Disruption of neurite morphology parallels MS progression

Objectives: To apply advanced diffusion MRI methods to the study of normal appearing brain tissue in MS and examine their correlation with measures of clinical disability. Methods: A multi-compartment model of diffusion MRI called neurite orientation dispersion and density imaging (NODDI) was used to study 20 patients with relapsing-remitting (RR-) and 15 with secondary progressive (SP)-MS, and 20 healthy controls. Maps of NODDI were analyzed voxel-wise to assess the presence of abnormalities within the normal appearing brain tissue, and the association with disease severity. Standard diffusion tensor imaging (DTI) parameters were also computed for comparing the two techniques. Results: MS patients showed reduced neurite density and increased orientation dispersion compared to controls in several brain areas (P<0.05), with SPMS patients having more widespread abnormalities. DTI indices were also sensitive to some changes. In addition, SPMS patients showed reduced orientation dispersion in the thalamus and caudate nucleus. These abnormalities were associated with scores of disease severity (P<0.05). The association with the MS functional composite score was higher in SPMS compared to RRMS patients. Conclusions: NODDI and DTI findings are largely overlapping. Nevertheless, NODDI helps to interpret previous findings of increased anisotropy in the thalamus of MS patients, and are consistent with the degeneration of selective axon populations.