During the projection of X-rays through an examination object, dependent upon the spectrum used and the materials to be penetrated, different absorption and scattering behavior occurs, wherein the low energy components in the spectrum are absorbed faster than the high energy components (radiation hardening). The X-ray radiation detected behind the examination object therefore contains information concerning the material properties of the examination object. In computed tomography systems (CT systems) with simultaneous detection of a plurality of spectra of the X-ray radiation, following the acquisition of projection data, image data sets can be reconstructed which contain information regarding the materials contained in the examination object. “Different X-ray energies” is to be understood as the detection of X-ray radiation with different X-ray energy spectra. This can be achieved, firstly, in that the X-ray source is operated with different voltages. Therefore, normally an X-ray energy is also simply denoted with the X-ray voltage (in kV) that has been set. Secondly, individual regions of the projected X-ray spectrum can be detected selectively in portions of a detector intended therefor. An energy pre-determined for the representation or evaluation must not necessarily correspond to the actually detected energy of the projected X-ray spectrum, but can also be determined freely by way of an interpolation within a particular range. Herein the image data sets should be reconstructed as free as possible from artifacts caused by the radiation hardening.
From the document DE 10 2005 008 767 A1, there is known a raw data-based correction method wherein from a number of different detected spectral regions of the X-ray radiation, an image can be reconstructed in which radiation hardening artifacts are substantially suppressed. For this purpose, for each material component a transmission thickness is determined as the measured value, wherein the different components have different energy-dependencies of the absorption coefficients. The transmission thicknesses are consequently summed weighted to a pseudomonochromatic measured value. From these pseudomonochromatic measured values, an image is then reconstructed in which the radiation hardening artifacts are substantially suppressed. For simplification of the image reconstruction, therefore, the spectrally different measured values for the material components are grouped together to a pseudomonochromatic measured value.
Another image data-based approach known from practice is the separate radiation hardening correction for each measured region of the X-ray spectrum. For this purpose, initially a first reconstruction is undertaken and a base material is selected. In the first reconstruction, with the aid of a segmentation, those voxels which have admixtures of other base materials are identified. Although this succeeds in reducing the radiation hardening artifacts, the accuracy of the quantitative representation of the material properties (attenuation contributions or proportions) is restricted so that the images with reduced radiation hardening do not necessarily correspond to a consistent monochromatic image. This limitation applies particularly to the separation of more than two materials. Furthermore, variations of the spectrum over the field of view, for example, through shaping filters cannot be taken into account in practice.