Weak-diffusion theory of NMR signal in magnetically heterogeneous media

A general theory is developed for the effect of molecular diffusion on the NMR signal obtained from magnetically heterogeneous media in the limit of weak diffusion. The theory is based on a rigorous expansion in the diffusion constant D, with the correction to first order in D being given explicitly for unrestricted, isotropic diffusion. The expansion allows for an arbitrary sequence of field gradients and 180 degrees spin-flip pulses, making it applicable to a wide variety of NMR protocols. The theory may be useful for estimating the magnitude of diffusion effects and in determining some of a medium's microscopic magnetic properties. (C) 1997 Academic Press.