Modelling of X-ray tube spot size and heel effect in Arion

X-rays produced in X-ray tubes originate from a focal spot on the target material. This spot is not infinitely small, but has a finite size. This finite size of the spot will affect the radiographic projections taken during X-ray Compted tomography. In order to simulate correct radiographic projections, this finite spot size needs to be taken into account during the simulations. This can be done by modelling a two dimensional profile of the spot and use this model to convolve the simulated radiographic projections simulated with an infinitely small spot size. A second effect, the heel effect that originates in directional X-ray tubes will also have an influence on the final projections. This effect can also be modelled and this model can be used to correct the simulated projections for this effect

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

Evaluation of the absorbed dose in X-ray microtomography

It is widely known that a sample receives a radiation absorbed dose during a CT-scan. Although this can have unwanted effects on the sample such as discolouration, little can be found in literature about the absorbed dose in micro-CT applications (except for small animal micro-CT). This research aims to validate the accuracy of dose simulations to be able to predict the dose before scanning the sample. Both Monte Carlo simulations with BEAMnrc and simulations with the in-house developed Setup Optimizer are compared with measurements with an ionisation chamber. The simulations nearly always underestimate the experimental values with a maximal deviation of 40%. In contrast the dose reduction after a layer of material obtained with the simulation programmes is relatively accurate

Arion: a realistic projection simulator for optimizing laboratory and industrial micro-CT

Optimal scanning conditions in a X-ray Computed Tomography scan are determined by the source and detector of the CT-scanner and composition, size and density of the sample. Because all these components have an energy-dependent behaviour, optimizing a CT scan is not straightforward. In order to ease this process a GPU-accelerated realistic projection simulator, Arion, is developed. Arion allows the user to simulate realistic radiographic projections for a certain geometry while taking into account the polychromatic behaviour of X-ray tube, detector and sample. This allows the user to produce realistic CT datasets that can be used to optimize the scanning conditions for a certain sample

Evaluation of the absorbed dose in X-ray microtomography

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