1. Department of Electronics (DEEI), University of Trieste, Via Valerio 10, I-34127 Trieste, Italy
2. Department of Radiology, University of Trieste, Cattinara Hospital, I-34149 Trieste, Italy
3. Department BBCM, University of Trieste, Via Giorgieri 1, I-34127 Trieste, Italy
4. PROTOS Research Institute, P. O. Box 972, I-34100 Trieste, Italy