Structural information in neuronal tissue as revealed by q-space diffusion NMR spectroscopy of metabolites in bovine optic nerve

¹H NMR diffusion experiments performed on the signal of the metabolites in bovine optic nerve showed that the signal decay due to diffusion is bi- exponential with a slow and a fast diffusing component. Diffusion was measured as a function of the diffusion time, and the data were analyzed as a function of b and q values. Bi-exponential fit was used to analyze the data, and the results were compared with the displacement distribution profiles obtained from the q-space analysis of the data. This q-space analysis showed that the fast diffusing component has a broad displacement distribution and appears not to be restricted. On the other hand, the slow diffusing component appears to be highly restricted to milieu in the order of 1-2 μm. The orientation of the sample with respect to the axis for which diffusion was measured affected mainly the relative sizes of the populations of each component, but had only a small effect on the extracted apparent diffusion coefficients. These results from both the b and the q value analyses suggest that the slow diffusing component is related to restricted diffusion of these metabolites in the axonal fibers, while the fast diffusing component represents diffusion of metabolites in cells and along the long axis of the nerve fibers. It is concluded that q-space analysis of metabolite diffusion enables extraction of structural information about the sample, and that the diffusion of the metabolites in optic nerve is dictated mainly by the cellular medium and microstructure of the tissue.