In recent years, x-ray micro-computed tomography (micro-CT) systems with amplitude modulated beams have gained global interest. These systems feature a modulator, that is, an x-ray opaque mask with periodically spaced apertures, in front of the sample, creating an array of spatially separated beamlets. The approach offers x-ray phase contrast imaging (XPCI), which improves the contrast-to-noise ratio and reveals the presence of sub-resolution inhomogeneities by capturing, respectively, refraction and ultra-small angle scattering (dark field signal) alongside x-ray attenuation. Additionally, the modulator can increase spatial resolution, as the narrow beamlets can transfer higher spatial frequencies without requiring geometric magnification. This brief communication reviews the working principle of the approach and comments on a remaining challenge (relatively long scan times).
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.
A rotating-anode x-ray source and custom-built sCMOS-based detector have been integrated into a lab-based micro-CT system to demonstrate full CT acquisition in as little as 132ms. This has been used to examine the expansion of a polymer foam in 4D, with a temporal resolution of 2Hz. The system is easily adapted to carry out fast phase-sensitive multi-contrast CT with sub-10s CT acquisition times. This is made possible through the beam-tracking technique, which is capable of multi-contrast CT using only a single shot per projection angle, while also being compatible with incoherent sources. This paves the way to dynamic, phase-sensitive, multi-contrast micro-CT in the laboratory.
A preliminary investigation into the use of cycloidal computed tomography for intraoperative specimen imaging is presented. Intraoperative imaging is applied in time-sensitive clinical settings, where obtaining a high-resolution, highquality image within minutes is paramount in evaluating the success of operations and/or the need for additional surgery. As a flexible imaging method that is compatible with x-ray phase contrast imaging, cycloidal computed tomography can provide both high spatial resolution and high image contrast, whilst keeping scan times short thanks to an effective under-sampling approach. To gather early evidence, the method was tested on resected breast and oesophageal tissue. The results, although preliminary, indicate that cycloidal imaging may indeed be beneficial for intraoperative specimen imaging, although further studies are required to confirm this potential.
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