Micro-CT scanners create 3D images of small specimens’ X-ray attenuation. However, when attenuation is low, or there is little difference in attenuation between adjacent features, such scanners may provide insufficient image contrast. X-ray Phase Contrast Imaging (XPCI), originally introduced in synchrotron systems, is a method of imaging the refractive index of the specimen and is a useful method when there is little attenuation contrast. Because phase is not directly detectable, most XPCI methods have to retrieve the phase from interference patterns that occur when relatively shifted rays combine. Edge illumination phase contrast X-ray imaging (EIXPCI, developed at UCL), however, is a method of directly detecting the refraction of the X-rays. This uses two X-ray masks comprising vertical bars with spaces between them. One mask is placed between the source and the specimen and the other between the specimen and the detector. The bar spacing of the detector mask is set such that the bars perfectly align with the detector pixels. For the source mask, the spacing is set to match the detector mask after allowing for geometric magnification. When the X-ray beams are refracted by a specimen, the alignment is effectively altered such that the detected image will vary in intensity according to the amount of refraction.
Although EIXPCI is simpler than interferometry methods, one disadvantage is the need to align the detector mask precisely with the detector pixels. Not only is this practically difficult to achieve, but it also means that the mask dimensions must be dictated by the pixel dimensions. Another disadvantage is that tomographic images are susceptible to mask defects and imperfections.
Equiangular time-delay integration (EATDI) X-ray imaging (developed at QMUL) is a method whereby the specimen is moved in an arc about the X-ray source whilst simultaneously reading out the CCD X-ray camera (a scintillator coated curved fibre-optic faceplate is used to convert the equiangular X-rays to equispatial light). By synchronising the specimen movement with the shifting of the image on the CCD, the recorded image averages out all defects and irregularities. If this method is used with EIXPCI, it will make the shadows of the masks disappear, eliminating the need to align the masks with the detector pixels and making it immune to mask defects. This will form an entirely new type of micro-CT scanner with improved imaging capabilities.
In this project, the feasibility of combining EIXPCI with EATDI well be tested by modifying the QMUL-designed EATDI scanner. The geometry of the system will be altered and the two X-ray masks and associated alignment systems will be added. It will then be tested on custom designed phantoms for characterisation of the imaging process, followed by tests with biological and cultural heritage samples of a type that yield poor results with conventional X-ray attenuation contrast imaging.