## 2 Materials and Methods  
This study was performed using a 3D Accuitomo F17 (Dental CBCT system, [PERSON], Kyoto, Japan) equipped with a flat-panel detector (FPD) of an indirect type made of cesium iodide (CsI). Dental CBCT examinations were performed by positioning a pediatric whole-body phantom (PBU-70; Kyoto Kagaku Co., Ltd., Kyoto, Japan) in the same position used for the patient examinations. The experimental layout is shown in Figure 1. The phantom was modeled after a 5-year-old child who was 110 cm in height. The phantom consists of a synthetic skeleton with the synthetic lungs, liver, mediastinum, and kidneys embedded in a soft-tissue substitute. This equipment requires manual setting of the exposure factors. In our previous study [49], we determined the tube voltage characteristics of an FPD installed in a CBCT. Among the tube voltages examined, the X-ray photon detection rate for the FPD was the highest at 90 kV, and the radiation dose was the lowest. Therefore, a tube voltage of 90 kV was used at all time points. In addition, in our previous study [50], we examined the optimal protocols for diagnosing ectopic eruptions and impacted teeth in children using different tube currents, FOVs, and rotation angles while using a tube voltage of 90 kV. Based on radiation doses and physical and visual evaluations, the optimal protocols were determined to be 2 mA, 4 \(\times\) 4 cm FOV, and 180\({}^{\circ}\) rotation. However, 1 mA was not suitable for diagnosis due to image noise and artifacts. Furthermore, tube currents of 2 and 3 mA were examined because tube currents greater than 3 mA could significantly exceed National Diagnostic Reference Levels (NDRLs) for the UK [51]. In this study, eight combinations of tube voltage (90 kV), tube current (2 and 3 mA), FOVs (4 \(\times\) 4 cm and 6 \(\times\) 6 cm), and rotation angles (360\({}^{\circ}\)and 180\({}^{\circ}\)) were used. Imaging conditions are presented in Table 1. Exposure time is 17.5 s for 360\({}^{\circ}\) rotation and 9.0 s for 180\({}^{\circ}\) rotation. A 180\({}^{\circ}\) rotation can be used to reduce the exposure doses, and the X-ray tube rotates around the posterior aspect of the phantom. X-ray irradiation image is shown in Figure 2.  
Figure 1: Experimental layout. Generally, in dental CBCT, the center of FOV and the center of rotation are at the same position because the target tooth is placed at the center of rotation. The maxillary median and maxillary right canine were centered in the FOV on scout images because the maxillary incisors and canines are common sites for ectopic eruption and impacted teeth in children [25; 52; 53; 54; 55; 56; 57; 58; 59; 60]. [PERSON] et al. [61] reported that 90-98% of ectopic eruptions and impacted teeth occur in the maxilla, with a particular predilection for the premaxilla. The positions of the center of the FOVs on the scout images are shown in Figure 3. CBCT scans were performed thrice for each imaging condition and position. An optical cable-type real-time dosimeter (RTD) (RD-1000; Toreck Co., Ltd., Yokohama, Japan) was used to measure the entrance surface dose. This RTD consisted of photoluminescence sensors (Y\({}_{2}\)O\({}_{2}\)S: Eu, Sm), an optical fiber cable, a photodiode, and a digital display that included the power supply, and can measure using a maximum of four sensors \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Tube voltage (kV) & 90 \\ Tube current (mA) & 2, 3 \\ Field of view (cm) & \(4\times 4\), \(6\times 6\) \\ Rotation angle (\({}^{\circ}\)) & 360, 180 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Imaging conditions. Figure 3: The positions of the center of the FOVs on the scout images: (**A**) anteroposterior scout and (**B**) lateral scout. Figure 2: X-ray irradiation image. Solid line indicates 360\({}^{\circ}\) imaging: X-rays are radiated at 360\({}^{\circ}\) angle; dotted line indicates 180\({}^{\circ}\) imaging: X-rays are radiated at 180\({}^{\circ}\) + fan angle. (ch.1, ch.2, ch.3, and ch.4) in one measurement.