Walter Dr. Hillen

Imaging performance of a digital storage phosphor system

The imaging performance of a storage phosphor system (SPS) for digital projection radiography is studied in which the x-ray image is temporarily stored in a light-stimulable phosphor plate which is subsequently read out by a scanning laser beam. The imaging performance of this system has been analyzed for two types of imaging plates. The spatial resolution is described by the modulation transfer function and the signal-to-noise (S/N) ratios of the recorded image data are measured in terms of noise-equivalent quanta and detective quantum efficiency. Their dependence on detector entrance dose and spatial frequency is discussed. A detailed analysis of the different sources of image noise is given to outline the intrinsic features and limits of the system. Finally, the S/N behavior of the SPS is compared with published data of screen-film systems.

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.

A Selenium-Based Detector System For Digital Slot-Radiography

A research system for digital radiography is described, which is based on a selenium detector with capacitive probe readout. The detector, in which a selenium drum is used as the primary image receptor, is exposed by a scanning fan beam. Scatter reduction and primary transmission by slot-radiography as well as the imaging properties of the selenium detector are discussed. The spatial resolution and the noise behaviour of the detector are analysed. The signal-to-noise ratios expressed in terms of noise equivalent quanta and detective quantum efficiency are calculated and compared with competitive systems.

Image Quality In Selenium-Based Digital Radiography

Capacitive sensing of x-ray generated charge images on amorphous selenium layers represents an attractive technique for the electronic recording of projection radiographs. In order to evaluate the practical limits of image quality with such a selenium detector we have set up a laboratory system that allows the recording of small size (70 x 70 mm2) radiographic images. We have measured MTF and noise power spectra and have calculated from these data noise equivalent quanta (NEQ) and detective quantum efficiency (DQE) as a function of dose and spatial frequency. On the basis of these quantities the imaging performance of the system is discussed.