The quality control of video camera based electronic portal imaging systems used in radiotherapy and standardization of the obtained images

Purpose: The purpose of this study is analyzing the images obtained by using?QC-3V? digital phantom by PIPSpro software program and evaluation the performance of the electronic portal imaging device (EPID) according to data. Material and Methods: For acquired test images four different set-ups were performed such as phantom location at gantry 0° angle, around the dedector of the EPID and at isocenter, at 90°-270° gantry angles isocenter. The measurements were performed for 30 days along at video camera based Siemens Beamview Plus EPID which is assembled to Siemens Primus linear accelator. Automatic analysis of portal images was performed by using PIPSpro software program which belongs to phantom. As a result of analysis relative modulation transfer function (RMTF), regional signals, contrast noise ratio (CNR), spatial resolution, the values of standard deviation in regions were generated as a report by software program. The performance of the EPID was evaluated as a result of examination of the image parameters. Results: At gantry 0° on the EPID with 6 MV photon energy the average value of RMTF_a RMTF_b which belong to a and b images was measured respectively as 0.462(±0.360) and 0.470(±0.360). On the other hand, at 18 MV photon energy exposure, the values were obtained as 0.419(±0.400) and 0.417(±0.370). At the isocenter the average RMTF value for the 6 MV photon energy was calculated as 0.501(±0.360) and 0.493(±0.370), for the 18 MV photon energy exposure the values were obtained as 0.474(±0.360) and 0.476(±0.410). At gantry 90° and at the isocenter the average RMTF for 6 MV photon energy was found as 0.487(±0.360) and 0.490(±0.470) respectively. For the gantry 270° the values were obtained as 0.490(±0.360) and 0.484(±0.360). When we evaluate the average standard deviation of signals from phantom regions, we can that all measurements are in ± %5 acceptable clinical value. By setting up the phantom on the EPID for 6 MV and 18 MV the average CNR and spatial resolution were measured as 30.0(±4.5)-36.7(±8.3) and 0.212-0.203 lp/mm respectively. At gantry 90°-270° by setting up the phantom at the isocenter with 6 MV photon energy exposure the average resolution values were maeasured as 0.222 lp/mm ve 0.223 lp/mm respectively. Conclusion: As we know the effect of dose rate, screen, mirror, relative movement of the camera on the quality of image, in this study the effect of energy, magnification, gantry rotation angle and different factors can be seen. Therefore, optimal display performance for EPID systems, quality control program is required on a regular basis. Keywords: EPID, quality assurance, quality control, modulation transfer function, spatial resolution, contrast.