Direct quantitative observation of transport processes with Positron-Emission-Tomography


Direct quantitative observation of transport processes with Positron-Emission-Tomography

Kulenkampff, J.; Gründig, M.; Wolf, M.; Lippmann-Pipke, J.; Richter, M.; Enzmann, F.

Positron-Emission-Tomography (PET) enables direct and quantitative monitoring of the spatio-temporal distributions of dissolved inert and/or reactive PET-nuclides and PET-nuclide-labeled colloids during their passage through decimeter-scaled material samples. We apply PET exclusively to geomaterial samples and reach the physical resolution limit of about 1 mm with our small-animal-PET scanner (ClearPET, Raytest). We suggest our GeoPET has unrivalled sensitivity and selectivity for our tracer concentrations to some 107 tracer atoms/µl and thus is ideally suited for direct flow and transport process observations in geomaterials. This lower limit of the tracer concentration in the order of about 1 kBq/µl outranges other process observation methods (e.g. NMR or resistivity tomography) by many orders of magnitude. Like in the common medical practice, a combination with µCT for structural imaging would be advantageous for improving the spatial significance.
In the past we demonstrated the feasibility of the method, applying in-house developed and medical PET-scanners (Richter et al., 2003, Gründig et al., 2007). The installation of ClearPET in our controlled area made possible long-term experiments, like diffusion of long-living PET-tracers (like 58Co, decay time 71 days) and flow observations over several days (with 124I-labelled compounds, decay time 4 days) (Kulenkampff et al. 2008, Wolf 2010). The installation of a new cyclotrone will extend the availability of short-living PET-isotopes for fast process observations (e.g. 11C, decay time 20 min).
The density of geomaterials may cause more than 50% of Compton-scattered events, which degrade image quality mainly by inhomogeneous reconstruction artifacts. These artifacts caused explicitly by geomaterials (and only insignificantly in medical applications) are currently being addressed by model-based scatter-correction procedures.
Application examples include the fluid flow visualization in saliniferous rock cores. Different types of flow regimes are identified: in rather dense and low porous rock samples we observe slowly propagating diffuse “clouds of tracer”, whereas in fractured halite and sandstone samples (Fig. 1) we identified networks of extremely localized pathways, each with locally varying propagation velocities (Fig. 2). Quantitative parameterization of pathway and velocity distributions is possible, but still pending. However, with GeoPET-process observations we are capable of evaluating the results of Lattice-Boltzmann simulations, based on structural information from voxel-wise segmented µCT of the same sample. With respect to localized pathways in the fracture system of the halite, the model yields comparable results.
Nevertheless, even the high resolution of µCT may be insufficient to visualize the pore-space characteristics in tighter material, and more complex compositions may impair segmentation results from µCT-images. Then, stochastic models which are generated from other information, like pore-size distributions (with missing network information) and integral transport parameters (with missing distribution information), can still be validated by comparing model results with GeoPET-observations.

Keywords: PET; transport; tracer; geomaterial; tomography

  • Lecture (Conference)
    Transport in porous materials, 19.-20.8.2010, Villigen, Schweiz

Permalink: https://www.hzdr.de/publications/Publ-14176
Publ.-Id: 14176