1. Department of Information Engineering, University of Pisa, 56126 Pisa, Italy
2. Research Centre “E. Piaggio”, University of Pisa, 56126 Pisa, Italy
3. EndoCAS Center for Computer-Assisted Surgery, University of Pisa, 56126 Pisa, Italy
4. Department of Neurosurgery, Azienda Ospedaliera Universitaria Pisana, 56126 Pisa, Italy