1. Department of Medical Physics, Faculty of Physics, Ludwig-Maximilians-Universität München, 85748 Garching, Germany
2. Dipartimento di Fisica, Università degli studi di Pavia, 27100 Pavia, Italy
3. Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW 2522, Australia
4. Max-Planck-Institut für Quantenoptik, 85748 Garching, Germany