Clemens Herrmann

Reflective and transmissive CR ScanHead technology on needle image plates

The image quality of needle-image-plate (NIP) Computed Radiography (CR) scanners based on ScanHead technology was optimized. In order to get the best image quality for different applications, the influence of the phosphor layer thickness on the detective quantum efficiency (DQE) for different beam qualities was investigated. We compared a cassette-based, reflective CR-NIP-scanner to a new, transmissive flat-panel CR scanner with fixed, mounted NIP. The image quality was analyzed by DQE- and modulation transfer function (MTF) measurements supported by an observer study. The NIP systems reached DQE values up to three times higher than that of high-quality, state of the art CR scanners independent of the scanning principle. This allows a dose reduction by a factor of two to three without loss of image quality for both scanning systems. For high tube voltages, the variation of the phosphor layer thickness results in a DQE maximum at relatively large thicknesses. For lower tube voltages the DQE is less dependent on the layer thickness, reaching excellent values already at considerably lower thicknesses. Consequently, CR scanners can be adapted to different applications by using NIPs with different thicknesses. This could be easily realized for the cassette based system, but not for the flat-panel system with fixed IP. The latter demands a compromise with respect to the phosphor thickness, to yield superior image quality for all applications.

Optimization of a CR system comprising line-scanning and needle image plate technology with respect to examinations of extremities

A CR system based on line-scanning and needle image plate technologies (CsBr), was modified by means of an improved line light source for stimulation and a high aperture line optics for light collection. For comparison, two types of needle image plates were used, i.e. standard and high resolution plates. The image quality performance was investigated in terms of modulation transfer function, detective quantum efficiency and visual impression of both anthropomorphic and technical phantom images, with respect to application for X-ray examinations of extremities. MTF and DQE at beam quality RQA 3 were clearly improved by the modified scanning components. Further improvement especially with respect to MTF at spatial frequencies beyond 2 LP/mm was obtained via the high resolution image plate, but at reduced low frequency DQE values. The visual inspection of images of a high contrast resolution pattern and an anthropomorphic hand phantom revealed a clear preference for the modified system. The use of the high resolution image plate alone resulted in images with higher sharpness but increased noise. The best performance - at constant X-ray exposure dose - was delivered by the combination of improved optics components with the adapted needle image plate.