Monte Carlo detective quantum efficiency and scatter studies of a metal/a-Se portal detector

Simulation studies of a portal imaging detector based on amorphous Selenium (alpha-Se) are reported. The a-Se is deposited on a metal substrate which is turned toward the megavoltage x-ray beam to act as an effective buildup layer. The thickness and material of the metal layer are studied using Monte Carlo methods by consideration of the effect of dose deposition in the alpha-Se layer for a 6-MV beam hardened by a 20-cm polystyrene phantom. The detective quantum efficiency, DQE(f), due to energy absorption is modeled using the hardened primary beam. It is found that although DQE(0) increases with metal thickness up to dmax, there is a crossover near 1 cycle/mm which indicates that smaller metal thickness are more useful to visualize edges and small objects. Tungsten front plates offer the best DQE of the metals studied. The scatter fraction and scatter-to-primary ratio are also modeled to understand the effect of patient scatter on the various metal/a-Se combinations. The scatter fraction is measured experimentally for four metal plate/a-Se combinations and agrees with the Monte Carlo results within experimental uncertainties.