## 2 Materials and Methods  
In this study, radiation dose exposure of the medical staff members who assist in head, chest and upper abdomen CT scans was evaluated. The assistant actions included acts, such as monitoring injection of the contrast media (e.g., initial supervision in bolus tracking method), restraining the patient (e.g., fixation of head or arms) and operating the medical device (e.g., operating of bag valve mask). Medical staff phantom built up has been shown in Figure 1. Medical staff phantom was constructed with anthropomorphic head phantoms (ALDERSON) and X-ray water phantom for chest and abdomen (JIS Z 4915-1974, Miwa Electric Medical Co., Ltd., Aichi, Japan). External layer of the water phantom was filled with water. The water phantom was set on a desk, and the head phantom was set on the water phantom. The height of medical staff phantom was 158 cm; it simulates the average body height of a Japanese woman. The medical staff phantom was covered with radiation protection apron of lead equivalent of 0.35 mmPb (Dream Light, Maeda & Co., Ltd., Tokyo, Japan). Figure 1: Medical staff phantom and layout of personal dosimeter and Pb glasses. DOSIRIS (Chiyoda Technol Corporation, Tokyo, Japan) for \(H_{\text{p}}\)(3) and glass badge (Chiyoda Technol Corporation, Tokyo, Japan) for \(H_{\text{p}}\)(10) and \(H_{\text{p}}\)(0.07) [19] were used as personal dosimeter. DOSIRIS is consist of thermoluminescent dosimeter, and glass badge is composed of radiophotoluminescence glass dosimeter. These dosimeters are of the passive type, and a radiation dose reading is performed by manufacturer. Radiation monitoring system of the manufacturer complies with ISO/IEC 17025: 2017 and traceability is ensured [20,21]. With glass badge, \(H_{\text{p}}\)(10) and \(H_{\text{p}}\)(0.07) can be measured simultaneously. DOSIRIS and glass badge were set on medical staff phantom. DOSIRIS was set near the left lateral canthus using a ready-made head set prepared by manufacturer. Glass badges were set both, outside (neck-chest position) and inside (abdomen) the radiation protection apron. It has reported that DOSIRIS and glass badge have a good angular response [19,22]. Light type of Pb glasses (Panorama Shield, Toray Medical Co. Ltd., Tokyo, Japan) whose lead equivalent was 0.07 mmPb was used. With or without Pb glasses, individual dose measurement was performed. Dose reduction effect of Pb glasses was calculated as follows:  
\[\text{Dose reduction effect (\%) = \{1-(H_{\text{p}}\text{(3) with Pb glasses)}/(H_{\text{p}}\text{(3) without Pb glasses)}\}\times 100.} \tag{1}\]  
An 8-channel multi-detector row CT scanner (LightSpeed Ultra, GE Healthcare Japan Corporation, Tokyo, Japan) was used for measuring the dose after performing the daily calibration. Anthropomorphic phantom (PBU-50, Kyoto Kagaku Co., Ltd., Kyoto, Japan) was laid down on the bed of the CT scanner as patient phantom. In head scan, the hands of patient phantom were set on the abdomen. In chest and upper abdomen scans, the arms of the patient phantom were raised and fixed. Scanning was performed using CT-auto exposure control. Detail scan parameters and dose information (CT dose index: CTDIvol dose length product: DLP) indicated on CT scanner console are shown in Table 1. These values (CTDIvol or DLP) are reasonable setting compared to national diagnostic reference levels in Japan (Japan DRLs 2020) [23]. Medical staff phantom was made to stand straight, facing the iso-centre of the CT scanner. The distance between the iso-centre and the nasal root was 75, 85 and 95 cm in the head, chest and upper abdomen scans, respectively. The layout in dose measurement is shown in Figure 2. Scanning was performed 10 times in the head scan, 20 times in the chest and upper abdomen scans. The scattered X-ray (mainly derived from patient phantom) was measured with a personal dosimeter (i.e., DOSIRIS and glass badge). Each personal dose equivalent per examination was calculated by dividing the accumulated dose by the number of scan times.