Multiple sclerosis pathology in the normal and abnormal appearing white matter of the corpus callosum by diffusion tensor imaging

Lesions in the corpus callosum in multiple sclerosis (MS) include those that are hyperintense on T2-weighted images, which can be either focal (isolated) or connected, but there is evidence that the corpus callosum, similar to other white matter regions, contains normal appearing white matter (NAWM) which is abnormal based on quantitative MR methodologies. In this pilot study, diffusion tensor based measures were determined in corpus callosum from 10 patients with MS and 12 age and gender matched controls. T2-hyperintense lesions were carefully segmented out from normal appearing corpus callosum to minimize contamination of the NAWM fraction with these lesions. The orientationally averaged diffusion coefficient was increased and the fractional anisotropy reduced in the NAWM fraction of the MS patients. These results confirm prior studies which suggest that pathology in the NAWM occurs independent of focal MS lesions, and are not likely the result of sample contamination through or across slices. This injury to the NAWM may be the result of focal, microscopic T2-invisible lesions and/or secondary degeneration related to distant lesions whose related fibres cross the corpus callosum.