1. Orbis Medical Centre, Dr. H. van der Hoffplein 1,6130 MB, Sittard-Geleen, The Netherlands
2. Centre for PET, Department of Nuclear Medicine, KU Leuven, Leuven
3. Department of Neurology, Headache Research Unit, University of Liège, Liège, Belgium [Correction added after online publication 9 April 2009: JWM ter Berg's affiliation address has been changed]