## 4 Discussion

Table 3: \(H_{\text{p}}\)(3) of the medical staff per DLP in normalised position of 75 cm (without Pb glasses). examination, when the medical staff does not wear Pb glasses owing to the small amount of scattered X-ray or infrequent assistance. Although X-ray energy used in CT examination is relatively high, the dose reduction effect of Pb glasses was 63-69%, similar to that in IR reported previously [13; 14; 15]. Air dose distribution and energy distribution of scattered X-ray in CT examination is expected to be very complicated owing to the rotation of the X-ray tube. Thus, it was believed that the dose reduction effect was not lower than that in IR. The effective doses per examination of the medical staff member associated with assistance actions in CT with protective apron were 0.087, 0.026 and 0.015 mSv in the head, chest and upper abdomen, respectively. \(H_{\text{p}}\)(10) inside the protective apron was about 1/10 th of \(H_{\text{p}}\)(10) outside the protective apron. The trunk of the body was adequately protected by the apron. Therefore, the effective dose was much smaller than \(H_{\text{p}}\)(10) without the apron. Thus, the use of a radiation protective apron is essential while performing assistive actions during X-ray irradiation. \(H_{\text{p}}\)(3) of the medical staff per DLP was determined as 3.6 \(\times\) 10\({}^{-4}\), 7.4 \(\times\) 10\({}^{-4}\) and 7.3 \(\times\) 10\({}^{-4}\) mSv/DLP in head, chest and upper abdomen scans, respectively. In spite of the largest \(H_{\text{p}}\)(3) of 0.44 mSv/exam in the head scan, \(H_{\text{p}}\)(3) per DLP was the smallest among the 3 kinds of scans. This may be owing to the size difference of CTDI phantom used in CTDI measurement. DLP indicated on the console of the CT scanner is the product of CTDI\({}_{\text{vol}}\) and scan range (cm). This CTDI\({}_{\text{vol}}\) was a value measured by the manufacturer in advance. In the measurement of CTDI\({}_{\text{vol}}\), small size phantom (diameter of 16 cm) for head and large size phantom (diameter of 32 cm) for trunk of the body are used. If the output of X-ray from X-ray tube is same, CTDI\({}_{\text{vol}}\) obtained by small size CTDI phantom is larger than that obtained with large CTDI phantom. Therefore, output of X-ray from X-ray tube per CTDI\({}_{\text{vol}}\) in head scan is smaller than that in chest and upper abdomen scans. Consequently, scattered X-ray per CTDI\({}_{\text{vol}}\) in head scan is also smaller than that in chest and upper abdomen scan. Moreover, the amount of scattered X-ray depends on the size of the scanning subject; therefore, the scattered X-ray from the anthropomorphic head phantom was smaller than that from the chest or abdomen phantom. Thus, it was thought that \(H_{\text{p}}\)(3) per DLP in head scan was the smallest. We believe that \(H_{\text{p}}\)(3) per DLP shown in Table 3 can be universally used as a crude indicator (i.e., conversion factor) to estimate the \(H_{\text{p}}\)(3) of the medical staff in any CT examination, multiplying the value of \(H_{\text{p}}\)(3)/DLP and DLP indicated on the console, unless there are significant differences in the patient physique or scan range. In this case, it needs to be distinguished from the head and trunk of the body as the body site of the scan (i.e., it needs to be distinguished whether the DLP indicated on the console obtained from 16 cm or 32 cm CTDI phantom). Furthermore, the estimated radiation dose needs to be corrected by distance based on the inverse square law. This indicator may need to be additionally reviewed for use in several situations. For example, the difference in the filter used for the X-ray tube may need consideration because it influences the effective energy of X-ray that affects the amount of scattered X-ray radiation. This study has certain limitations. First, only female medical staff members with standard height of 158 cm were enrolled. This height is not representative of all medical staff members because the average height of male medical staff members would be more.