Department of Neurology, Universitätsmedizin Mannheim UMM, University of Heidelberg, Germany. Electronic address: achim.gass@medma.uni-heidelberg.de. Neuroimaging Research Unit, Institute of Experimental Neurology, Division of Neuroscience and Department of Neurology, San Raffaele Scientific Institute, Vita-Salute San Raffaele University, Milan, Italy. Department of Brain Repair and Rehabilitation, University College London, Institute of Neurology National Institute for Health Research, University College London Hospitals, Biomedical Research Centre, London, UK. NMR Research Unit, Queen Square Multiple Sclerosis Centre, University College London, Institute of Neurology National Institute for Health Research, University College London Hospitals, Biomedical Research Centre, London, UK. University of Bristol, Clinical Research and Imaging Centre, Bristol, UK. Department of Neurology, University Hospital Basel, Basel, Switzerland. Department of Neurology, Universitätsmedizin Mannheim UMM, University of Heidelberg, Germany. Department of Radiology and Nuclear Medicine, VU University Medical Center, Amsterdam, Netherlands.