Studies of restricted diffusion in heterogeneous media containing variations in susceptibility.

Measurements of water self-diffusion in heterogeneous media by pulsed gradient spin-echo methods depend on the precise choice of experimental parameters. This variation may reflect the presence of restricted or anisotropic diffusion, or the presence of intrinsic magnetic field gradients. In experiments at 2.0 T, restricted diffusion effects have been demonstrated in freshly excised tissues with an effective barrier radius of order 10 microns. In samples with intrinsic susceptibility variations such as tissues or gels containing dispersions of iron oxide particles, the reduction of apparent diffusion coefficient with diffusion interval can be used to estimate the degree of heterogeneity of the magnetic field in the sample. The implications of these effects for diffusion-weighted MRI as well as their use to derive novel tissue parameters are discussed.