Ulrich Schiebel

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.

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.

Digital image acquisition and processing in medical x-ray imaging

This contribution discusses a selection of today’s techniques and future concepts for digital x-ray imaging in medicine. Advantages of digital imaging over conventional analog methods include the possibility to archive and transmit images in digital information systems as well as to digitally process pictures before display, for example, to enhance low contrast details. After reviewing two digital x-ray radiography systems for the capture of still x-ray images, we examine the real time acquisition of dynamic x-ray images (x-ray fluoroscopy). Here, particular attention is paid to the implications of introducing charge-coupled device cameras. We then present a new unified radiography/fluoroscopy solid-state detector concept. As digital image quality is predominantly determined by the relation of signal and noise, aspects of signal transfer, noise, and noise-related quality measures like detective quantum efficiency feature prominently in our discussions. Finally, we describe a digital image processing algorithm for the reduction of noise in images acquired with low x-ray dose.

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.

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.