## 4 Discussion
The automatic recording and detection of the magnetic transmitter on the patient's skin has always been effective and has never prevented the use of the navigation system. In a randomized study on 180 patients using a navigation system based on optical tracking in the field of lung biopsies, [PERSON] et al. found a significant reduction in the procedure time, the number of control scans and the radiation dose [20]. In another study, this optical navigation system called SIRIO (MASMEC S.p.A., Modugno, BA, Italy) has been shown to be more accurate for percutaneous lung biopsies of lesions <20 mm [21]. With this navigation system, [PERSON] et al. showed better efficiency in terms of dose reduction, procedural time and diagnostic success for lesions <10 mm compared to the conventional method, with fewer post-procedural complications [19]. These optical and electromagnetic navigation systems both have their advantages and disadvantages [30]. The optical-based systems use fiducial markers located on the instrument and on the patient's skin that are detected by stereoscopic cameras, which requires a direct line of view between the cameras and the fiducials and may limit their application in clinical practice. Although electromagnetic guidance systems do not suffer from these kind of constraints, they can be affected by magnetic susceptibility artifacts. One of the advantages of the SIRIO optical navigation system is the synchronization to the patient's respiratory movements, which is particularly interesting for lung biopsies and may partly explain the significant decrease in the number of control acquisitions, radiation dose and procedure time compared to the standard method. To our knowledge, six studies about the use of the CT-Navigation(tm) system on patients have been published [13; 14; 18; 29; 31; 32]. [PERSON] et al. 's retrospective study with 12 patients, the CT-Navigation(tm) system has been used as a guidance system for radiological percutaneous osteosynthesis and cementoplasty of malignant pathological fracture of the proximal femur [31]. Although the study's main subject was not focused on the navigational system, an excellent technical success of 100% was found [31]. In [PERSON] et al. 's study, the feasibility of percutaneous fixation by internal cemented screw for the prevention or palliation of pelvic or femoral neck fractures using the CT-Navigation(tm) system was evaluated [18]. With a total of 76 screws inserted, technical success was achieved in 96% (48/50 patients), with high accuracy even when a significant craniocandal angulation was needed. With a total of 120 enrolled patients, a prospective randomized trial assessed the accuracy and usability of this system by comparing conventional and navigated procedures in a full range of routine CT interventions on the chest, abdomen, pelvis and bones, such as biopsy, drainage, tumor ablation, sympathicolysis and joint infiltration [13]. A significant improvement of the accuracy was found with the use of the system, with a gain in accuracy remaining significant in both the easy and difficult intervention subgroups and in a subgroup analysis based on operator experience [13]. The retrospective study of [PERSON] et al. focused on the radiation doses delivered to patients during percutaneous vertebroplasty [29]. With 15 consecutive patients who underwent a conventional CT guided procedure, and 22 patients who underwent a procedure with the use of the CT-Navigation(tm) system, a significant reduction in the DLP by a 3.2 factor was found. The use of this navigation system also demonstrated an important reduction of the procedure duration (50%) [29]. Two studies focused on the use of the CT-Navigation(tm) system for hepatic tumor ablation [14; 32]. In [PERSON] et al. 's study, 27 percutaneous ablations of small hepatic tumors (<2 cm) that were invisible on ultrasounds and difficult to reach on CT, requiring a double-oblique approach, were performed using both CT-Navigation(tm) system and high-frequency jet-ventilation (HFJT) [14].