Visualization of Heterogeneous Diffusion Processes with PET aligned with 3D FE simulation results

Positron-Emission-Tomography (PET) provides directly the spatial distribution of tracer concentrations with ultimate sensitivity and thus is particularly suited for visualizing the propagation of chemical species in opaque media (Kulenkampff et al. 2013). The method of sequential PET-imaging of the tracer distribution for quantifying diffusion parameters of heterogeneous opaque materials overcomes fundamental problems of common analytical methods, which either are destructive or without spatial resolution. However, these parameters are crucial for the safety analysis of geologic waste repositories in clay (Van Loon et al. 2004), for instance.
We apply longer-living nuclides than in common clinical PET-applications, like ⁵⁸Co (T_1/2=70.86 d) and ²²Na (T_1/2=2.603 a), and we have to consider the higher density of our specimens, which causes significant attenuation and scattering of photon radiation. These attenuation and scattering effects of the material, as well as the effects of parasitic gamma-radiation of the non-standard PET-nuclides have been quantified with Monte-Carlo-Simulations and are corrected (Zakhnini et al. 2013).
For instance, Opalinus clay rock cores were charged with solutions labelled with the PET-nuclide, and then the tracer distribution was sequentially imaged with PET (Kulenkampff et al. 2015). We aligned the 4D data set with FE simulation results (Comsol Multiphyiscs) and obtained the anisotropic diffusion coefficients. The residuals from the optimized model are attributed to structural inhomogeneities and heterogeneous composition.