Evaluation of an edge method for computed radiography and an electronic portal imaging device in radiotherapy: Image quality measurements

Regular quality assurance (QA) of image quality is essential for reasonable patient dose and accurate treatment. Thus, QA should be performed as a routine for correction. The purpose of this study was to evaluate the modulation transfer function (MTF), the noise power spectrum (NPS) and the detective quantum efficiency (DQE) of the computed radiography (CR) system and the digital radiography (DR) system by using the edge method in megavoltage X-ray imaging (MVI). We used an edge block, which consisting of tungsten with dimensions of 19 (thickness) × 10 (length) × 1 (width) cm3 and measured the pre-sampling MTF by using a 6-megavolt (MV) energy. Computed radiography with an image plate (CR-IP) showed the values of 0.4 mm−1 and 1.19 mm−1 for MTF 0.5 and 0.1. In the DR group, Elekta iViewGT showed the highest value of 0.27 mm−1 for MTF 0.5, and Siemens BEAMVIEWPLUS showed the highest value of 0.98 mm−1 for MTF 0.1. In CR, the NPS of CR-IP showed a favorable noise distribution. Thus, in the DR group, the NPS of Elekta iViewGT showed the highest noise distribution. CR-IP showed values at peak DQE and 1 mm−1 DQE of 0.0013 and 0.00011, respectively. In the DR group, Elekta iViewGT showed the best efficiency at a peak DQE of 0.0009, and Siemens BEAMVIEWPLUS showed the best efficiency at a 1-mm−1 DQE of 0.000008. The edge method produced fast assessments of the MTF and the DQE. We could validate the evaluation of the edge method by comparing of the CR system to the DR system. This study demonstrated that the edge method can be used for not only traditional QA imaging but also quantitative MTF, NPS and DQE measurements in detector development.