Many fields, from aerospace engineering to cultural heritage, can benefit from x-ray micro computed tomography (micro-CT). However, access to x-ray imaging tools remains limited for non-expert users. The UK’s National X-Ray Computed Tomography facility (NXCT) therefore aims to provide access and expert support to academia and industry. As part of the NXCT, at UCL we have developed a unique user facility with multi-scale and multi-contrast x-ray micro-CT capabilities. Our custom system has an x-ray generator with Molybdenum and Copper targets, which can be changed to adapt the energy to the needs of an imaging experiment. The x-rays are emitted on both sides of the source allowing for two imaging stations: one at mm-sized field-of-view (FOV) with resolutions of around 1μm, the “high-resolution station”; and one at cm-sized FOV with resolutions of around 10μm, the “large FOV station”. The high-resolution station is fitted with a custom mirror which gives a monochromatic beam at 17.5keV (for Mo) and 8keV (for Cu). Both stations can be operated with phase-contrast methods such as free-space propagation or beam tracking. Access to this new imaging facility, dedicated to academic and industrial users, is supported through free-at-the-point-of-access and paid schemes.
Tissue imaging is a pivotal component of both biomedical research and clinical practice. In order to identify tissue structures down to the cellular level, it requires the capability to image mm-size unstained tissue specimens with micron to sub-micron resolution. Tissue imaging is normally performed either using x-rays or visible light. While the latter is limited by light scattering in relatively thick tissues, the former often suffers from poor contrast in absorption-based systems. Phase-contrast x-ray microscopes exist but they often lack the required quantitativeness, entail acquisition times of the order of tens of hours for 3-D imaging and are limited to narrow fields of view. We propose a novel multi-modal phase-based x-ray microscope capable of imaging mm-thick tissue samples on a mm-size field of view using intensity-modulation masks. They act as optical elements allowing the quantitative retrieval of tissue properties such as transmission, refraction and scattering. Additionally, given that the system’s spatial resolution depends only on the mask aperture size, a multi-resolution approach is possible by selecting masks with aperture size matching the resolution requirements (micron and sub-micron) of specific samples. The design and optimization of the x-ray microscope is presented in this paper together with exemplar images of a thin foam sample resulting from the retrieval of the three contrast channels. The final paper will include details of the system parameter optimization (e.g., propagation distance, mask aperture and period), their effect on the retrieval algorithm and imaging performance as well as the first images of biological samples.
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