Phase-contrast and dark-field computed tomography

Vergleich von Darkfield-Kontrasten
© Faserinstitut FIBRE e.V./Robert Gaizsch
Darkfield contrast in comparison: Left normalized divided to the absorption contrast and right with the sample tilted by 90°. The significant differences in brightness show the sensitivity of the darkfield to the local orientation of the carbon fibers.

Grid-based phase contrast and particularly the dark-field contrast contained within it offer the unique opportunity to map microscopic structure information (0.5 µm-50 µm) in large objects (up to 15 cm) two- and three-dimensionally. The main field of application is structural/damage analysis of fiber-reinforced plastic composites and testing micro-porous bio-implants.

Based on the X-ray Talbot interferometer, first presented in 2002, established micro-CT technology can be expanded for phase-contrast imaging. Here, microscopic incremental grids are used whose self-mapping is described by the Talbot effect and which are used in the interferometer for phase stepping. Instead of the usual X-ray weakening image three images form: 1. the absorption image, 2. the differential phase-contrast image, and 3. the dark-field image. In the last of these, the visibility loss caused by dissipation on microscopic limit surfaces and particles is coded. All three image contrasts can be used both for 2D component testing and for 3D micro-CT.




System solution Darkfield imaging

Together with Friedrich-Alexander-Universität Erlangen-Nürnberg, Fraunhofer EZRT is continuing to develop phase-contrast and dark-field imaging for applications in materials research/materials testing. Here, the usual micro-CT set-up is expanded by the method explained above with the aim of realizing a compact system that allows both micro-CT and phase-contrast imaging. Within the software, methods are developed such as orientation-dependent dark-field contrast, where imaging can be used on fiber-composite materials to reconstruct the orientation and density of the fibers vectorally in volume.

  • Damage and orientation analysis on carbon fiber-reinforced plastic components and other fiber-reinforced plastics
  • 2D and 3D testing of microporous bio-materials ( (TCP, HAp, Al2O3, ZrO2, Ti and Mg basis)
  • Food control
  • Security
  • Mammography (medicine)

  • Component size up to 15 cm (with stitching)
  • Phase-contrast set-ups both with rotary anode and micro-focus X-ray source (also Liquid Metal Jet)
  • Choice of flat panel (127 µm) or photon-counting detector (55 µm)
  • 25 kV design energy (Umax typ. 60-80 kV)
  • Visibility above 20%

Advantages and customer value

Compared with existing damage analysis of carbon fiber-reinforced components – ultrasound and thermography – dark-field imaging is much easier to manage and provides a local resolution many degrees of magnitude higher with a comparable detectability of defects. In the area of security, both differential phase contrast and dark-field contrast can detect structures and properties of materials that would appear harmless – or not appear at all – in a normal radiographic image. The same applies to the area of food control. The technology is not significantly more complicated or more expensive than state-of-the-art radiographic testing or micro-CT.