小児 CT 検査における自動管電流調整機能を最適化するための新手法について

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Purpose: To compare the radiation dose and image quality using the conventional method for performing the front and side scout view and a new method for performing the side scout view, and then correct the table height at the scan isocenter and perform the front scout view. Methods: We retrospectively analyzed fifty-six children who had underwent computed tomography (CT) examination between June 2014 and August 2018. We divided them into two groups. The conventional method was performed in 3 steps: 1. obtain the front scout view, 2. obtain the side scout view, and 3. main scan. Without table position correction, the new method was performed in 4 steps: 1. obtain the side scout view with table position correction, 2. patient correction at the scan isocenter, 3. obtain the front scout view, and 4. main scan. We used a 64-row CT scanner (LightSpeed VCT; GE Healthcare). Scan parameters were tube voltage 80 kV, automatic tube current modulation, noise index 16, slice thickness 5 mm, rotation time 0.4 s/rot, helical pitch 1.375, and reconstruction kernel standard. We recorded the volume dose index (CTDI_vol) and dose length product (DLP) on the CT console and compared the radiation dose in both groups. To evaluate the image quality in both groups, the mean standard deviation of CT number (SD value) was measured within an approximately 5–10 mm²  circular region of interest. We measured the scan length of the pediatric patient and accuracy of pediatric positioning at the CT examination. A grid was displayed on the CT axial image, taken to evaluate the error from the scan isocenter during alignment, and the error between the height of half the body thickness and the scan isocenter was recorded. Results: Scan lengths were median (minimum–maximum) values of 16.2 cm (10.8–21.5 cm) and 16.8 cm (11.5–23.0 cm). There were no significant differences in the scan length between both groups (p=0.47). In the group with table position correction, median (minimum–maximum) values for CTDI_vol, DLP and SD value were 0.40 mGy (0.3–0.7 mGy), 7.6 mGy･cm (4.4–11.5 mGy･cm), and 24.0 HU (18.3–37.5 HU), respectively. In the group without the table position correction, median (minimum–maximum) values for CTDI_vol, DLP and SD value were 0.40 mGy (0.3–0.6 mGy), 7.1 mGy･cm (4.2–13.8 mGy･cm), and 20.3 HU (11.3–28.8 HU), respectively. There were no significant differences in the CTDI_vol and DLP values between both groups (p=0.42 and p=0.44, respectively); however, there were significant differences in the SD value in both groups (p<0.01). The error for the accuracy of pediatric positioning was 0 mm (0 to 0 mm) and 10 mm (–16 to+59 mm) using the conventional and new methods (p<0.01), respectively. Conclusions: It was suggested that the optimum image could be obtained during CT scan with automatic tube current modulation by using this potential new method (1. obtain the side scout view, 2. patient correction at the scan isocenter, 3. obtain the side scout view, and 4. main scan).