Transcranial Electrical Stimulation in Multiple Sclerosis

First the investigators will examine if the blood flow in the brain is different in patients with Multiple Sclerosis compared to healthy volunteers. HOW? - The blood flow of the subjects is imaged by use of a special MRI-technique. Secondly, the investigators will examine if the blood flow can determine disease progression. HOW? - Multiple Sclerosis patients are asked for a follow-up visit after two years. The main goal is to understand the effect of transcranial Direct Current Stimulation (tDCS) on the cerebral blood flow. * Does tDCS result in a different blood flow, in which brain regions? * Is there a difference between healthy volunteers and Multiple Sclerosis patients? HOW? - Participants get two session of tDCS. Before, during and after stimulation MRI scans are performed.