Peter Quadflieg

Signal-to-noise performance in cesium iodide x-ray fluorescent screens

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.

Simulation of the imaging performance of x-ray image intensifier/TV camera chains

The signal and noise propagation through a generalized x-ray image intensifier/TV camera chain is modeled in terms of the modulation transfer function, two dimensional noise power spectrum, and detective quantum efficiency. The model covers effects of energy dependent x- ray absorption in the cesium iodide entrance screens, K-fluorescence and K-reabsorption, electron optics, output screens, lenses and also sensing with electronic pick-up tubes (plumbicons) or CCD sensors. Several Philips x-ray image intensifiers in combination with plumbicon TV-chains and with laboratory type CCD cameras are compared with the model. Based on the model results we further present an image simulation tool. The image simulation allows a direct evaluation of the impact of the individual components of the imaging chain on image quality. It is shown that future high resolution CCD systems can exhibit a superior image quality as compared with electronic pick-up tube systems.

Image quality assessment of monochrome monitors for medical soft copy display

Soft-copy presentation of medical images is becoming part of the medical routine as more and more health care facilities are converted to digital filmless hospital and radiological information management. To provide optimal image quality, display systems must be incorporated when assessing the overall system image quality. We developed a method to accomplish this. The proper working of the method is demonstrated with the analysis of four different monochrome monitors. We determined display functions and veiling glare with a high-performance photometer. Structure mottle of the CRT screens, point spread functions and images of stochastic structures were acquired by a scientific CCD camera. The images were analyzed with respect to signal transfer characteristics and noise power spectra. We determined the influence of the monitors on the detective quantum efficiency of a simulated digital x-ray imaging system. The method follows a physical approach; nevertheless, the results of the analysis are in good agreement with the subjective impression of human observers.

A High‐Resolution 2k Monochrome Monitor based on the Triple‐Gun Principle

We have studied a 2048 lines 2k high-resolution monochrome monitor concept operating with the three guns of a modified color CRT and a standard graphics controller. The major application for the monitor system is medical image presentation. The attainable performance has been analyzed and proven with the help of a prototype system. Major advantages over the standard single-gun monitor architectures are the reduced bandwidth and the attractive system price.