Tomography – 3D imaging

3D imaging of solid samples, often with resolution in the millimetre to nanometre range; the technique is similar to CT (computerised tomography) scanning, used for medical purposes, but the resolution is much higher. This requires specialised equipment.

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3D animation of tomography data from a bryozoan limestone from Denmark

Laboratory X-ray Microtomography (µCT)

Information obtained

  • 3D greyscale image of the internal structure of a sample for visualization or input into computer simulations
  • Postprocessing, uses image analysis software to determine porosity, surface area, connectivity, etc.
  • Importing the 3D structure into simulation software to derive mechanical, fluid and heat flow, electrical and other properties

Principle

The sample is irradiated with X-rays from a conventional laboratory X-ray source. A 2D detector, placed behind the sample, registers the transmitted X-rays leading to a 2D projection of the internal structure of the sample, as in a medical X-ray radiograph. By rotating the sample through many small increments, a large number of projections is collected, from which the 3D structure can be reconstructed. The greyscale values in the resulting 3D image represent the relative X-ray absorption coefficient and thus the relative density of the material at every point or voxel (volume pixel). Best achievable resolution of the image is usually ~1 µm on state-of-the-art instruments.

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Tomograms reconstructed from X-ray tomography data from a laboratory source showing the front (where the spines attach) and the back of a sea urchin shell.

Synchrotron Radiation Nanotomography (nCT)

Information obtained

  • the same as above but much higher resolution and faster data acquisition.

Principle

Mostly the same principle as above but at far higher resolution, by using the high brilliance (high number of X-ray photons with the same wavelength) of synchrotron radiation. The geometry of the beam-sample-detector can be changed to optimise the type of data collected (coherent, noncoherent, ptychography, etc.).

Special features

Holotomography uses X-ray adsorption and phase information to enhance contrast. Ptychography uses X-ray diffraction information and reaches the highest resolution currently achievable (~10 nm).

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Synchrotron radiation tomography data of a bryozoan limestone from Denmark in two levels of resolution (left: 100 nm and center: 25 nm voxel size)