Realistic CT image simulation tools for laboratory based X-ray CT at UGCT

In laboratory based X-ray Computed Tomography (CT), the grey values in the resulting CT image depend on several scanning conditions such as the emitted spectrum, the response characteristics of the detector and beam filtration. Furthermore, due to beam hardening also the morphology and composition of the sample itself will have a significant influence. Therefore, to optimise scanning conditions simulations which incorporate all factors determining the imaging process are required. In this paper, two programs developed at the Centre for X-ray Tomography of the Ghent University (UGCT) are presented which allow a complete and realistic simulation of the obtained CT image

Characterizing analogue caldera collapse with computerized X-ray tomography

Analogue models of caldera collapse were imaged by computerized X-ray tomography (μCT). Interval μCT radiography sequences document ‘2.5D’ surface and internal model deformation in an unprecedented way, and carry the potential for a better understanding of the kinematics of various volcano-tectonic processes, of which caldera collapse is a mere illustration. A semi-automatic subsidence velocity analysis was carried out on radiographs. The developed method is a step towards the quantitative documentation of volcano-tectonic modelling that would render data interpretations immediately comparable to monitoring data available from recent deformation at natural volcanoes

Modifications of iterative reconstruction algorithms for the reduction of artefacts in high resolution X-ray computed tomography

X-ray Computed Tomography is a non destructive technique which allows for the visualization of the internal structure of complex objects. Most commonly, algorithms based on filtered backprojection are used for reconstruction of the projection data obtained with CT. However, the reconstruction can also be done using iterative reconstructions methods. These algorithms have shown promising results regarding the improvement of the image quality. An additional advantage is that these flexible algorithms can be modified in order to incorporate prior knowledge about the sample during the reconstruction, which allows for the reduction of artefacts. In this paper some of these advantages will be discussed and illustrated: the incorporation of an initial solution, the reduction of metal artefacts and the reduction of beam hardening artefacts

Modelling scattering contributions in X-ray micro-CT scanners with variable geometry

It is commonly known that scattered X-rays (both Compton- and Rayleigh scattering) produce a disturbing contribution to the projection images taken during a CT-scan. In medical CT a scatter-grid and collimators are used to decrease the contribution of scatter. In high resolution micro-CT scanners such as those at UGCT, the "Centre for X-ray Tomography" of the Ghent University, this approach is not possible because of the inherent structure of the used detector systems (e.g. flatpanels) and the highly variable geometry of the scanners. At UGCT a wide variety of samples is scanned, requiring different geometries. Very small samples are positioned close to the X-ray source, while larger samples have to be positioned close to the detector. The samples also have a wide range of densities, from organic material, such as apples, to geological stones and metals. For several reasons it is important to determine the specific amount of scattered X-rays that reach the detector in micro-CT. This amount is dependent on the distance between the object and the detector, the composition and size of the sample… . Also the geometry of the scanner and the kind of X-ray source (pencil beam, parallel beam or cone-beam) can have a relevant effect. As such, every different sample in its optimal geometry will cause a different amount of scattered photons reaching the detector plane. To study the characteristics of scattered radiation, the Monte Carlo based simulation program BEAMnrc is used . BEAMnrc is based on the EGS-code developed for coupled transport of photons and electrons . In BEAMnrc each photon can be ‘followed’ during the complete simulation. For each photon tallied at the detector plane one can determine whether this photon has scattered in the sample or not, which yields the number of scattered photons, next to the number of unscattered photons. The final goals of this research are to add a scatter-tool in our set-up optimizer and to be able to correct projection images for the scattering contribution. The used methodology and obtained results of this work will be presented

Optimization of scanner parameters for dual energy micro-CT

Two materials of different composition can have very similar grey values in an X-ray Computed Tomography (CT). This is because X-ray CT uses polychromatic sources in combination with energy-integrating detectors and the materials have a mass attenuation coefficient that is dependent on composition and photon energy. A distinction between different materials with similar grey values can be made by combining information from scans performed with different spectra, which can be achieved by varying the tube voltage and filtration. However, the polychromatic behaviour of laboratory based X-ray CT complicates the choice of the appropriate scanning conditions for such dual energy methods. Here, the programme Arion, for simulating realistic radiographic projections is used to determine optimal scanning parameters