Detective quantum efficiency of an amorphous selenium detector to megavoltage radiation

The spatial frequency dependent detective quantum efficiency (DQE(f)) of a high-resolution selenium-based imaging system has been measured at megavolt age energies. These results have been compared with theoretical calculation s. The imaging system was a video tube with a 5 mu m amorphous selenium (a- Se) target which was irradiated by 1.25 MeV gamma-rays. The modulation tran sfer function (MTF) decreased rapidly with spatial frequency (determined by spread of electrons in the build-up material) while the noise power spectr um was constant as a function of spatial frequency. The DQE obtained from t hese MTF and noise power measurements was compared with a Monte Carlo model of the pulse height spectrum of the detector. The DQE(0) model accounted f or the interaction of x-rays with the detector as well as the energy-depend ent gain (charge generated/energy deposition). Good agreement between the c alculated and measured DQE(0) was found. The model was also used to estimat e the DQE(f) of a metal plate + a-Se detector which was compared with a met al plate + phosphor system of the same mass thickness. The DQE(f)s of both detectors are very similar, indicating that the choice of which detector is better will be based upon criteria other than DQE(f such as read-out appro ach, ease of manufacture or sensitivity.