It was pointed out that the gradient approximation to the magnetic field failed for nuclear magnetic resonance self-diffusion measurements which were performed in a very strong non-uniform magnetic field. The standard formula for self-diffusion attenuation of the spin-echoes then had to be replaced by a new 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 the particle migration also increased the non-uniformity of the attenuation distribution.

J.Stepišnik: Zeitschrift für Physikalische Chemie, 1995, 190[1], 51-62