Practical method for photon fluency evaluation of digital X-ray image system

Quantum detective efficiency (DQE) is referred in literature as the most robust object-independent parameter for characterization of radiation detectors efficiency. DQE evaluation is particularly important in digital X-ray detectors, where the efficiency loss implies in image quality degradation and increase of patient exposed radiation dose. However, DQE cannot be evaluated directly. Its evaluation is derived of four other measurements: the modulation transfer function (MTF), the noise power spectrum (NFS), the gamma factor (γ) and the photon fluency (Φ). The photon fluency has a not easy experimental evaluation, being calculated by the integral of the X-ray radiation emission spectrum, using spectrophotometers or X-ray tube mathematical modeling. These approaches may lead to errors. In this study, photon fluency was extrapolated from the measurements of absorbed dose and of half value layer, using monochromatic radiation. Assuming this restriction, the photon fluency can be evaluated using only a conventional dosimeter. Therefore, this technique is compatible with the investments allocated for regular services of preventive maintenance and image quality control. As an evaluation of the viability in using this methodology in clinical environment, a DQE evaluation routine was implemented for the Digora (Soredex, Helsink, Finland) dental digital X-ray system with very good results that agreed with those obtained with conventional techniques.