Frederik Barkhof1, Olga Ciccarelli2. 1. Department of Radiology and Nuclear Medicine, VU University Medical Centre, PO Box 7057, 1007 MB Amsterdam, Netherlands. Electronic address: f.barkhof@vumc.nl. 2. Queen Square Centre, University College London Institute of Neurology, University College London, London, UK; National Institute for Health Research Biomedical Research Centre, University College London Hospitals, London, UK.