MONTE-CARLO STUDIES OF X-RAY-ENERGY ABSORPTION AND QUANTUM-NOISE IN MEGAVOLTAGE TRANSMISSION RADIOGRAPHY

The subject contrast of bony anatomy in megavoltage medical radiograph s is very low, making detection of bony landmarks difficult if additio nal noise sources are introduced into the images. One source of noise, which is inherent to the x-ray detection process, is x-ray energy abs orption noise. X-ray energy absorption noise results from variations i n the amount of energy deposited in the imaging detector per interacti ng x-ray. These variations increase the noise content of the image. In this study, EGS4 Monte Carlo simulations of x-ray interactions in met al plate phosphor screen detectors have been performed to determine th e distribution of energy absorption events within the phosphor screen. From these ''absorbed energy distributions (AEDs)'', the x-ray energy absorption noise and the quantum absorption efficiency of the detecto r are determined. These calculations are performed for a range of dete ctor thicknesses (0.1-4 mm) and x-ray energies (0.1-10 MeV). A number of conclusions can be drawn from these investigations. (i) The x-ray a bsorption noise reduces the detective quantum efficiency (DQE) of meta l plate/phosphor screen detectors by as much as 50% at energies used i n megavoltage imaging (1-10 MeV). (ii) It is important to include seco ndary particle (electron) transport in estimating the quantum absorpti on efficiency of these detectors. For instance, the quantum efficiency of a typical portal detector is approximately 2%, even though 4%-5% o f the incident photons are attenuated. (iii) The metal ''conversion'' plate commonly used in megavoltage imaging enhances the DQE of the pho sphor screen by increasing the quantum absorption efficiency and reduc ing the magnitude of the x-ray absorption noise.