## 4 Discussion
Compared to 6 \(\times\) 6 cm FOV and the midline, 4 \(\times\) 4 cm FOV and canine teeth showed greatly reduced lens doses in adults, whereas thyroid doses were greatly reduced in children. The difference in the scattered radiation dose due to differences in head and neck size between adults and children may have influenced the reduction rate. The reduction rates of the dose by decreasing the rotation angle were higher for the lenses and thyroid lobes than for the salivary glands, and similar results were obtained for both adults and children. As mentioned above, this is thought to be related to the location of the X-ray tube, lenses, thyroid lobes, and salivary glands. The reduction rates by FOV and rotation angle were different owing to different methods and phantom sizes but were all high, indicating that reducing the FOV and rotation angle was also useful in children. [PERSON] et al. [37] reported the average organ-absorbed doses with various CBCT devices using a 10-year-old phantom. The results of the study by [PERSON] et al. were similar to those of this study, such that salivary glands irradiated by primary radiation received significantly higher doses than other organs. In a previous study by [PERSON] et al., the average salivary gland dose was >2.5 mGy and the average thyroid doses was 0.6 mGy. The average radiation doses to highly radiosensitive organs with optimal conditions (90 kV, 2 mA, 4 \(\times\) 4 cm FOV, 180\({}^{\circ}\) rotation) in this study were 0.67 mGy for parotid glands, 0.13 mGy for sublimingual glands, and 0.09 mGy for thyroid lobes, which were much lower than those reported by the study by [PERSON] et al. [PERSON] et al. used higher tube currents, larger FOVs, and 360\({}^{\circ}\) rotation. Special pediatric imaging conditions were not used in some of the devices, and the same imaging conditions for adults were used, which may have resulted in the radiation doses to highly radiosensitive organs. The target area included the maxillofacial and alveolar bones in addition to the teeth. A direct comparison of the doses for each imaging condition was not possible because the radiation doses to highly radiosensitive organs at the FOVs (4 \(\times\) 4 and 6 \(\times\) 6 cm) and rotation angles (360\({}^{\circ}\) and 180\({}^{\circ}\)) used in this study were not compared. In our previous study [50], we investigated the optimal imaging conditions suitable for CBCT examinations for the diagnosis of ectopic eruption and impacted teeth in children, based on radiation doses and subjective and objective image quality evaluations. The optimal imaging conditions were 90 kV, 2 mA, 4 \(\times\) 4 cm FOV, and 180\({}^{\circ}\) rotation. It was shown that reducing the tube current from 3 to 2 mA, the FOV from 6 \(\times\) 6 to 4 \(\times\) 4 cm, and the rotation angle from 360\({}^{\circ}\) to 180\({}^{\circ}\) resulted in poorer image quality due to image noise; however, the image quality was acceptable for diagnosis. However, clinical studies are needed in the future because the experiment was conducted using a phantom. Compared with the maximum imaging conditions (90 kV, 3 mA, 6 \(\times\) 6 cm FOV, 360\({}^{\circ}\) rotation), the reduction rates of doses using the optimal conditions were 87.9% for lenses, 88.4% for thyroids, 60.1% for parotid glands, and 94.6% for sublingual glands. In this study, by measuring the radiation doses to highly radiosensitive organs, we substantiated the usefulness of reducing tube currents, FOVs, and rotation angles.