X-ray nanotomography (nano-CT)

About this technique

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X-ray nanotomography (namo-CT) takes X-ray tomography into new realms at the sub-micrometre scale. It is a non-destructive technique, because its aim is to image internal structure without the need to section or otherwise damage the specimen. This ability to see into the specimen is not limited to one particular view – X-ray nanotomography is naturally three-dimensional in its context – so the resulting data may be viewed and/or re-sliced at any angle to see the features of most interest.

The technique allows internal structural detail to be imaged either as 2-D slices or as 3-D rendered volumes. With specimen thickness in the order of 200 μm, 3-D sub-volumes of 64 μm cubed or 16 μm cubed can be imaged at resolutions of 150 nm or 50 nm respectively.

Although functionally similar to microtomography, the higher resolution achieved in nanotomography demands that particular attention be paid to the alignment of the projection images that are acquired as the specimen is incrementally rotated over 180° in steps of between 0.1° and 1°. To facilitate this alignment a 3 μm gold particle is placed on the specimen and subsequently used as a fiducial marker for correction of spatial misalignments. Following alignment of the projection image series, the projection data are subsequently converted into a stack of axial slices and viewed in 3-D.

Specimen preparation is a critical part of this process, with samples of somewhat less than 0.5 mm being mounted on steel pins for scanning. Typically a low-resolution micro-CT scan will be done as a preliminary assessment to establish the orientation of the sample and to find regions of interest for the higher resolution nano-CT scan. Scan durations are long, ranging from 12 hours for a simple preliminary scan to up to 168 hours (one week) for a very high-quality scan. In addition to the standard X-ray absorption mode of operation, the scanner can also be used in a phase-contrast mode that provides further enhancement of structural details, particularly at material boundaries.