Assignment of the water slow-diffusing component in the central nervous system using q-space diffusion MRS: Implications for fiber tract imaging

Diffusion-weighted NMR spectroscopy (MRS) was performed on isolated bovine optic nerve and rat brain (in vitro) to characterize the multiexponential w ater signal decay in diffusion experiments. q-Space analysis of the diffusi on data was used to obtain structural information about the investigated ne uronal tissues. This analysis provided displacement distribution profiles o f the water in the sample, Two diffusing components were identified from th ese profiles, thus enabling us to obtain the following information about th e slow decaying component: 1) displacement of this component is restricted to a diffusing distance of approximately 2 mu m; 2) it has a longer T-2 tha n the rapidly diffusing component; and 3) the population fraction of this c omponent depends on the orientation of the nerve fiber. When the diffusion was measured perpendicular to the long axis of the bovine optic nerve, the weighting of this population was 41 +/- 2%, whereas parallel to the long ax is of the nerve it was found to be 14 +/- 2%. In the randomly oriented brai n tissue, the population of this component was only 7 +/- 3%. These observa tions led to the conclusion that the slow-decaying component originates mai nly from restricted water diffusion in the neuronal fibers. In view of thes e findings, in vitro and in situ diffusion-weighted images with high b valu es (with long Delta) were acquired to obtain highly detailed images of whit e matter fiber tracts in,the central nervous system. These images provide d etailed information on white matter fiber tract location and allow spinal c ord maturation to be followed with high accuracy. (C) 2000 Wiley-Liss, Inc.