The primary aim of this study is to explore the effect of dimethyl fumarate on gray matter (GM) pathology, as measured by changes in diffusion-tensor imaging (DTI) of the thalamus in patients with relapsing multiple sclerosis (MS). The secondary objective of this study is to investigate the effect of dimethyl fumarate on evolution of microstructural changes in normal appearing white matter (NAWM), as measured by DTI.