A simple matrix formalism for spin echo analysis of restricted diffusion under generalized gradient waveforms.

A simple mathematical formalism is presented which allows closed form expressions for the echo attenuation, E(q), in spin echo diffusion experiments, for practically all gradient waveforms and for the case of restricted diffusion in enclosing pores, with or without wall relaxation. The method, which derives from the multiple propagator approach of A. Caprihan et al. (1996, J. Magn. Reson. A 118, 94), depends on the representation of the gradient waveform by a succession of sharp gradient impulses. It leads to E(q) being expressed as a product of matrix operators corresponding quite naturally to the successive sandwich of phase evolution and Brownian migration events. Simple expressions are given for the case of the finite width gradient pulse PGSE experiment, the CPMG pulse train used in frequency-domain modulated gradient spin echo NMR, and the case of a sinusoidal waveform. The finite width gradient pulse PGSE and CPMG pulse trains are evaluated for the case of restricted diffusion between parallel reflecting planes. The former results agree precisely with published computer simulations while the latter calculation provides useful insight regarding the spectral density approach to impeded Brownian motion. Copyright 1997 Academic Press. Copyright 1997Academic Press