersion coefficient can be used to infer fluid velocity for Peclet number Pe > 1. When measuring flow in biological systems we determined that with realistic experimental protocols the limits are roughly 1 μm/s for velocimetry and 300 μm/s for diffusometry.

Limits to flow detection in pulsed gradient spin echo NMR