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 a 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 a therapeutic X-ray portal imaging system, such as quantum and energy absorption efficiencies, Swank factor, and detective quantum efficiency (DQE). In addition, we 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 a large output signal. However, considering the noise properties, the presence of a metal plate degrades the DQE at nonzero spatial frequencies because the detector absorption noise is dominated by the quantum absorption. We have verified our simulation results by comparison with experimental measurements and results from other works.