## 4 Discussion  
In this study, the SSDE\({}_{\text{Dw}}\) was estimated for 274 pediatric patients aged 0 to 15 years, and factors influencing the SSDE\({}_{\text{Dw}}\) were evaluated. The SSDE based on the water-equivalent diameter (SSDE\({}_{\text{Dw}}\)) was calculated using the conversion factor for the CTDI\({}_{\text{vol}}\) to SSDE\({}_{\text{Dw}}\) for CT head exams, which was obtained from AAPM Report No. 293. The Dw was derived from the middle slice of the scan length, with the scan region defined as the area extending from the base of the skull to the vertex. The axial cross-sectional CT image from infants and older children encompassed several major components, including brain tissue, the cerebral cortex, ventricles, white matter, gray matter, and skull bones, as illustrated in Figure 1. Each of these components exhibited distinct CT numbers (HU). Additionally, radiation doses, including the CTDI\({}_{\text{vol}}\), DLP, and organ dose for the brain CT protocol were determined. As shown in Table 1, head size, represented by the effective diameter and the Dw, increased with age. This led to an increase in scan length and, consequently, in the DLP. The effective diameter and the Dw exhibited slight differences in mean values among all the age groups; however, the Dw was computed from the attenuation area product of the patients, whereas the effective diameter considers only the physical dimensions. This finding aligns with [PERSON] et al. 's [PERSON] et al. (2020) study on pediatric patient sizes across age groups, which reported that both the effective diameter and the Dw increased with age. Table 2 reveals that the CTDI\({}_{\text{vol}}\) was higher than the median SSDE\({}_{\text{Dw}}\) for all groups except the 0-to-6-month age group. This discrepancy is attributed to a conversion factor greater than 1 in the 0-to-6-month age group, where the Dw is less than 14 cm. Conversely, other age groups had a conversion factor less than 1 when the Dw exceeded 14 cm. The percent relative differences in the radiation dose from the CTDI\({}_{\text{vol}}\) to the SSDE\({}_{\text{Dw}}\) were as follows: 0 to 6 months, 8.4%; 6 months to 3 years, 5.0%; 3 to 6 years, 8.3%; 6 to 12 years, 11.74%; and 12 to 15 years, 14.3%. These differences are due to variations in head size and X-ray attenuation factors. [PERSON] et al. [PERSON] et al. (2017) similarly found that the SSDE values differ between small and large patients, even with the same CTDI\({}_{\text{vol}}\). Figure 3: Scatter plot showing the relationship between age (years) and water-equivalent diameter (cm). The black dots represent individual data points, each corresponding to a specific age and its associated water-equivalent diameter. The trend is highlighted by a red line representing a locally weighted scatterplot smoothing (LOWESS) curve. The CTDI\({}_{\rm vol}\) is measured using a cylindrical phantom with a diameter of 16 cm; however, the pediatric head is not cylindrical and varies in size and density. [PERSON] et al. [7] reported the Dw (mean \(\pm\) SD) and SSDED\({}_{\rm Dw}\) (median (IQR)) for patients aged 0 to 6 months and 6 months to 3 years: The Dw was 12.4 \(\pm\) 1.2 cm and 14.9 \(\pm\) 0.9 cm, respectively, whereas the SSDED\({}_{\rm Dw}\) for both groups was comparable, with values of 23 (21, 25) mGy and 23 (22, 25) mGy, respectively. The CTDI\({}_{\rm vol}\) values were 23 (19, 23) and 23 (23, 25) mGy, respectively. Our study found that the mean \(\pm\) SD of the Dw for the 0-to-6-month age group was lower than that for the 6-month-to-3-year age group, with values of 12.47 \(\pm\) 1.13 and 15.13 \(\pm\) 1.01 cm, respectively. Similarly, the SSDED\({}_{\rm Dw}\) was lower in the 0-to-6-month age group compared with the 6-month-to-3-year age group. The median (IQR) SSDED\({}_{\rm Dw}\) values were 15.4 (14.8, 15.8) and 20.1 (19.6, 20.6) mGy, respectively. The CTDI\({}_{\rm vol}\) values were 14.1 and 21.1 mGy, respectively (Table 2).