Monte Carlo simulations of the imaging performance of metal plate/phosphorscreens used in radiotherapy

The imaging performance of metal plate/phosphor screens which are used for the creation of portal images in radiotherapy is investigated by using Mont e Carlo simulations. To this end the modulation transfer function, the nois e power spectrum and the detective quantum efficiency [DQE(f)] are calculat ed for different metals and phosphors and different thicknesses of metal an d phosphor for a range of spatial resolutions. The interaction of x-rays wi th the metal plate/phosphor screen is modeled with the EGS4 electron gamma shower code. Optical transport in the phosphor is modeled by simulating sca ttering and reabsorption events of individual optical photons. It is shown that metals with a high atomic number perform better than lighter metals in maximizing the DQE(f). It is furthermore shown that the DQE(f) for the met al plate/phosphor screen alone is nearly x-ray quantum absorption limited u p to spatial frequencies of 0.4 cycles/mm. In addition, it is argued that t he secondary quantum sink of optical photons imposed by the optical chain ( mirror, lenses and video camera) leads to a significant degradation of the signal-to-noise ratio at spatial frequencies which are most important for s uccessful registration of portal images. Therefore, the conclusion is that a replacement of the optical chain by a flat array of photodiodes placed di rectly under the phosphor will lead to a substantial improvement in image q uality of portal images. (C) 1999 American Association of Physicists in Med icine. [S0094-2405(99)01710-1].