1. Department of Technical Physics II; TU Ilmenau; Ilmenau Germany
2. Research Unit of Medical Imaging, Physics and Technology; University of Oulu; P.O. Box 5000 Oulu Finland
3. Medical Research Center; University of Oulu and Oulu University Hospital; P.O. Box 50 Oulu Finland
4. Department of Diagnostic Radiology; Oulu University Hospital; P.O. Box 50 Oulu Finland