Study of Neuroinflammation in Multiple Sclerosis by PET-MRI Imaging Using the Radiotracer ([18F]-DPA-714) : a Multicentre Cohort Study (INFLANET)

Inclusion Criteria: RRMS patients: * Age between 18 and 55 years old * RRMS according to the 2017 Mc Donald criteria * At least 9 supra-tentorial white matter lesions on T2/FLAIR MRI * Last treatment with methylprednisolone should have been performed at least 1 month before PET examinations * No current disease modifying therapy * Indication for a highly active disease modifying treatment: Natalizumab, anti CD20 antibody, Alemtuzumab, sphingosine-1 phosphate modulator, or cladribine. This will consist either as patients with an active form of relapsing MS or patients who have experienced two relapses during the previous year * Affiliation to a social security scheme or beneficiary of such a scheme (except "Aide Médicale d'Etat") Healthy Volunteers: * Age between 18 and 55 years old * Without any evolutive pathology * Able to understand the study objectives and procedures * Affiliation to a social security scheme or beneficiary of such a scheme (except "Aide Médicale d'Etat") Exclusion Criteria: For all participants: * Any reasons, which does not allow to perform MRI, including claustrophobia, the implant of a pace-maker or the presence of an intra-ocular foreign body, (a contra-indication questionnaire will be filled in beforehand) * Realization of any test using radioactivity within the last 12 months * Low Affinity Binding profile (analyse of TSPO polymorphism done at screening visit) * Pregnancy, breast-feeding, lack of efficient contraception for women of childbearing potential * Current symptoms of severe or uncontrolled renal, hepatic, hematological, gastrointestinal, pulmonary or cardiac disease, or any other chronic neurological diseases * Unwillingness to be informed in case of unexpected MRI abnormality (with a significant medical anomaly) * Patient under legal protection * Participation in another interventional study or being in the exclusion period at the end of a previous study RRMS patients: * Hypersensitivity to gadoteric acid * Meglumine or any drug containing gadolinium * Severe renal insufficiency (creatinine clearance \< 60mL/min and GFR \<30ml / min / 1.73m2).