## 3 Results  
All DE-CTA examinations were successfully performed without any adverse events. VNC images were compared to baseline TNC scan images. No significative motion or breath artifacts were registered. Intravascular attenuation measurements were performed using ROIs as previously described and reported as HU \(\pm\) SD for TNC and VNC images. The data were subsequently compared using a test for paired measures. At the level of the infrarenal abdominal aorta, the mean intravascular attenuation value in TNC images was 47.1 \(\pm\) 6.6 HU and the corresponding value in VNC images was 33.0 \(\pm\) 8.2 HU (_p_ < 0.00). For common iliac artery, the mean intravascular attenuation values were 47.8 \(\pm\) 8.9 HU in TNC and 34.1 \(\pm\) 11.4 HU in VNC images (_p_ < 0.00). At the level of the superficial femoral artery, the mean intravascular attenuation value was 48.5 \(\pm\) 9.0 HU in TNC images while the corresponding value in VNC images was 38.2 \(\pm\) 10.4 HU (_p_ < 0.001). At the level of the popliteal artery, the mean intravascular attenuation values were 49.4 \(\pm\) 7.8 HU in TNC and 40.7 \(\pm\) 9.6 HU in VNC images (_p_ < 0.001). Finally, for the external iliac artery, the same values were 46.6 \(\pm\) 9.4 HU in TNC images and 33.1 \(\pm\) 6.4 HU in VNC images (_p_ = ns). The results of the attenuation comparison of TNC and VNC images at the examined levels have been summarized in Table 3.  
In all the examined regions, TNC images showed significantly higher attenuation values (_p_ < 0.001) than VNC images, with a mean difference of attenuation ranging from 14.1 and 8.7 HU, except for the level of the external iliac artery, where no statistically significant differences in terms of attenuation were registered (_p_ < 0.27). In detail, a mean intravascular attenuation difference of 14.1 HU between TNC and VNC images at the level of the infrarenal aorta, a difference of 13.7 HU at the level of the common iliac artery, a difference of 13.5 HU at the level of the external iliac artery, a difference of 10.3 HU at the level of the superficial femoral artery, and a difference of 8.3 HU at level of popliteal artery were detected. The cumulative results of this analysis have been reported in Figure 3. Furthermore, the comparison of intravascular attenuation detected in TNC and VNC scans demonstrated less difference in HU value as the vessel caliber decreases, from the abdominal aorta to the popliteal segment, as shown in the diagram in Figure 4.  
Figure 4: Differences in attenuation values (HU) between true non-contrast and virtual non-contrast images: the difference in HU value reduces as the vessel caliber decreases from the infrarenal aorta to the popliteal segment. AIC = common iliac artery; AIE = external iliac artery; AFS = superficial femoral artery; AP = popileal artery. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Anatomical Region** & **Contrast Phase** & **Mean Attenuation (HU)** & \multicolumn{2}{c}{**Standard**} & \\  & & **Deviation (DS)** & \(p\) \\ \hline Aorta & TNC & 47.1 & 6.6 & \textless{}0.000 \\  & VNC & 33.0 & 8.2 & \\ AIC & TNC & 47.8 & 8.9 & \textless{}0.000 \\  & VNC & 34.1 & 11.4 & \\ AIE & TNC & 46.6 & 9.4 & \textless{}0.27 \\  & VNC & 33.1 & 6.4 & \\ AFS & TNC & 48.5 & 9.0 & \textless{}0.001 \\  & VNC & 38.2 & 10.4 & \\ AP & TNC & 49.4 & 7.8 & \textless{}0.000 \\  & VNC & 40.7 & 9.6 & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Results of the comparison between true non-contrast (TNC) and virtual non-contrast (VNC) images in the different analyzed arterial segments: abdominal aorta, common iliac artery (AIC), external iliac artery (AIE), superficial femoral artery (AFS), popliteal artery (AP). Figure 3: Bar graphs showing the comparison between mean attenuation values (HU) of true non-contrast (TNC) and virtual non-contrast (VNC) images in the different arterial segments analyzed: abdominal aorta, common iliac artery (AIC), external iliac artery (AIE), superficial femoral artery (AFS), popliteal artery (AP).