X-ray microtomography

X-ray microtomography measurements were carried out at Diamond Light Source on the Diamond-Manchester Imaging Branchline I13-2, using a monochromatic X-ray beam (energy 13 keV and bandwidth of 0.1 eV). This enabled in-line phase contrast imaging with a parallel-beam setup where attenuation and refractive effects are decoupled to image interfaces between materials of similar attenuation²¹. Projection images were collected using a pco.4000 CCD camera (pixel size 11 µm × 11 µm) coupled with a 4 × objective lens. A further 2 × magnification given by the beamline setup (total magnification of 8×) gave an effective pixel size of 1.12 µm (2.24 µm scaled with 2 × binning) and a maximum field of view of 4.5 mm × 3.0 mm. A single scan comprised 1800 projections around a 180° rotation of the sample with an exposure time per projection of 1 s. Reconstruction of the set of normalized projections was performed using a filtered backprojection algorithm²² to produce 3D volumetric datasets of X-ray attenuation consisting of a sequence of evenly spaced slices of thickness 1 pixel. For image analysis and interpretation, a single-distance phase backpropagation algorithm²³ was applied to the projections to reduce interference fringe effects and to enhance contrast between the matrix material and the capsule contents. In order to quantify the filled/healed crack and the filled capsules a 3D median filter, with a 26-voxel neighbourhood, was first applied to the original X-ray tomographic reconstructions to reduce the background noise. The different material components in each tomographic slice were then segmented using greyscale thresholding. Image processing, segmentation and visualization were performed using Avizo software (from ThermoFisher Scientific).