Pp. Mitra et Bi. Halperin, EFFECTS OF FINITE GRADIENT-PULSE WIDTHS IN PULSED-FIELD-GRADIENT DIFFUSION MEASUREMENTS, Journal of magnetic resonance. Series A, 113(1), 1995, pp. 94-101
The effects of finite gradient-pulse widths on NMR diffusion measureme
nts for fluids in restricting geometries are studied, It is shown that
the echo amplitude is the spatial Fourier transform of a ''center-of-
mass'' propagator, which reduces to the usual diffusion propagator in
the limit of zero pulse widths, A finite gradient-pulse width delta ef
fectively changes the pore shape, making isolated pores appear smaller
than their actual size. The diffraction analogy still holds for long
diffusion times, with the fluid density rho(r) being replaced by p(cm)
(r, delta). This quantity, the ''center-of-mass distribution function,
'' is the spatial probability distribution of the center of mass of Br
ownian trajectories of duration delta in the pore space, For a periodi
c pore space, ''Bragg'' peaks still appear in the amplitude at the rec
iprocal lattice vectors. The heights of these peaks are enhanced for s
mall delta but reduced for large delta, A number of results valid for
small delta and piecewise smooth pore surfaces are presented. (C) 1995
Academic Press, Inc.