# Image Quality and Information Parameters of Electronic Portal Imaging Devices
## 3 Results
More specifically, at 0.1 lp/mm, the maximum differentiation of the SNFR graphs is 27.48%, with the 115 cm distance values being higher than the 100 cm values, except for  
Figure 4: CTEnorm 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). Figure 3: CTFnorm 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). the SNFR value for 2 MUs at a 200 DR at 0.27 lp/mm. Since the SNFR is a function of the CTF, in the numerator, and the NPS, calculated by Equation (4), in the denominator, the increased numbers at 115 cm follow the corresponding CTF increase shown in Figures 3 and 4. In Figure 6, the IC (f) graphs for 100 cm and 115 cm source-phantom distances with 2 MUs at a 200 DR, a 400 DR and a 600 DR are presented. Using IC (f), or the IC before the integration, shown in Equation (6), over the frequency range was considered more practical to present a metric that helped comparisons with the SNFR. From 0.1 lp/mm to 0.27 lp/mm, the IC values show a small variation of about 0.35-0.58%. From 0.27 lp/mm to 0.789 lp/mm, the variation is about 1.26-1.41%. The IC values for the 115 cm distance are higher than those for the 100 cm distance, except for the IC value at 0.27 lp/mm for 2 MUs at a 200 DR. The increased IC (f) values are due to the corresponding increase in the aMTF shown in Figures 1 and 2. In Figure 7, the SNR graphs, calculated by Equation (3), for 100 cm and 115 cm source-phantom distances with 2 MUs and 100 MUs at a 200 DR, a 400 DR and a 600 DR are shown. There is a noticeable differentiation of about 27.50% between the corresponding SNR values, where the SNR for 100 cm is higher. Upon inspecting Equation (3), it is seen that the SNR is proportional to the mean pixel value, corresponding to the signal, and inversely proportional to the standard deviation, expressing noise. The higher SNR at 100 cm irradiation with respect to 115 cm irradiation may be attributed to the combined effect of the signal and noise in the detector response. Considering the fact that the LINAC-to-EPID distance is the same, the corresponding difference in the SNR shown in Figure 7 may be attributed to the different spatial contributions of scatter radiation due to the closer distance of the phantom to the EPID in the 115 cm setup. The scatter spread toward the EPID under 100 cm irradiation conditions is larger, meaning that more X-rays are propagating toward the EPID periphery. The QC-3V phantom is placed at an angle [3], meaning that the part of the area of air where the SNR was calculated is toward the EPID periphery. Under the 100 cm irradiation conditions, it was exposed to primary radiation and scatter from the phantom. This scatter was larger under the 100 cm conditions than under the 115 cm conditions. Therefore, the radiation incident at the region of air where the SNR is measured was higher at 100 cm than at 115 cm. The higher radiation produces higher signal values and better noise statistics, that is less noise. Both of them yield a higher SNR value at 100 cm than at 115 cm from the LINAC head, as shown in Figure 7. The EPID signal and noise values are finally affected by the software algorithms imposed on the image before its final presentation [3]. In Table 1a,b and Table 2a,b, the summation of the SNR over spatial frequencies, called hereafter the ISNR, and the IC calculated by Equation (6) are shown. The ISNR as a single parameter allows for a direct comparison with the IIC. It can be shown from the tables that neither the ISNR nor the IIC significantly vary with respect to the MU and DR [3; 23; 24]. In addition, both values are higher under 115 cm exposure conditions. The relative average increase in the ISNR from 110 cm to 115 cm for all MUs and DRs is 12.5%, with a minimum value equal to 5% and a maximum of 21%. The corresponding  
Figure 7: SNR graphs of 100 cm and 115 cm with 200 DR, 400 DR and 600 DR (1 DR = 1 MU/min). relative increase values for the IC parameter are a 1.1% average, a 0.01% minimum and a 2% maximum.