A study of some energy dependent characteristics of X-ray screens used in diagnostic radiology : screen-film sensitivity, MTF and some related factors

Abstract: Fluorescent x-ray screens are used in medical x-ray diagnostics to absorb x-ray photons and convert these x-ray photons to visible light. The light distribution from these screens are then registered on photographic film to give an x-ray image. Both the sensitivity and the resolution characteristics of these systems are dependent on the x-ray photon energy. To enable a study of these and some other energy dependent characteristics of x-ray screens a number of almost monoener-getic radiation sources were constructed, tested with regard to their purity and calibrated. Both film and a photo-multiplier tube were used as light detectors.The sensitivity of screens with three different screen phosphors were studied as a function of the photon energy and large variations in sensitivity was found for different photon energies and screen phosphors. The light from the screens has been compared to the absorbed energy in the screens and this comparison shows that the energy dependence of the screens can approximately be predicted by calculations of the absorbed energy, except at low photon energies where other effects like increased light absorption in the screens is present.The modulation transfer factor (MTF) was studied both experimentally and theoretically as a function of photon energy. Two effects were shown to influence the energy dependence of the MTF. At low energies an increased light diffusion will destroy the MTF and at energies above the K-edge of the high-Z elements in the screens the production and re-absorption of K-radiation will deteriorate the MTF.Both the energy dependence of the screen-film sensitivity and the MTF have been calculated for some normally used spectral distributions from x-ray tubes and significant changes due to choice of kV and filtration of the beam were found. Other effects such as the number of interacting photons in the screens per unit area, contribution of K-radiation from one screen to the other, and light contribution to the front emulsion of the film compared to the back emulsion have also been investigated as a function of photon energy.Optimization of x-ray systems and clinical routines to give the lowest possible radiation dose to the patient with an acceptable image quality is an important task to carry out. The energy dependent characteristi es of x-ray screens studied in this work is a lead in the optimizing of the system with regard to choice of x-ray screens, film and radiation quality.