Effects of susceptibility variations on NMR measurements of diffusion

A pulsed-gradient spin-echo method has been used to measure the water self-diffusion coefficient in various media containing susceptibility variations arising from the presence of superparamagnetic iron oxide particles. These include aqueous suspensions, polyacrylamide gels, and rat livers. The results for each medium indicate that, contrary to intuition, the apparent diffusion coefficient decreases as the iron particle concentration increases. A simple theoretical model has been developed to interpret the data. This model relates the changes in apparent diffusion to the variance of the internal gradient distribution generated by the iron particles. A computer simulation of nuclei undergoing a random walk in an inhomogeneous medium was performed and the results provide qualitative confirmation of the experimental observations. The relevance of these phenomena to measurements of molecular diffusion in heterogeneous systems and their use to estimate internal gradient strengths are discussed.