Evaluation of X-ray beam quality and film-screen combinations for detection of small low-contrast objects

Abstract

The detection of small low-contrast objects in transmission radiography has been simulated by phantom experiments for specific anatomical regions (phantom thicknesses). Improvement of image quality in combination with a low patient dose is addressed to a complicated optimization problem. It has been shown that in the low-contrast case, the image resolution is largely determined by the low-frequency part of the noise spectrum and by the absorption contrast between detail of interest and surrounding medium. Improvement of image resolution is performed more efficiently via beam quality (by enhancement of absorption contrast). This is also true in the case of photoelectrically recorded photons (DLR). Conversely, the detector MTF (modulation transfer function) is of minor importance for the low-contrast case. The role of X-ray scattering in the phantom as well as light propagation in the phosphor is outlined. Numerous commercially available film-screen combinations of different speed and phosphors have been tested. In summary, fast screens of Gd2O2S phosphors are most appropriate for detection of low-contrast objects.