1. Research Unit of Medical Imaging Physics and Technology, University of Oulu, P.O. Box 5000, Oulu 90014, Finland
2. Department of Applied Physics, University of Eastern Finland, Kuopio 70210, Finland
3. Department of Diagnostic Radiology, Oulu University Hospital, Oulu 90014, Finland
4. Medical Research Center, University of Oulu and Oulu University Hospital, Oulu 90014, Finland