In multiple sclerosis (MS), significant pathology correlating to disease progression, to expanded disability status scale (EDSS) and to cognitive decline, takes place outside the plaque areas, i.e. in areas of normal appearing white matter and gray matter. Neuropathological studies suggest that mechanisms involved in this widespread pathology include activation of microglial cells, oxidative stress and deficiency in mitochondrial functions. Activated microglia can be detected in vivo with a translocator protein (TSPO), expressed in activated, but not resting microglia) binding radioligands and positron emission tomography (PET). 11Carbon-PK11195 radioligand is one such radioligand. Importantly, the possible effect of MS therapies on microglial activity can be evaluated in patients in vivo with PET-imaging performed before and after the treatment.