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 MRI data.

Inclusion Criteria:

• Major Patient
• Patient with relapsing remitting multiple sclerosis and eligible for treatment with Tysabri
• Patient affiliated to a social security scheme
• Patient who signed informed consent
• Patient who have been informed of the results of the prior medical examination

Exclusion Criteria:

• Patient not currently eligible for treatment with Tysabri
• Contraindication to MRI scanning
• Unability to provide informed consent (subject in an emergency situation, difficulties in understanding , ...)
• Patient under judicial protection
• Patient under guardianship or curatorship
• Pregnancy (women of childbearing age in the absence of effective contraception)
• Breastfeeding