X-ray microtomography (µCT)
High-resolution x-ray tomography (µCT) has become the predominant method for porous media characterization. X-ray tomography yields the three-dimensional spatial distribution of mass attenuation coefficients, which are directly related to electron density and thus bulk density of the material. This information can be segmented into pore space and matrix components of different densities. Most comprehensive image parameters (e.g. pore size and grain size distributions, shape parameters, fracture width distribution, tortuosity, anisotropy) are then derived with statistical 3D image analysis methods. The spatial distribution of the components can also provide the substructure for geochemical transport simulations on the pore scale. Image segmentation is limited by resolution and contrast; therefore µCT-scanners are optimized to accomplish both high spatial resolution in the micrometre range and below, as well as high contrast.
In the HZDR department Reactive Transport, µCT is used in combination with positron emission tomography as GeoPET/µCT, like PET/CT in nuclear medical applications. In this combination, spatial resolution of µCT is a less significant feature, because PET data correction and fusion of coregistrated PET and CT density or porosity images require PET-resolution in the order of 1 mm.
µCT applies a microfocus or even nanofocus x-ray source with a small spot size. A large number of attenuation images (cone-beam projections) of the rotating sample are recorded with a detector screen. A three-dimensional tomogram, which can be interpreted as density image, is then reconstructed from these projections.
The HZDR department Reactive Transport uses a Nikon XTH 225 scanner that is suited for the study of common GeoPET samples with a diameter up to 100 mm.
- Open microfocus x-ray source, power 225 kV at max. 1 mA permanently, minimum focal spot size 3 µm at 80 kV, 3 W.
- Varian 4030, size 400×300 mm, 2000×1500 pixels, effectiv 14 bit
- max. field of view: D=285 mm, H=225 mm
- max. resolution at 100 mm sample diameter ca. 30 µm
- max. resolution at 20 mm sample diameter ca. 6,5 µm
- interior space: L×W×H=990×500×890
- Rekonstruktion Nikon Metrology CT Pro 3D
- Presentation, segmentation, registration and 3D image analysis: FEI Avizo for Materials Science
- Segmentation and FEM-meshing: Simpleware ScanIP
GeoPET process tomography during a conservative flow experiment with a [18F]KF solution through a gravel pack, fused with the µCT image (column length 100 mm, diameter 50 mm, still image after complete injection of the tracer pulse of 5 mL, 60 min after start of injection).