Monte Carlo analyses of X-ray absorption, noise and detective quantum efficiency considering therapeutic X-ray spectrum in portal imaging detector

The bremsstrahlung spectrum from 6 MV linear accelerator (LINAC) was obtained and used as an input X-ray source in the simulation to estimate several important physical quantities of the detector in therapeutic X-ray portal imaging system, such as quantum and energy absorption efficiencies, Swank factor and the detective quantum efficiency (DQE) etc. In addition, the authors have obtained a spatial distribution of energy deposit within the detector, resulting in a spatial-frequency dependent DQE. From the simulation results, it is found that the use of metal plate largely enhances the energy absorption, leading to the large output signal. However, considering the noise properties, the presence of metal plate degrades the DQE at non-zero spatial frequencies because the detector absorption noise is dominated by the quantum absorption. For the verification of their simulation results, the authors have compared with experimental measurements and results of other literatures