## 3 Results  
### Diagnostic Accuracy  
Seventeen different inserts were used for this experiment, whose dimensions ranged from \(20\times 14\) mm to \(49\times 34\) mm.  
The two different readers measured the longest insert length, with both EOS and conventional radiography obtaining values ranging from 7 mm to 40 mm with the EOS system and from 6 mm to 36 mm with conventional radiography. Both conventional radiography and EOS underestimated the true length of the inserts by an average of 6.6 mm (25.2%) and 6.1 mm (23.9%), respectively, when compared with the true length of a respective insert on its longest axis. The comparison of the mean difference between the measurements for both modalities and the true length is reported in Table 1.  
Both observers were able to detect the same number of inserts per phantom on EOS and conventional radiography: This was equal to the number of inserts placed on the phantom in 4/5 cases (80%), while in one case, 1 out of 17 inserts was not seen. On CT, both observers always detected the correct number of inserts, as compared to the real number (Table 2).  
Intermodality reliability, quantified as the mean of the difference between the two modalities' measurements (bias) and the repeatability coefficient (CoR), showed excellent agreement for EOS, conventional radiography, and CT with a bias calculated to be 0.14 mm (88% reproducibility; CoR 2.9; 95% CI from \(-\)2.8 to 3.1 mm; \(p\) = 0.5) and 0.15 mm (81% reproducibility; CoR 5.2; 95% CI from \(-\)5.5 to 5.2 mm; \(p\) = 0.7) when comparing EOS to conventional radiography and CT, respectively. The assessment of interobserver reliability in the EOS modality showed 71% reproducibility between different observers, with a bias as low as 0.6 mm (CoR 7.2; 95% CI from \(-\)6.6 to 7.8 mm; \(p\) = 0.25) (Figure 1).  
\begin{table}
\begin{tabular}{c c c} \hline \hline  & **EOS Imaging System** & **Conventional Radiography** \\ \hline Difference from true length of & & \\ phantom inserts (mm) \({}^{\star}\) & \(-\)6.1 (from \(-\)18 to \(+\)1) & \(-\)6.6 (from \(-\)19 to \(+\)1.5) \\ \hline Relative magnification \({}^{\star\star}\) & \(-\)23.9\% & \(-\)25.2\% \\ \hline \hline \end{tabular} \({}^{\star}\) The values indicate the difference from the true length of the phantom inserts as provided by the manufacturer and are expressed as the mean, with the range in parentheses. \({}^{\star\star}\) The relative magnification was derived by dividing the difference between the measured length and the true length by the true length.  
\end{table}
Table 1: Insert length measurements with EOS and CR.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline  & & **Observer 1** & & & **Observer 2** & \\ \hline
**Phantom** & **n. Inserts CT** & **n. Inserts EOS** & **n. Inserts CR** & **n. Inserts EOS** & **n. Inserts CR** \\ \hline \# 1 & 16 & 16 & 16 & 16 & 16 \\ \# 2 & 17 & 16 & 16 & 17 & 16 & 16 \\ \# 3 & 3 & 3 & 3 & 3 & 3 \\ \# 4 & 6 & 6 & 6 & 6 & 6 \\ \# 5 & 9 & 9 & 9 & 9 & 9 \\ Total & 51 & 50 & 50 & 51 & 50 & 50 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Number of inserts detected per phantom on the three imaging modalities by two different observers who reviewed all images independently.