## 2 Materials and Methods  
This study utilized 1 y/o anthropomorphic phantom (ATOM, 704 model CIRS, Norfolk, VA, USA). The body weight, height and thorax dimension of the phantom were 10 kg, 75 cm, and 12 \(\times\) 14 cm, respectively. This phantom was made from tissue equivalent material, which consisted of the lung, soft, and bone tissue. The phantom was made up of 28 slabs with 2.5 cm slice thickness, which cover from head to lower leg and each slab contains a hole about 5.0 mm in diameter for inserting dosimeter, such as TLD, to measure the absorbed dose at the specific area of sensitive organs and for the simulation study of a blood vessel filling with contrast material, as shown in Figure 1.  
The CT CAP examination was performed on 64 slices using a multidetector CT scanner (MDCT) Siemens Definition AS (Siemens Sector Healthcare, Forchheim, Germany) by applying nine different CT protocols (P1 to P9). The tube potential of P1, P2, and P3 were fixed at 100 kVp and P4, P5, and P6 were fixed at 80 kVp with various tube current reference values (ref. mAs: 210, 180, and 150), while in P7, P8, and P9 the modification was made on P1 with changes on slice collimation, pitch factor, and tube current modulation (TCM), respectively. The details of the CT acquisition parameter used in this study were shown in Table 1. The scan range starts from 1 cm above the apex of lungs and ends at 1 cm below symphysis puis with a total length of 32 cm for all protocols involved. The source to detector (SDD) and source to object distance (SOD) used in this study were 100 cm and 50 cm, respectively.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multicolumn{8}{c}{**Protocols**} \\ \cline{2-10}  & **P1** & **P2** & **P3** & **P4** & **P5** & **P6** & **P7** & **P8** & **P9** \\ \hline Slice thickness recon (mm) & 3 & 3 & 3 & 3 & 3 & 3 & 3 & 3 & 3 \\ Tube potential (kVp) & 100 & 100 & 100 & 80 & 80 & 80 & 100 & 100 & 100 \\ Effective tube current (mAs) & 27 & 23 & 19 & 33 & 31 & 26 & 27 & 26 & - \\ Ref. mAs & 210 & 180 & 150 & 210 & 180 & 150 & 210 & 210 & 210 \\ Number of detector & 64 & 64 & 64 & 64 & 64 & 64 & 32 & 64 & 64 \\ Slice collimation (mm) & 0.6 & 0.6 & 0.6 & 0.6 & 0.6 & 0.6 & 1.2 & 0.6 & 0.6 \\ Pitch & 1.4 & 1.4 & 1.4 & 1.4 & 1.4 & 1.4 & 1.4 & 1.2 & 1.4 \\ Dose Modulation & On & On & On & On & On & On & On & On & Off \\ Table Feed & 53.8 & 53.8 & 53.8 & 53.8 & 53.8 & 53.8 & 53.8 & 56.0 & 53.8 \\ Exposure time (s) & 0.5 & 0.5 & 0.5 & 0.5 & 0.5 & 0.5 & 0.5 & 0.5 & 0.5 \\ Kernel filtration & B30f & B30f & B30f & B30f & B30f & B30f & B30f & B30f \\ CTDI\({}_{\text{vol}}\) (mGy) & 1.8 & 1.5 & 1.3 & 1.0 & 0.9 & 0.8 & 1.7 & 1.7 & 6.9 \\ DLP (mGy.cm) & 58 & 48 & 42 & 32 & 29 & 26 & 54 & 54 & 221 \\ E (DLP x \(k\) factor) & 1.6 & 1.3 & 1.2 & 0.9 & 0.8 & 0.7 & 1.5 & 1.5 & 6.2 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Routine and modified CT CAP acquisition parameters used in this study.  
Figure 1: (**a**) One-year-old anthropomorphic phantom was set up before the scan. (**b**) The anthropomorphic lung tissue slab comprises a-holes matrix about 5.0 mm in diameter for dosimeter insertion, such as TLD.