Sci-Sat AM(1): Imaging-02: Comparison between experimental measurements and Monte Carlo simulations for the off-focal...

In a typical x-ray tube, off-focal radiation is mainly generated by the backscattered electrons that re-enter the anode outside the focal spot. In an earlier study, the EGSnrc/BEAMnrc system was modified to be able to properly transport the anode backscattered electrons, and to tally their subsequent generation of off-focal x-rays. In the current study, a diagnostic system and a recent digital mammography system are simulated using the modified BEAMnrc code, and the simulation results are compared with experimental measurements from the literature. Simulation results show excellent agreement with experimental measurements for the spectral shape of both the primary and the off-focal components, and also for the integral off-focal-to-primary ratio. The spectrum of the off-focal component at the patient plane is softer than the primary, which causes a slight softening in the overall spectrum. For a given configuration, the off-focal component increases with tube voltage because of the increased probability for off-focal x-rays to escape the anode self-filtration and the total filtration. This study validates our earlier implementation of off-focal radiation in EGSnrc/BEAMnrc, and provides a well-benchmarked tool that simulates x-ray tubes more realistically. The macro to add this feature to BEAMnrc is available from the authors.