1. Microelectronics Research Unit, Faculty of Information Technology and Electrical Engineering, University of Oulu, P.O. Box 4500, FIN-90014Oulu, Finland
2. Department of Applied Physics, Aalto University, FIN-00076Aalto, Finland
3. Research Unit of Medical Imaging, Physics and Technology, Faculty of Medicine, University of Oulu, Aapistie 5A, 90220Oulu, Finland