Magnetic field inhomogeneity effects on spin echo NMR diffusion measurements

In this work, we evaluate the impact of field inhomogeneity on nuclear magnetic resonance (NMR) measurement of diffusion via the spin echo technique. Computer simulations, using a model that accounts for diffusion within samples, indicate that echo ratio measurements can differ from Carr-Purcell theory predictions by as little as a few tenths of a percent and by as much as 30% for a 0.073 m diameter sample under a 0.035 T/m gradient. Experiments with water as the test sample corroborate that measured echo ratios deviate from Carr-Purcell theory with larger sample sizes and larger applied magnetic field gradients, regardless of whether diffusion or T₂ decay is the dominant mechanism acting on echo ratio. This difference can be attributed to imperfect flipping angles of magnetization across the samples, associated with the finite duration of the 180° rf refocusing pulses