Heinz Horbaschek

Amorphous silicon x-ray image sensor

The design and the performance of a 20 cm by 20 cm flat panel x-ray detector for digital radiography and fluoroscopy is described: Thin film amorphous silicon (aSi) technology has been used to build a 1024 by 1024 photodetector matrix, each pixel including both a photodiode and a switching diode; the pixel size is 196 by 196 micrometers2. A high resolution and high absorption CsI(Tl) scintillator layer covers the top of the photodetector matrix in order to provide for x ray to light conversion. For low electronic noise and 30 fr/s operating rate we developed a custom design charge readout integrated circuit. The detector delivers a 12 bit digital output. The image quality, signal to noise ratio, and DQE are presented and discussed. The flat panel detector provides a MTF in excess of 30% at 2 lp/mm and a high contrast ratio without any distortion on the whole imaging area. The x-ray absorption is 70% for 50 KeV photons. The readout amplifier is optimized to reduce the electronic noise down to 1000 e-. This low noise level, combined with high sensitivity (1150 e-/incident x-ray quantum) provides the capability for fluoroscopic applications. The digital flat panel detector has been integrated in a C-arm system for cardiology and has been used on a regular basis in a European hospital since February 1995. The results are discussed for several operating modes: radiography and fluoroscopy. Conclusions on present detector performances, as well as further improvements, are presented.

Amorphous silicon X-ray detectors

Abstract Amorphous silicon (a-Si) has proven to be the most suitable semiconductor for large-area devices. Our detector prototype with a pixel pitch of 200 μ m and an active area of 20×20 cm 2 uses one PIN photodiode and one PIN switching diode per pixel for readout. Cesium iodide is used as scintillator. Evaluation of the detector was performed in the laboratory as well as in a clinical site where it was integrated in a C-arm for cardiological investigations. In this paper, modulation transfer function, dynamic behavior, noise figures, and quantum yield will be discussed. The performance of these detectors represents a first step towards the goal of replacing existing fluoroscopic or radiographic X-ray systems for medical diagnosis.