# Image Quality and Information Parameters of Electronic Portal Imaging Devices
## 2 Materials and Methods  
The study aimed to systematically evaluate the imaging performance of an electronic portal imaging device (EPID), applying the SNFR and IC of an EPID used in the Infinity\({}^{\text{TM}}\) LINAC manufactured by Elekta AB, Stockholm, Sweden, under literature-based irradiation conditions [3]. Available SNFR published data for a QC-3V phantom at a distance of 100 cm were used and compared with IC values that have been calculated in this work. In addition, a new experimental setup where the source-to-phantom distance was set at 115 cm was employed in order to examine the effect of changes in image geometry compared to a previous work with irradiation at 100 cm [3]. The imaging performance evaluation at different source-to-phantom distances can simulate varying treatment scenarios encountered in clinical practice. Alongside the SNFR and IC, other parameters like the signal-to-noise-ratio (SNR) have been calculated. The QC-3V phantom, a standard tool for EPID image quality evaluation, was used in this study [3]. DICOM RT format images with a pixel size of 0.40 \(\times\) 0.40 mm\({}^{2}\) were acquired as previously described [3], while image analysis was conducted using ImageJ version 1.52a and custom MATLAB version R2021b software. The following methods and formulas were used to determine the image quality and system performance metrics. The exposure conditions were from 2 MU to 100 MU so as to not only examine typical EPID imaging under low MU values, but also investigate the response of EPID in conditions closer to treatment settings. In addition, the chosen dose rates (DRs) were 200 MU/min, 400 MU/min and a LINAC maximum dose rate (DR) of 600 MU/min so as to check the fast dose delivery treatment conditions. The CTF was used as an index for determining the response of spatial frequency, \(f\), as follows [4; 7; 8; 12; 13; 14; 15; 16; 17]:  
\[\mathrm{MTF(f)}=\frac{\pi}{4}\bigg{[}\mathrm{CTF(f)}+\frac{\mathrm{CTF(3f)}}{ 3}-\frac{\mathrm{CTF(5f)}}{5}+\ldots\bigg{]} \tag{1}\]  
\[\mathrm{CTF(f)}=\frac{\mu_{\mathrm{max}}(f)-\mu_{\mathrm{min}}(f)}{\mu_{ \mathrm{max}}(f)+\mu_{\mathrm{min}}(f)} \tag{2}\]  
where the local maximum pixel value is \(\mu_{\mathrm{max}}\) (f), and the local minimum pixel value is \(\mu_{\mathrm{min}}\) (f), for a given frequency f, corresponding to five regions of the phantom [18]. The contrast transfer function (CTF) was computed for each corresponding set of line pairs located in the QC-3V phantom for each frequency [10; 14; 15]. The CTF and MTF describe the resolution in the spatial frequency domain. High values of these parameters at each spatial frequency indicate the ability of the EPID to image small objects. Checking these parameters during an EPID QC determines how well the EPID can delineate an organ or image a small-sized area [19; 20]. The SNR was calculated using mean pixel values and standard deviations of regions of interest (ROIs), allowing for the estimation of a signal within a noisy background [21]. \[\mathrm{SNR}=\frac{\mu_{\mathrm{ROI}}}{\sigma} \tag{3}\]  
where \(\mu_{\mathrm{ROI}}\) represents the mean pixel value of the region of interest (ROI) and \(\sigma\) denotes the standard deviation associated with that ROI. The SNR is used to estimate the signal present within the noise. The SNR provides a means for characterizing image clarity. High-SNR EPID images effectively assert the accuracy of patient positioning. The NPS was utilized to analyze noise characteristics in the spatial frequency domain [10; 22]. The ROI used was in air. \[\mathrm{NPS(f_{\mathrm{x}},f_{y})}=\frac{\left\langle\left|\mathrm{FFT}\left( \sum_{\mathrm{n}_{\mathrm{x}}=0}^{\mathrm{N}_{\mathrm{x}}-1}\sum_{\mathrm{n}_{ \mathrm{y}}=0}^{\mathrm{N}_{\mathrm{y}}-1}\mathrm{P(x,y)}\right)\right|^{2} \right\rangle}{\mathrm{N}_{\mathrm{x}}\mathrm{N}_{\mathrm{y}}}\Delta\mathrm{x }\Delta\mathrm{y} \tag{4}\]  
The NPS is a measure of the noise distribution in the spatial frequency domain, giving a limit of image clarity, especially in small contrast tissues.