# Image Quality and Information Parameters of Electronic Portal Imaging Devices
## 4 Discussion  
Portal images are formed through photon beam projection on a detector, which detects a photon flux consisting of (a) primary radiation in the form of a diverging beam that attenuates according to the inverse square law and causes magnification of the imaged objects, and (b) scattered radiation originating from the phantom, which is scattered at relatively small angles. The parameters of the SNFR and IC, defined on the basis of the aMTF and CTF, decrease with spatial frequency, although at a different rate, as can be observed from Figures 1-4. The shape of these variations is determined by two factors: (i) the relation between the signal (as expressed through the aMTF and CTF) and the noise (NPS) in the spatial frequency domain (see Equations (5) and (6)), and (ii) the mathematical relationship that defines the IC and SNFR. As has been previously mentioned [25], the MTF, and similarly the CTF, decreases with frequency faster than the NPS, and this affects the shapes of the SNFR and IC curves in the frequency space. On the other hand, the IC is defined by means of a logarithm, which compresses the range of available numerical values. For this reason, the IC appears to decay more slowly with frequency compared to the SNFR. IC has been defined as a measure of information, which is more specifically expressed by the signal corrupted by noise [26], in the context of mathematical information theory. Its use in medical imaging serves as a measure of comparison between imaging systems and is usually expressed as a single index (integrated over the frequency spectrum). In each case, the values of the IIC and ISNFR, as shown in Table 1a,b and Table 2a,b, were higher at the 115 cm irradiation distance, while the corresponding IC (f) and SNFR values were found to \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c} \hline \multicolumn{11}{c}{**100 cm Source–Phantom Distance**} \\ \hline  & \multicolumn{4}{c}{**200 DR**} & \multicolumn{4}{c}{**400 DR**} & \multicolumn{4}{c}{**600 DR**} \\ \hline MU & 2 & 10 & 50 & 70 & 100 & 2 & 10 & 50 & 70 & 100 & 2 & 10 & 50 & 70 & 100 \\ ISNFR (\(\times 10^{6}\)) mm\({}^{-2}\) & 2.49 & 2.64 & 2.64 & 2.66 & 2.63 & 2.48 & 2.74 & 2.59 & 2.48 & 2.56 & 2.28 & 2.60 & 2.51 & 2.44 & 2.61 \\ \hline \multicolumn{11}{c}{**115 cm Source–Phantom Distance**} \\ \hline  & \multicolumn{4}{c}{**200 DR**} & \multicolumn{4}{c}{**400 DR**} & \multicolumn{4}{c}{**600 DR**} \\ \hline MU & 2 & 10 & 50 & 70 & 100 & 2 & 10 & 50 & 70 & 100 & 2 & 10 & 50 & 70 & 100 \\ \hline ISNFR (\(\times 10^{6}\)) mm\({}^{-2}\) & 2.67 & 2.82 & 2.82 & 2.90 & 2.91 & 2.79 & 2.88 & 2.89 & 2.98 & 2.74 & 2.96 & 3.02 & 2.95 & 2.86 \\ \hline \end{tabular}
\end{table} 
Table 1: (a) Tabulated ISNFR data for 100 cm source–phantom distance with 200 DR, 400 DR and 600 DR (1 DR = 1 MU/min). (b) Tabulated ISNFR data for 115 cm source–phantom distance with 200 DR, 400 DR and 600 DR (1 DR = 1 MU/min). \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c c} \hline \multicolumn{11}{c}{**100 cm Source–Phantom Distance**} \\ \hline  & \multicolumn{4}{c}{**200 DR**} & \multicolumn{4}{c}{**400 DR**} & \multicolumn{4}{c}{**600 DR**} \\ \hline MU & 2 & 10 & 50 & 70 & 100 & 2 & 10 & 50 & 70 & 100 & 2 & 10 & 50 & 70 & 100 \\ IIC (bits/mm\({}^{2}\)) & 71.98 & 72.34 & 72.30 & 72.65 & 72.27 & 72.25 & 72.98 & 73.09 & 72.28 & 73.08 & 72.05 & 72.82 & 72.15 & 72.19 & 72.72 \\ \hline \multicolumn{11}{c}{**115 cm Source–Phantom Distance**} \\ \hline  & \multicolumn{4}{c}{**200 DR**} & \multicolumn{4}{c}{**400 DR**} & \multicolumn{4}{c}{**600 DR**} \\ \hline MU & 2 & 10 & 50 & 70 & 100 & 2 & 10 & 50 & 70 & 100 & 2 & 10 & 50 & 70 & 100 \\ IIC (bits/mm\({}^{2}\)) & 72.43 & 73.46 & 73.15 & 73.16 & 73.59 & 73.15 & 73.31 & 73.31 & 74.09 & 72.85 & 73.52 & 73.59 & 73.24 & 73.16 \\ \hline \end{tabular}
\end{table} 
Table 2: (a) Tabulated IIC data for 100 cm source–phantom distance with 200 DR, 400 DR and 600 DR (1 DR = 1 MU/min). (b) Tabulated IIC data for 115 cm source–phantom distance with 200 DR, 400 DR and 600 DR (1 DR = 1 MU/min).