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 water signal decay in diffusion experiments. q-Space analysis of the diffusion data was used to obtain structural information about the investigated neuronal tissues. This analysis provided displacement distribution profiles of the water in the sample. Two diffusing components were identified from these profiles, thus enabling us to obtain the following information about the slow decaying component: 1) displacement of this component is restricted to a diffusing distance of approximately 2 microm; 2) it has a longer T2 than the rapidly diffusing component; and 3) the population fraction of this component 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 axis of the nerve it was found to be 14 +/- 2%. In the randomly oriented brain tissue, the population of this component was only 7 +/- 3%. These observations led to the conclusion that the slow-decaying component originates mainly from restricted water diffusion in the neuronal fibers. In view of these findings, in vitro and in situ diffusion-weighted images with high b values (with long delta) were acquired to obtain highly detailed images of white matter fiber tracts in the central nervous system. These images provide detailed information on white matter fiber tract location and allow spinal cord maturation to be followed with high accuracy.