Herfried Karl Wieczorek

Fluoroscopic x-ray imaging with amorphous silicon thin-film arrays

The dream of an all-solid state large area x-ray image sensor with digital readout and full dynamic performance will most probably find a first realization in 2D thin-film amorphous silicon arrays. In this paper we address in particular the evaluation of the limits of the signal/noise ratio in this concept. Using small prototype detectors measurements of MTF and noise power spectra have been made as a function of x-ray dose. The results are given in terms of the detective quantum efficiency as a function of dose and spatial frequency. We further present an analysis of the different noise sources and their dependence on the detector parameters, and we provide estimates on the maximum signals that may be achieved per unit dose. The intrinsic lag of the amorphous silicon photodiodes causes a second problem area with this type of x-ray detectors. Especially in radiography/fluoroscopy mixed applications, memory effects may not be negligible.

Dynamic X-ray imaging system based on an amorphous silicon thin-film array

In this paper we address design concepts and performance characterization with our laboratory x-ray detector system. Key component is a 1k2 pixel TFT switched photodiode array with a pixel pitch of 200 micrometer. It is built of a-Si:H with a CsI:Tl scintillator layer. The detector system can be used for radiography and fluoroscopy applications with up to 30 images/s. It shows a S/N-ratio better than 23dB at a dose of 10nGy/frame and reaches a value for DQE of more than 60% at low spatial frequencies. We have developed a new evaporation process for CsI:Tl deposition directly on the array. It yields an x-ray sensitivity close to the theoretical limit and a spatial resolution on a sufficiently high level. An optimized plate design in combination with a dedicated charge sensitive readout amplifier chip lead to a very low level of electronic noise. In particular sources and properties of electronic noise and signal crosstalk have shown to be crucial for the clinical use of the new technology. The visual impression of the remaining noise in the images from our system is isotropic. This means especially that synchronous noise has been reduced to the edge of visibility.

Dynamic X-Ray Imaging System based on an all-solid-state Detector

New digital detector systems based on all-solid-state large area elec-tronics offer a number of advantages for the user, like no image distortions, flat and light weight housing, no veiling glare, and large dynamic range. On the other hand, they require a dedicated image preprocessing to exploit their full image quality. In this paper we address design concepts, performance charac-terization and resulting image preprocessing aspects of our experimental de-tector system. The system comprises the detector frontend and a realtime image preprocessing unit with an interface to a commercial digital video system. It is intended for clinical evaluation of this new technology.