## 4 Discussion  
This study examined the effects of FOVs and rotation angles on radiation doses to highly radiosensitive organs in children undergoing dental CBCT. Based on the results of this study, radiation doses to highly radiosensitive organs differed greatly depending on the FOV and rotation angle. It is self-evident that reducing the FOV and rotation angle reduces the radiation dose. However, as the rate of dose reduction varies among organs, it is important to consider the relative positions of different organs with respect to the FOV and the trajectory of the X-ray tube. Although this study measured surface doses rather than organ doses, it provides additional information that may be useful in reducing exposure dose in dental CBCT examinations. Based on the results of this study, the following conclusions were drawn. The positional relationship between FOVs and RTDs is shown in Figure 5. The parotid gland doses were significantly higher than the other organ doses examined in the 4 \(\times\) 4 cm FOV. Because the parotid glands are located at approximately the same height as the maxillary incisors and canines, the parotid glands were partially or fully irradiated by the primary X-ray beam. Compared with the 4 \(\times\) 4 cm FOV, the sublingual gland doses increased significantly at the 6 \(\times\) 6 cm FOV. This was because the primary X-ray beam irradiated the sublingual glands little or not at all in the 4 \(\times\) 4 cm FOV and either partially or fully in the 6 \(\times\) 6 cm FOV. The FOV should be as small as possible because sublingual gland doses can be significantly reduced by reducing the FOV from 6 \(\times\) 6 cm to 4 \(\times\) 4 cm. Since the lens and thyroid doses were small, lens and thyroid were not likely irradiated by the primary X-ray beam at both the 4 \(\times\) 4 cm and 6 \(\times\) 6 cm FOV. Therefore, the lens and thyroid doses were reduced from the 6 \(\times\) 6 cm FOV to the 4 \(\times\) 4 cm FOV because the scattered radiation doses decreased. All measured lens doses were significantly lower than the eye dose threshold for cataract induction, so children undergoing dental CBCT examinations to diagnose ectopic eruptions and impacted teeth are at a very low risk of developing cataracts. The lens dose decreased the most among the organs examined when the rotation angle was reduced from 360\({}^{\circ}\) to 180\({}^{\circ}\). This is related to the positions of the X-ray tube and lenses. At a 180\({}^{\circ}\) rotation, the X-ray tube rotated around the posterior aspect of the phantom. The lenses were placed in front of the head. Therefore, the lenses were irradiated by a reduced X-ray beam passing through the head and neck, which explains the reduced lens doses. The parotid glands are located on the side of the head and behind the center of rotation of the X-ray tube. Therefore, some of the X-ray beams directly irradiated the parotid glands without passing through the head; thus, the reduction rates in the parotid gland doses were the lowest. The dose reduction rates of the 180\({}^{\circ}\) rotation relative to the 360\({}^{\circ}\) rotation were  
Figure 5: The positional relationship between FOVs and RTDs for (**A**) lenses and thyroid lobes and (**B**) parotid glands and sublingual glands. highest for the lens, followed by the thyroid, sublingual gland, and parotid gland, similar to the study by [PERSON] et al. [67] using a large FOV (17 \(\times\) 12 cm FOV). Compared to the maxillary median, when the maxillary right canine was placed at the center of rotation, there was little difference in each dose at a 4 \(\times\) 4 cm FOV. However, at a 6 \(\times\) 6 cm FOV, the right sublingual gland dose was slightly higher. The right sublingual gland was included in the 6 \(\times\) 6 cm FOV, which may have increased the effect of the primary X-ray beam. Careful consideration of the relative positions of the FOV and its organs is necessary, particularly when the FOV is large. [PERSON] et al. [45] reported the effects of FOVs and rotation angles on organ-absorbed doses in adult males.