1Department of Neurobiology, A.I. Virtanen Institute for Molecular Sciences, University of Eastern Finland, Kuopio, Finland, 2Department of Applied Physics, University of Eastern Finland, Kuopio, Finland, 3Finland Diagnostic Imaging Center, Kuopio University Hospital, Kuopio, Finland, 4Department of Neurology, Clinical Neurosciences Research Laboratory, Hospital Clínico Universitario, Health Research Institute of Santiago de Compostela (IDIS), University of Santiago de Compostela, Santiago de Compostela, Spain, 5Department of Medical Physics & Biomedical Engineering, University College London, London, United Kingdom