1. HUS Medical Imaging Center, Helsinki University Central Hospital, Helsinki, Uusimaa, Finland
2. Department of Physics, University of Helsinki, P.O. Box 64, 00014 University of Helsinki, Finland
3. STUK-Radiation and Nuclear Safety Authority, Laippatie 4, Helsinki 00880, Finland
4. Department 6.2 Dosimetry for radiation therapy and diagnostic radiology, Physikalisch-Technische Bundesanstalt, Bundesallee 100, Braunschweig 38116, Germany