SPIN-ECHO ATTENUATION BY DIFFUSION IN NONUNIFORM FIELD GRADIENTS

The diffusion-dependent decay of spin-echo signals from samples distributed over non-uniform field gradients is analyzed. This decay deviates from the linear semilogarithmic dependencies on t3 or t obtained, respectively, in Hahn spin-echo or Carr-Purcell echo trains from spins in constant field gradients. It is demonstrated here that when the average diffusion distance is small compared to the spatial variation of the gradient, the spin-echo decay can be modeled by a simple superposition of signals from a large number of microdomains, each characterized by a different gradient. This is illustrated by two examples, a water sample in a deliberately created quadratic variation of the external field, and a saturated water/ sand mixture with large internal field gradients. © 1990.