The influence of a nonconstant magnetic-field gradient on PFG NMR diffusion experiments. A Brownian-Dynamics computer simulation study

The influence of a nonconstant magnetic-field gradient on the performance of pulsed-held-gradient NMR diffusion experiments was investigated by performing Brownian-dynamics simulations for two different coil designs using two different active sample volumes for each coil. The active sample volumes were chosen in order to represent different degrees of deviation from a perfectly constant magnetic-field gradient, The results show that one can actually tolerate a rather large deviation from a perfectly constant magnetic-field gradient and still obtain accurate values of the selfdiffusion coefficients. The dependence of Delta (the observation time) and the dependence of D (the self-diffusion coefficient) while keeping D and Delta constant, respectively, were also investigated for three of the active sample volumes. The simulations show that the effects due to a nonconstant field gradient depend only slightly on the values of Delta and D. In conclusion, the Brownian-dynamics computer simulations procedure is a quantitative way of investigating the performance characteristics of a gradient coil of a certain design before it is built. (C) 1997 Academic Press.