1. Department of Diagnostic Imaging, Azienda Ospedaliera Universitaria Senese, Viale Bracci 10, 53100 Siena, Italy
2. Department of Medical, Surgical and Neuro Sciences, Diagnostic Imaging, University of Siena, Viale Bracci 10, 53100 Siena, Italy
3. Department of Physics, Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Florence, Italy