Quantitative observation of transport processes in soils with high-resolution PET

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-labelled compounds during their passage through decimetre-scaled material samples. We apply PET exclusively to geological 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 tracer concentrations to some 107 tracer atoms/µl and thus is ideally suited for direct flow and transport process observations in soils. 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. The installation of the PET scanner in our controlled area makes possible long-term experiments and the application of non-standard and long-living PET-nuclides (like 124I, decay time 4 days, and 58Co, decay time 71 days). The installation of a new cyclotron will also extend the availability of short-living PET-isotopes for fast process observations (e.g. 11C, decay time 20 min). Application of these nuclides extends the common radiopharmaceutical practice of labelling organic compounds with 18F (decay time 110 min).
The density of geomaterials may cause more than 50% of Compton-scattered events, which degrade the image quality. The quantification of the resulting artefacts is under way by Mont-Carlo model-based tools and will significantly empower the scatter-correction procedures.
Transport observation studies have been conducted on consolidated, partially fractured rocks and on soils, often showing more or less localized transport pathways. The effective volume and transport network, as well as the effective surface area for specific process conditions can be derived directly and quantitatively from the spatio-temporal tracer distribution in the sample, which can be visualized as 3D-movie, in contrast to indirect observations based on break-through curves at the endpoint of the sample. Comparisons with Lattice-Boltzmann simulations based on structural information obtained by µCT are indicating, that any actual transport field very much depends on elusive boundary conditions and therewith represents just one instance of a variety of possible spatio-temporal distributions with similar effective parameters.