# Image Quality and Information Parameters of Electronic Portal Imaging Devices
## 4 Discussion
decrease with spatial frequency. The latter behavior can be attributed to the changes in the CTF and MTF. The corresponding decrease in the CTF and MTF with spatial frequency depends on various factors, including radiation fluence, source size, X-ray scattering, detector properties such as phosphor thickness, light scattering within phosphor mass, the type of optical sensor and pixel size, as well as image magnification, which is of interest in our case. In the experimental setup presented, the properties of the detector do not affect the comparison of the aMTF and CTF results, since in both cases (100 and 115 cm) the detector is the same (flat panel imager with Gd2O2S:Tb phosphor). The same holds for the source dimensions. However, this behavior could be attributed to two factors: (i) the effect of Compton scattering, and (ii) magnification. In radiation therapy energies (in the range of a few or several MeVs), the scattered photons are oriented within low-angle directions. That is, they impinge on the detector within a spread width that increases as a function of the phantom-detector distance. In the case of the short source-phantom distance (100 cm), the phantom-detector distance increases, and this has a similar effect on the spread width, which in turn degrades spatial resolution and the MTF. The second factor that affects parameters expressing spatial resolution is magnification. This effect is more significant in the case of 100 cm (phantom-detector distance equal to 60 cm vs. 45 cm). Magnification increases the penumbra, i.e., the geometric unsharpness around imaged structures, which in turn degrades spatial resolution and decreases the MTF at 100 cm. In the case of the zero-frequency SNR, shown in Figure 4, the source-phantom distance of 100 cm produces higher values, apparently due to the inverse square law (i.e., radiation intensity is proportional to the inverse of the square of the distance), which corresponds to higher \(\upmu_{\text{ROI}}\) values at lower distances (see relation (3)) (recall that if Poisson statistics are assumed, the denominator of the SNR is equal to the square root of \(\upmu\) and therefore increases at a slower rate than \(\upmu_{\text{ROI}}\)). Based on the previous discussion, we can state that increasing the source-phantom distance from 100 cm to 115 cm results in image quality improvement at multiple levels. Image quality enhances across nearly all spatial frequencies, as evidenced by the aMTF, CTFnorm, SNR and IC values. Furthermore, increasing the MUs to the maximum (100 MU) further ameliorates image quality. The results of our study provide a solid methodology relying on typical quality control measurements for calculating multivariate parameters like the SNFR and IC in order to check the EPID performance of a LINAC in terms of the signal-to-noise ratio in the spatial frequency domain. These values can be used for checking deviations in EPID performance, which is also useful in the application of EPID dosimetry [27]. In addition, our results with respect to distance may be of importance for better realization of three-dimensional organ images when a two-dimensional EPID image is acquired.