## 3 Results
For different contrast objects (i.e., air, PMP, LDPE, polystyrene, acrylic, Delrin, Teflon) of the CTP404 phantom module, summary noise level values in CT imaging using FBP and ASiR-V/ASiR with different blending levels of reconstruction are reported in detail in Table 3. Figure 7 shows the noise reduction properties of ASiR-V and ASiR reconstruction algorithms with respect to conventional FBP for different contrast objects and blending levels of reconstruction. For all contrast objects, when compared with conventional FBP, both ASiR-V and ASiR allowed a substantial reduction in noise level, which decreased with an increasing blending level of reconstruction (Table 3). The noise reduction properties of both ASiR and ASiR-V depended on the contrast object. In particular, higher contrast objects (i.e., air and Teflon) were characterized by a lower noise reduction degree (Figure 7). Moreover, for all contrast objects but air and Teflon, ASiR-V showed a greater noise reduction potential than ASiR when the blending level was \(\geq\)40%. Histograms of N-ASiR and N-ASiR-V maps, as well as of the N-FBP map, are shown in Figure 8. Moreover, values of histogram descriptors (median, interquartile range, kurtosis, skewness) are reported in detail in Table 4. While noise map histogram of the FBP-reconstructed CT image approximated a Gaussian distribution (kurtosis and skewness values of 3 and 0.23, respectively), noise map histograms of ASiR- and ASiR-V-reconstructed CT images leaned to deviate from a Gaussian distribution when increasing the blending level of reconstruction, with kurtosis/skewness values for pure ASiR- and ASiR-V-reconstructed CT images of 3.57/0.41 and 3.35/0.24, respectively. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & & **Air** & **PMP** & **LDPE** & **Polystyrene** & **Acrylic** & **Delrin** & **Teflon** \\ \hline FBP & & 9.0 (1.0) & 9.7 (1.1) & 9.8 (1.1) & 9.6 (1.0) & 9.7 (1.2) & 10.4 (1.1) & 11.3 (1.3) \\ \hline \multirow{4}{*}{ASiR} & 20\% &  
Figure 8: Histograms of noise maps (bin width of 0.1 HU) for FBP- (**a**), ASiR- (**b**), and ASiR-V-reconstructed (**c**) CT images of the homogenous CTP486 module with different blending levels of reconstruction (20%, 40%, 60%, 80%, and 100%). Median and interquartile range values are expressed as HU; kurtosis and skewness values are unitless. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & \multicolumn{2}{c}{**Median**} & \multicolumn{2}{c}{**Interquartile**} & \multicolumn{1}{c}{**Kurtosis**} & \multicolumn{1}{c}{**Skewness**} \\ \hline FBP & & 9.16 & 1.62 & 3.00 & 0.23 \\ \hline \multirow{4}{*}{ASiR} & 20\% & 7.81 & 1.39 & 3.07 & 0.27 \\  & 40\% & 6.86 & 1.17 & 3.10 & 0.26 \\  & 60\% & 5.88 & 0.99 & 3.13 & 0.25 \\  & 80\% & 5.00 & 0.83 & 3.14 & 0.22 \\  & 100\% & 4.51 & 0.80 & 3.57 & 0.41 \\ \hline \multirow{4}{*}{ASiR-V} & 20\% & 7.91 & 1.37 & 3.03 & 0.24 \\  & 40\% & 6.57 & 1.15 & 3.10 & 0.27 \\ \cline{1-1}  & 60\% & 5.51 & 0.92 & 3.11 & 0.25 \\ \cline{1-1}  & 80\% & 4.52 & 0.74 & 3.14 & 0.23 \\ \cline{1-1}  & 100\% & 3.60 & 0.57 & 3.35 & 0.24 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Descriptive metrics of noise map histograms for FBP-, ASiR-, and ASiR-V-reconstructed CT images of the homogenous CTP486 module with different blending levels of reconstruction (20%, 40%, 60%, 80%, and 100%). Median and interquartile range values are expressed as HU; kurtosis and skewness values are unitless. Figure 7: CTP404 phantom module with different contrast objects (i.e., air, PMP, LDPE, polystyrene, acrylic, Delrin, Teflon). Noise reduction properties of ASiR-V and ASiR reconstruction algorithms with respect to conventional FBP, _NR_(%), estimated as the mean (SD) value of PDN-ASiR-V and PDN-ASiR-V maps within a central circular ROI of 1 cm diameter placed in each contrast object.