Aspects of X-ray microtomography equipment design

We have developed a high definition X-ray microtomography scanner using time-delay integration CCD readout mode, whereby the camera is moved through the X-ray "shadow" during simultaneous readout. This method eliminates ring artefacts in the reconstructed image and allows the recorded image to be larger than the CCD itself. To maximise dynamic range, the reflective coated scintillator was lens coupled (two back-to-back 50mm f1.2 camera lenses). Equivalent X-ray photons per pixel were derived from noise measurements in specimen-absent projections. This was typically 600,000 for a 10 second exposure (90kV, 200 LA, 25 cm source to camera). To quantify relay lens efficiency, this was re-measured at aperture settings from f1.2 to f11. The results closely fitted a model based on two noise sources (one from finite X-ray photons and one from finite light photons per X-ray photon), yielding an efficiency of 60% at f1.2. Although higher efficiency is desirable, this is a good compromise that avoids CCD saturation. This suggests that when using the more efficient direct fibre-optic coupling, a reflective scintillator coating may be undesirable, as the marginal increase in efficiency would not justify any loss of resolution or dynamic range. Correction for beam hardening is currently carried out using a 7 step Al wedge to measure experimental attenuation vs theoretical attenuation for monochromatic radiation. We intend to modify this method to improve accuracy in a more diverse range of materials. Dishing artefacts were decreased further by using a moving X-ray aperture to reduce scattered radiation.