## 4 Discussion
In addition, when measuring the registration accuracy using anatomical landmarks on the patient's skin surface with the pointing tool, the lack of consistency was caused by the softness of the skin and the deformation due to pressure [43]. In this study, we designed and fabricated the phantom for evaluating our point-to-point registration method based on the standard phantom in ASTM F2554-10 for the first time [26]. Immediately after performing the registration using the developed point-to-point method, the FRE value was 0.14 mm. As a result of the accuracy evaluation using the phantom, the mean values of the absolute differences between the true and measured positions were 0.48 \(\pm\) 0.50, 0.34 \(\pm\) 0.23, and 0.41 \(\pm\) 0.33 mm on the x-, y-, and z-axes, respectively, and the TRE was 0.82 \(\pm\) 0.50 mm. Our method showed a higher accuracy than previous methods based on point-to-point registration using customized phantoms. We also demonstrated that our method showed a clinically acceptable level of accuracy (under 2 mm) [34,35]. Intraoperatively, the IGS systems should accurately and intuitively provide the surgeons with the 3D position of the pointing tool tip in real-time. Conventional IGS systems visualized the intraoperative position of the pointing tool tip in two-dimensional multi-planar (axial, sagittal, and coronal) views, and it was difficult for surgeons to easily recognize 3D spatial information with the 2D positional information provided [42]. In addition, even if the position of the tool tip was displayed on the 3D virtual model of the patient, it was still difficult for the surgeon to intuitively identify the position of the tool tip occluded by the skin or bone tissue models [30]. To overcome the limitations of the conventional methods, our system provided 3D slab volumetric view for the surgeons to intuitively identify the real-time position of the pointing tool tip inside the patients' nasal cavity (Figure 2). We performed the clinical comparative study to validate the performance of our developed system by enrolling eleven chronic sinusitis patients who needed the ESS with an IGS system. As a result of the study, we demonstrated that the registration time and image loading time of the developed system were significantly shorter than those of the commercialized system. This showed that it was more efficient to use the developed system than to use the commercially available system in the surgical preparation process. Although the average FRE value of the commercialized system was significantly lower than that of the developed system, there was no statistically significant difference between the average accuracy scores of the two systems when digitizing five anatomical landmarks, as evaluated by the surgeons. A previous study demonstrated that the FRE and TRE were not correlated to each other [44]. In conclusion, we developed an image-guided endoscopic sinus surgery system with 3D volumetric visualization of the nasal cavity and paranasal sinuses. The developed system provided an accurate point-to-point registration method based on the SVD algorithm and the leave-one-out strategy. According to the results of the clinical comparative study on eleven patients, we demonstrated that the developed system showed reliable potential for clinical application. However, the developed system could not perform the surface-based registration method. For future investigations, we will develop a surface-based registration method by modifying the conventional algorithms, such as iterative closest point (ICP) [27] and coherent point drift (CPD) [45]. In addition, we plan to recruit more patients to conduct clinical comparative studies, and to apply the developed system to additional cases of revision surgery or advanced skull base surgery. S.-J.L. : Contributed to conception and design, development of the system used in the work, in-vitro accuracy evaluation, data interpretation, and critically revised the manuscript; J.-Y.Y.