# Image Quality and Information Parameters of Electronic Portal Imaging Devices  
[PERSON]  
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2 Department of Radiotherapy, General Hospital of Athens Alexandra, 11528 Athens, Greece; [EMAIL_ADDRESS] (V.P.); [EMAIL_ADDRESS] (A.A.); [EMAIL_ADDRESS] (G.K.)  
[PERSON]  
2 Department of Radiotherapy, General Hospital of Athens Alexandra, 11528 Athens, Greece; [EMAIL_ADDRESS] (V.P.); [EMAIL_ADDRESS] (A.A.); [EMAIL_ADDRESS] (G.K.)  
[PERSON]  
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[PERSON]  
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1 Radiation Physics, Materials Technology and Biomedical Imaging Laboratory, Department of Biomedical Engineering, University of West Attica, 12210 Athens, Greece; [EMAIL_ADDRESS] (M.K.T.  
which can provide information about the signal and noise in the spatial frequency domain. Such a parameter is the detective quantum efficiency (DQE), whose calculation requires the MTF as well as the calculation of noise in the spatial frequency domain. The DQE calculation has been reported in the current literature [1; 4; 6; 7; 8]. These calculations, however, require data about X-ray fluence which are either difficult to obtain through measurements or are tabulated [1; 4; 6; 7; 8] and thus may not be case-specific.  
Among the array of metrics used for this purpose, the signal-to-noise frequency response (SNFR) has recently [3] appeared as a universal indicator of EPID image quality. SNFR determination requires the calculation of the contrast transfer function (CTF) and the noise power spectrum (NPS) [3]. It can be considered a substitute for detective quantum efficiency (DQE), which is often applied in X-ray medical imaging [1]. A DRawback of the aforementioned methodology is the need for a bar pattern phantom for the calculation of the CTF. However, this requirement can be easily offset in SNFR calculations by the availability of specific phantoms. Previously published work [3] has studied variations in the SNFR in terms of dose rate (DR) and monitor unit (MU) choice with an experimental setup, incorporating a QC-3V imaging performance phantom placed 100 cm from the source and the head of the LINAC. It was found that in clinical imaging settings, the image algorithms imposed by iViewGT\({}^{\text{TM}}\) version R3.4.1 b519 software of the EPID affects the final image quality metrics [1; 4; 5; 6].  
This work focuses on examining whether other holistic imaging performance metrics that can be calculated in clinical imaging radiotherapy conditions can provide useful data. Such a parameter is information capacity (IC), giving the available information in bits/mm\({}^{2}\)[9; 10]. Information capacity has been investigated within the framework of [PERSON]'s information theory to quantify the information signal that is transmitted through an electronic communication channel without errors [11]. The concept of information capacity has been previously used effectively as an imaging performance metric in diagnostic radiology and in nuclear medicine [12]. To our knowledge, it has not been previously used in portal imaging of radiation therapy. The calculation of this parameter includes NPS measurements and modulation transfer function (MTF) calculations; thus, it can be regarded as a prominent candidate for comparison with the SNFR [3]. In addition, a comparison of SNFR and IC usability is attempted.  
Furthermore, it might be of clinical use to determine the effect of patient positioning distance during EPID imaging. Thus, in this work, the SNFR and IC were studied at two phantom distances from the LINAC head at 100 cm and 115 cm, that is closer to the detector. This distance difference incorporates body shape sizes and may provide a deeper understanding of the effect of scatter and imaging geometry on EPID imaging. The results of this investigation may assist in the development of optimized QA protocols for LINACs in radiotherapy settings and may contribute to a better evaluation of patient positioning images.