# Image Quality and Information Parameters of Electronic Portal Imaging Devices
## 3 Results  
In Figure 1, the aMTF curves for source-phantom distances of 100 cm [3] and 115 cm, using 2 MUs at a 200 DR, a 400 DR and a 600 DR are demonstrated. The aMTF curves were approximated through the CTF using a method described in the literature [3]. The aMTF values from 0.1 lp/mm to 0.212 lp/mm are nearly identical, differing by only 0-2.37%. From 0.212 lp/mm to 0.789 lp/mm, the aMTF values for the 100 cm distance are lower than those for the 115 cm distance. Specifically, at 0.463 lp/mm, there is a difference of 10.84%, and at 0.789 lp/mm, a 10.20% increase is observed for the 115 cm distance. The lower values at 100 cm might indicate an increase in the X-ray spread due to the incident scattered X-rays on the detector (i.e., larger air gap). In the energy range applied in radiation therapy, X-ray scatter occurs at small scattering angles, i.e., mainly in forward directions. However, the proximity of the phantom to the EPID, under 115 cm irradiation conditions, results in a reduced spread of the scatter X-rays, i.e., scattered X-rays are distributed in a smaller area on the detector's entrance surface than in the case at 100 cm. This is manifested in a seemingly higher MTF. In Figure 2, the aMTF graphs for 100 cm and 115 cm source-phantom distances using 100 MUs at a 200 DR, a 400 DR and a 600 DR are shown. Similar to Figure 1, the aMTF values from 0.1 lp/mm to 0.212 lp/mm are almost equal, with only a 0-1% difference. From 0.212 lp/mm to 0.789 lp/mm, the aMTF values for the 100 cm distance are lower than those for the 115 cm distance. At 0.463 lp/mm, the difference is 10.10%, and at 0.789 lp/mm, it is approximately 18.13% for the 115 cm distance. The persistent degradation of the aMTF values for the 100 cm distance, for higher MUs, may suggest that this behavior is  
Figure 1: aMTF graphs of 100 cm using 2 MUs and 115 cm using 2 MUs with 200 DR, 400 DR and 600 DR (1 DR = 1 MU/min). more affected by X-ray physics than the software manipulation imposed in clinical EPID imaging [3; 23]. In Figure 3, the normalized CTF graphs, called hereafter CTrnorm graphs, for 100 cm and 115 cm source-phantom distances for 2 MUs under 200 DR, 400 DR and 600 DR irradiation conditions are demonstrated. It can be observed that from 0.1 lp/mm to 0.27 lp/mm, the CTrnorm values show a minor variation from 0% to 2.01%. However, the CTrnorm values for the 100 cm irradiation setup are 0.01% higher. For the 115 cm distance, from 0.27 lp/mm to 0.789 lp/mm, there is an increase in CTrnorm from 0.76% to 16.09%, compared with the irradiation conditions of 100 cm. This increase at 115 cm is qualitatively reflected in the corresponding aMTFs. Figure 4 illustrates the CTrnorm graphs for 100 cm and 115 cm source-phantom distances using 100 MUs at a 200 DR, a 400 DR and a 600 DR. The CTrnorm values for 115 cm from 0.1 lp/mm to 0.27 lp/mm present a small increase of about 0-3.25% compared to the corresponding values for the 100 cm irradiation setup. Similarly, the difference from 0.27 lp/mm to 0.463 lp/mm is about 1.27-2.11%. Furthermore, the difference in favor of the 115 cm distance from 0.463 lp/mm to 0.789 lp/mm has been found to be equal to 18.56%. In each case, the CTrnorm values for the 115 cm distance are noticeably higher for all of the spatial frequencies examined. In all cases, the standard error in the CTF calculation has been reported to range from 0.5% to 11%, depending upon the MU, DR and spatial frequency [3]. Figure 2: aMTF graphs of 100 cm using 100 MUs and 115 cm using 100 MUs with 200 DR, 400 DR and 600 DR (1 DR = 1 MU/min). In Figure 5, the SNFR graphs, calculated by Equation (5) for 100 cm and 115 cm source-phantom distances, with 2 MUs at a 200 DR, a 400 DR and a 600 DR, are shown. From 0.1 lp/mm to 0.27 lp/mm, the difference between the SNFR curves ranges from 4.98% to 27.48%. From 0.27 lp/mm to 0.789 lp/mm, the variation stabilizes at about 7.13-8.54%. The SNFR values for the distance of 115 cm are significantly higher than those for the 100 cm distance.