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 t echnique. Computer simulations, using a model that accounts for diffus ion within samples, indicate that echo ratio measurements can differ f rom Carr-Purcell theory predictions by as little as a few tenths of a percent and by as much as 30% for 0.073 m diameter sample under a 0.03 5 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 T-2 decay is the dominant mechanism acting on echo ratio. This difference can be attributed to imperfect flipping an gles of magnetization across the samples, associated with the finite d uration of the 180 degrees rf refocusing pulses.