A METHOD FOR EVALUATING ANISOTROPIC AND RESTRICTED DIFFUSION BY SIMULTANEOUS USE OF SPIN AND STIMULATED ECHOES

A new NMR method is described for evaluating anisotropic and restricted molecular diffusion in a single experiment by simultaneously exploiting both spin-echo (SE) and stimulated-echo (STE) NMR signals. The method employs a sequence of four RF pulses, θ-τ1-90°-τ2-180°-τ2-(SE)-τ3-90°-τ1-(STE), combined with two pairs of diffusion-sensitive gradient pulses, one pair of which is positioned in the τ1 intervals and the other of which is introduced into the 2 intervals. We show that the diffusion attenuation of the SE is affected only by the diffusion-sensitive gradient pulses positioned in the τ1 intervals, whereas the diffusion attenuation of the STE is influenced only by the diffusion-sensitive gradient pulses placed in the τ2 intervals. This method should be useful for anisotropic and restricted diffusion measurements of molecules which diffuse relatively freely in one direction while being impeded by barriers in another direction in systems in which the T1 of the diffusing substance is greater than T2. The SE signal is used to measure relatively unrestricted diffusion in the first direction, whereas the STE signal is used to measure more restricted diffusion in the second direction, where the barrier separations may be probed by larger spacings between the diffusion-sensitive gradient pulses. © 1992.