Performance of an edge block used in a configuration detector: Image quality measurements

In megavoltage X-ray imaging (MVI), computed radiography (CR) has required a very simple and general quality assurance (QA) method based on a radiotherapy imaging technique. The purpose of this study was to develop a general QA method for evaluating the modulation transfer function (MTF), the noise power spectrum (NPS) and the detective quantum efficiency (DQE) for MVI. Thus, a simple and compact edge block made of tungsten, with dimensions of 19 × 10 × 1 cm³, was efficiently designed and developed. The pre-sampling MTF was evaluated with the following settings: a 6 megavoltage (MV) energy, four different types of detectors, CR-IP (image plate: photo-stimulable phosphor screen), CR-IP-Lead (image plate + lead screen), CR-IP-Regular (fast front screen + image plate + fast front screen) and CR-IP-Fast (fast back screen + image plate + fast front screen). The MTF values at 1 mm^-1 for CR-IP-Regular showed the highest resolution of 0.18. The best noise distribution was observed in the measurement of the NPS with CR-IP-Regular. With increasing spatial frequency, our results showed that value of the DQE for the four types of detectors was approximately 1.0 mm^-1, which is comparable to the values reported in the literature. In this report, measurements for megavoltage imaging based on line-pair modulations by using an edge block are presented. When the wide side of the tungsten block was reduced, the accuracy of the edge measurement was improved to match those offered by edge techniques. The present study demonstrates that the QA method with our home-made edge block can be used to evaluate the MTF, NPS and DQE for MVI.