Simultaneous Acquisition of K-Edge Subtraction Images using a pnCCD

The recent developments in the semiconductor detector technology, in particular the pnCCD, makes it possible to measure energy and position of X-rays after scattering with the specimen at the same time. An interesting application for such a device is the measurement of a contrast medium in vivo or in vitro using the K-edge subtraction method. The pnCCD detectors developed at the semiconductor laboratory of Max-Planck-Institute are well suited for such applications. They show very high quantum efficiencies in the energy range between 1-15 keV and can be used in a single photon counting mode. The pnCCD used for these experiments comprises 256x256 pixels with a pixel size of 7505 mu m(2) and is equipped with a special analog frame buffer for high speed applications. The setup for measurements at the energy-dispersive EDR beamline at BESSY utilises a multilayer X-ray mirror under fixed angle of incidence in order to select at least two energy bands, one below and another above the K-edge of the contrast medium. Using the single photon counting mode of the pnCCD the image of the specimen can be measured below and above the K-edge simultaneously with optimum weighting functions, two energy bands can be selected from the continuous spectrum of an X-ray tube and it is shown to give simular results. This allows the quantitative and separate reconstruction of the distribution of contrast medium within the tissue images. First results and applications will be discussed in this paper.