X-ray microtomography (micro-CT)

About this technique

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X-ray microtomography (micro-CT) has emerged over the last few years as a leading frontier in characterisation across almost the entire range of medical, science and engineering disciplines. This form of microscopy is often referred to as non-destructive, since its aim is to image internal structure without the need to section or otherwise damage the specimen. Since this ability to see into the specimen is not limited to one particular view X-ray microtomography is naturally three-dimensional in its context.

This technique is based on a similar principle to medical CT-scanning, except with a much higher resolution. The specimen is incrementally rotated over 180° or 360° in a beam of X-rays in steps of between 0.1° and 1°. A series of projection images are digitally acquired at each rotational position that map the absorption of the X-rays in the specimen at that angle. This series of projection images are subsequently processed via a technique called filtered-back-projection to generate a stack of individual slices that include both the surface detail, as well as internal structural detail. These slices are referred to as the axial slices since they are perpendicular to the axis of specimen rotation in the original scan. The axial slices can be viewed individually or, since they are spatially aligned, a specialised visualisation software can be used to produce an interactive 3-D rendering that effectively reconstructs the original sample in a computational space that supports whole object viewing, cutting in any direction, animation, and analysis of individual components within the specimen. The technique allows internal structural details to be revealed in a broad range of materials but is particularly well suited to porous specimens or materials where there is a large density difference between adjacent structural components.

Differential staining of components within the samples can enhance density differences and therefore extend the range of samples that can be usefully imaged. The available resolution is a function of sample size and density but in general ranges from about 15 µm down to 1.5 µm. Sample size is usually in the order of 1 mm up to 10 mm in diameter, but larger samples can be used if the average density is not too large.

As well as modeling the structure of the material it is also possible to simulate deformation and failure of structures using measured or standard data. In the material sciences there is a constant need to validate simulation with physical measurement, and vice versa. However, there are a limited range of tools for the specific materials problem of calculating the transport and mechanical properties directly from tomograms. These are typically computationally intensive. Tool boxes include multi-phase segmentation, finite element modelling, fluid flow, conduction, morphological measures/filtering as well as the ability to perform direct image-based registration in which the tomograms of successively disturbed (dissolved, fractured, cleaned, etc) specimens can be correlated back to an original undisturbed state in 3-D.