Artefacts from off-focal X-rays in tomography of highly attenuating objects

Abstract

At CTLab we have recently commissioned a 300keV x-ray tomography system capable of imaging metallic parts and rock cores with diameters over 100mm. This new imaging regime manifests artefacts in the reconstructed tomographic volumes that resemble those from incoherent x-ray scatter from the object. However, through the experimental investigation outlined herein, we have determined these artefacts to be primarily due to off-focal, or secondary, x rays emitted from the x-ray source. All micro-focus x-ray sources tested, (both transmission and reflection targets), exhibited off-focal x rays to a greater or lesser extent (6 different x-ray source models from 4 different manufacturers). We also demonstrate that off-focal x rays may have received limited attention to date since the artefacts they cause are only severe for highly attenuating samples. Here our aim is to fully understand and characterise this phenomenon in order to incorporate it into the imaging model and mitigate the associated artefacts in computed tomography.