Through this phase IV study, multicenter prospective exploratory, uncontrolled, the
investigators propose to identify MRI predictive factors of treatment response, using
diffusion MRI sequences, in addition to conventional sequences.
The primary objective is to study the links between changes on MRI diffusion and response to
treatment with Tysabri to 2 years.
The secondary objective is to compare the evolution of diffusion MRI data with the volumetric