1. Institute of Clinical Physiology, National Council of Research; via G. Moruzzi 1 Pisa 56124 Italy
2. Fondazione G. Monasterio CNR-Regione Toscana; via G. Moruzzi 1 Pisa 56124 Italy
3. GE Healthcare; Park Alle 295 Broendby 2605 Denmark
4. Department of Electrical Engineering; the Technical University of Denmark; Kgs Lyngby 2800 Denmark