Strategies for the reduction of environmental scattering in X-ray Computed Tomography

Absorption computed tomography (CT) relies on the imaging of X-ray radiation transmitted through a specimen in different orientations. For photon energies exceeding 100 keV, a significant fraction of the X-ray radiation is scattered by the object and the environment (components of the instrument, detector, radiation protection, etc.). As the scattered radiation blurs the transmission images, the uncorrected CT-data will not represent the geometry of the sample accurately. Furthermore, small structures or defects may be unperceivable in the CT images. In the literature, the effect of radiation scattered by the object has been examined extensively. In this work, we focus on the quantitative assessment of the scattering from the environment. A simulation model for an industrial X-ray CT scanner based on the GEANT4 frame work has been developed and verified experimentally. This model permitted to study quantitatively the influence of the individual components of the scanner system on the intensity and distribution of the scattered radiation. An optimised setup has been implemented experimentally. A comparison of CTmeasurements on the optimised setup and on the former non-optimised one shows a significant improvement of the contrast. Besides instrumentation aspects, we investigated the influence of two measurement parameters (source collimation and source-object distance). Our results indicate that the scattering from the environment strongly depends on the area illuminated by the X-ray beam and may even for the optimised setup dominate object scattering. These findings allow formulating guiding principles in the design of new instrumentation and measurement procedures.