CT
We used a 64 detector-row CT scanner (Lightspeed VCT; GE Healthcare, Milwaukee, WI, USA) with scout view at different tube voltages (80, 100, and 120 kVp) from the apex of the lung to the subdiaphragm. We performed APSV from the front position (0°) and PASV from the back position (180°). The scan range with scout view was 150 mm for 5-year-old phantoms. Next, we performed a helical scan to evaluate the image noise in the main scan because the ATCM depends on the scout view. The scanning parameters were as follows: tube voltages of 80, 100, and 120 kVp, helical pitch of 1.375, beam width of 40.0 mm, section thickness of 5.0 mm (noise index 10), gantry rotation time of 0.4 s, and detector collimation of 64 × 0.625 mm. Moreover, the settings for the small scan field of view were 100 mm the matrix size was 512 ×512 for full mode, and standard reconstruction kernel.
Phantoms
We used a pediatric anthropomorphic phantom (ATOM Phantom, CIRS, Norfolk, Virginia, USA) that represents a 5-year-old child (Fig. 1). The assumed body weight and height for the 5-year-old were 19.0 kg and 110.0 cm, respectively. The phantoms were made of radiologically equivalent tissue materials with internal structures, including artificial skeletons, lungs, and soft tissues, and were formulated for the accurate simulation of clinical exposure.
Comparison of Radiation doses from 10 cm ionization chamber, computed tomography dose index volume, and RD-1000
Since it was necessary to confirm the traceability of the RD-1000, we compared the doses from the 10 cm ionization chamber and the RD-1000 by using a CT system. The RD-1000 was good for linearity of reference dose with the ionization chamber.
Dosimeters and dose measurement
A real-time skin dosimeter (RD - 1000; TORECK CO, Kanagawa, Japan) was used on the pediatric anthropomorphic phantom at the back of the mammary gland level surfaces, right mammary gland, and left mammary gland between the APSV and PASV protocols (Fig. 2). The phantom was scanned 10 times for each protocol. The measured dose values for the RD - 1000 were compared for each protocol.
Image noise
The tube current was varied based on a scout view and real-time feedback of the actual attenuation during rotation. The type of scout view used has a significant impact on the level of radiation exposure with the ATCM. To evaluate the image noise for the different scout methods, we measured the image noise [standard deviation (SD) of the CT number] in the center, front, back, left, and right sides of the phantom at the level of the mammary gland within a circumscribed 10.0 mm diameter region of interest in the main scan. For each tube voltage scan, we measured the image noise (SD of the CT number) five times, for a total of five marginal and central points.
Statistical analysis
The Mann–Whitney U test was used to analyze the measured dose values of RD-1000 in the different protocols. Statistical significance was set at P < 0.05 and statistical analyses were performed using the free statistical software “R” (R, version 3.2.2; the R Project for Statistical Computing; http://www.r-project.org/).