It was pointed out that the gradient approximation of the magnetic field failed for nuclear magnetic resonance measurements which involved a very strong non-uniform magnetic field. Thus, the usual formula for the self-diffusion attenuation of spin-echoes had to be replaced by one in which the role of the gradient of one field component was taken over by the gradient of the magnitude of the total magnetic field. This led to a distribution of the spin-echo attenuation in specimens, and to a non-exponential relationship between the spin-echo signal and the parameters of the applied non-uniform magnetic field. An anisotropy of particle migration further increased the non-uniformity of the attenuation distribution.

J.Stepisnik: Zeitschrift für Physikalische Chemie, 1995, 190[I], 51-62