The beamline ID19 is mainly devoted to X-ray imaging. Across a specimen, the various implemented techniques detect the spatial variations of features that affect the X-ray beam, viz. absorption, scattering, or the optical path-length.

X-ray computed microtomography

X-ray computed microtomography (µCT) consists in recording a number of projections from an object, with different angle of views, and reconstructing from these projections a 3D image with the help of an adapted algorithm. The ESRF allows improvements and new possibilities for µCT, because of :

  • a high photon flux in a homogeneous, parallel, monochromatic, and highly coherent beam (polychromatic, divergent and incoherent on laboratory setups);
  • recording of 'phase images' obtained by adjusting the sample to detector distance ('propagation technique');
  • adapted detector (FRELON camera, already described);
  • implementation of efficient reconstruction procedures.

The parallel and monochromatic beam allows to record absorption images and reconstruct the volume with an improved spatial resolution (up to 1 µm) of bones, foams, building materials, metallic alloys, rocks ... The present evolutions are :

  • to perform quantitative processings taking benefit of the monochormaticity of the beam (e.g. bone densitometry);
  • the implementation of 'local tomography', i.e. the high resolution reconstruction of a region of interest within a matrix, which is only reconstructed at a low resolution. This is a crucial point for many applications where it is not possible to extract a small sample from its matrix.

The use of coherence led to the development of 'phase' tomography, which dramatically extends the possibilities and application range of the method. Phase tomography is a unique technique to investigate features like holes in bulk quasicrystalline grains [10] composite materials where the various components display very similar X-ray attenuation, or damage assessment in the bulk of a non transparent material. The images are often recorded in the 'edge detection' regime, a case where the usual filtered back-projection algorithm used for absorption tomography appears to be acceptable approximation.

Another, more quantitative, approach is the retrieval, from a set of images recorded at several distances, of the optical phase. Algorithms, developed for electron microscopy, were successfully adapted to the X-ray case. The phase retrieval, combined with tomography (‘holotomography’ technique [12]), leads to high quality quantitative 3D results.