## 5 Experimental Results  
We conducted phantom experiments to test navigation system performance and to confirm both the accuracy of the U-Net-based segmentation and the feasibility of the developed bendable endoscope and surgical tool. Measurement of fiducial registration error (FRE) and target registration error (TRE) has been accepted as an important procedure for checking registration accuracy [31]. Initially, using the phantom model, we measured the FRE and TRE of the surgical navigation system. The FRE measurement identifies errors by matching five image coordinate points to five phantom points, as shown in Figure 15. In our study, the TRE measurement was performed in relation to the F-6 point, which was selected as the point nearest the ICA, as shown in Figure 26. Automatic segmentation of the ICA was performed using U-net and the experimental environment for FRE and TRE measurement is shown in Figure 27.  
Figure 26: Landmark points: (**a**) TRE experiment using landmark F-6 (Pink); (**b**) alarm warning system accuracy measurement experiment, using landmarks F-3 and F-7 (Green). Figure 25: 3-D design and prototype of the phantom used for experiments: (**a**) phantom design using patient CTA data; (**b**) phantom TSA for surgery experiments. We performed a second experiment to quantify the similarity between the U-Net-based prediction and the manual segmentation of the ICAs and optical nerves. In this experiment, we employed the Jaccard index to find the similarity. The Jaccard index is a way to measure similarity between two datasets; it has a value between zero and one, showing one if the sets are identical and zero if there are no common elements between them. Finally, the developed phantom was used to test whether the bendable surgical devices were appropriate for visualization of the target lesion and for specific tasks deep inside the brain. Results from the navigation system FRE and TRE measurement experiments may be seen in Figure 28. As shown in the results of Figure 28, if the navigation system provided FRE and TRE within 0.5 mm, this would indicate precise surgery and would reduce the possibility of surgical instruments invading the ICAs or optic nerves during TSA surgery. Based on these guidelines, it can be seen, from Figure 28, that the use of the proposed navigation system with its alarm function would significantly reduce the risk of complications in patients undergoing TSA surgery. We were also able to conclude that the proposed surgical navigation system in this study was as or more accurate than conventional surgical navigation systems [32; 33; 34; 35]. A warning system quantitative evaluation experiment was also conducted. As shown in Figure 29, distances between the bendable surgical tool and fiducial markers F-3 and F-7 were calculated by both the navigation system and as measured by the electromagnetic (EM) sensor (NDI Aurora). In this experiment, three EM sensors were attached--to the F-3 and F-7 fiducial markers in the phantom and to the distal end of the bendable surgical tool--with the surgical tool mounted on a motorized XYZ stage, to eliminate handshake errors during the experiment. In order to measure surgical navigation system alarm function error, the XYZ stage was moved at 1 mm intervals on each axis. Alarm function errors  
Figure 28: FRE and TRE results (n = 25). Figure 27: Experimental setup used to test navigation system performance. were measured for a total of 27 points. According to the experimental results depicted in Figure 30, we were able to confirm that the warning system had reasonable accuracy. The developed bendable endoscope and the surgical tool were inserted into the phantom's nasal area, as shown in Figure 31. It was confirmed that all areas were identifiable by the endoscope and reachable using the surgical tool, which would not have been possible using a rigid endoscope. Figure 31: Phantom experimental environment for the proposed bendable endoscope and bendable surgical tool.