High b-value q-space diffusion MRS of nerves: structural information and comparison with histological evidence.

High b-value q-space diffusion MRS was used to study the diffusion characteristics of formalin-fixed swine optic and sciatic nerves over a large range of diffusion times (3.7-99.3 ms). The very short diffusion time range was studied with a 1 ms resolution. The displacement distribution profiles obtained were fitted to a bi-Gaussian function, and structural parameters were extracted from the q-space diffusion MRS data. This structural information was correlated with axon sizes obtained by histological examination. It was found that high b-value q-space diffusion MRS can easily distinguish between the two nerve types. The root mean square displacements (rmsds) of both the slow and fast diffusing components of the optic nerves were found to be smaller than those of the sciatic nerves. When the rmsd was plotted against the square root of the diffusion time (t(d)(1/2)), it was found that all four components showed an increase in rmsd; this increase was significantly smaller than expected from the Einstein equation. However, the most restricted component is the slow diffusing component of the optic nerve. This is also the only diffusing component that shows a large change in the slope (i.e. a 'breaking point') of the plot of rmsd as a function of t(d)(1/2). This rmsd is very similar to the mean axon size of these optic nerves determined histologically. Such a change in slope was less apparent for the slow diffusing component of sciatic nerves, which showed a wider distribution of axon size in histological images. The fast diffusing components of both nerve types showed only a small gradual change in the slope of rmsd plotted against t(d)(1/2). These findings are discussed in the context of component assignment, origin of restriction, and relationships between the structural information extracted from q-space diffusion MRS and histological examination.