<title>Signal-to-noise performance in cesium iodide x-ray fluorescent screens</title>

Abstract

The signal-transfer and noise behavior of cesium iodide (CsI) layers, which are used as input screens of X-ray image intensifiers, have been investigated. Experimentally the performance of the CsI screen has been studied with a laboratory-type X-ray image intensifier as well as with a medical device. The scintillation spectrum (gain distribution), the spatial resolution (MTF), the image noise and the corresponding signal-to-noise (S/N) ratios have been determined. The measurements show that the DQE of the X-ray image intensifier is almost exclusively determined by the input screen performance for a wide range of spatial frequencies. The experimental data are compared with the results of a simulation. The screen model includes the primary X-ray absorption process, the effects of K-fluorescence escape and reabsorption, as well as the optical properties of the dedicated (needlelike) CsI screen structure. The simulated signal-to-noise (S/N) ratios are in good agreement with the experimental results. The analysis shows the influence of the various physical processes on the S/N-performance of the screen. The observed drop of the DQE below the absorption limit, which is most pronounced at higher spatial frequencies, is strongly related to the escape and the reabsorption of K-fluorescence.