Electroencephalographic source imaging: a prospective study of 152 operated epileptic patients

Electroencephalography is mandatory to determine the epilepsy syndrome. However, for the precise localization of the irritative zone in patients with focal epilepsy, costly and sometimes cumbersome imaging techniques are used. Recent small studies using electric source imaging suggest that electroencephalography itself could be used to localize the focus. However, a large prospective validation study is missing. This study presents a cohort of 152 operated patients where electric source imaging was applied as part of the pre-surgical work-up allowing a comparison with the results from other methods. Patients (n = 152) with >1 year postoperative follow-up were studied prospectively. The sensitivity and specificity of each imaging method was defined by comparing the localization of the source maximum with the resected zone and surgical outcome. Electric source imaging had a sensitivity of 84% and a specificity of 88% if the electroencephalogram was recorded with a large number of electrodes (128–256 channels) and the individual magnetic resonance image was used as head model. These values compared favourably with those of structural magnetic resonance imaging (76% sensitivity, 53% specificity), positron emission tomography (69% sensitivity, 44% specificity) and ictal/interictal single-photon emission-computed tomography (58% sensitivity, 47% specificity). The sensitivity and specificity of electric source imaging decreased to 57% and 59%, respectively, with low number of electrodes (<32 channels) and a template head model. This study demonstrated the validity and clinical utility of electric source imaging in a large prospective study. Given the low cost and high flexibility of electroencephalographic systems even with high channel counts, we conclude that electric source imaging is a highly valuable tool in pre-surgical epilepsy evaluation

Distributed, high-resolution modelling of 18O signals in a meso-scale catchment

A combined simulation of runoff dynamics and distributed transport of the stable isotope oxygen-18 (18O) was performed in the mountainous Dreisam basin (258 km2) in the southwest of Germany. For this purpose, a mixing cell solute routing scheme was implemented in the fully distributed, process-orientated catchment model TACd. Perfect mixing in the catchment reservoirs was assumed in order to keep the model simple. In addition, effects like fractionation, diffusion into and out of immobile phases, as well as kinematic effects were neglected. Although results show a generally good agreement between simulated and measured concentrations of 18O in stream discharges, we discovered that the initial objective of validating the process representation of TACd by incorporating conservative solute transport cannot be achieved with the given data. Simulation misfits cannot be clearly attributed to process descriptions, mainly due to large uncertainties in the input regionalization of precipitation and 18O, and due to the lack of data for model initialization. Nevertheless, we critically evaluate the model’s conceptualization and parameterization, in order to deliver insight to the potential and limitations of distributed modelling of 18O signals on the meso-scale. In addition, the large influence of surface runoff generated on a small fraction of the total catchment area, and fast interflow components on the 18O values in total stream discharge could be demonstrated for the investigated events. This paper documents the first attempt to simulate the distributed 18O balance in a mesoscale catchment with temporal resolution on an hourly basis in order to optimize future measurement campaigns and modelling attempts

Effect of mineral reactions on the hydraulic properties of unsaturated soils: Model development and application

The selective radius shift model was used to relate changes in mineral volume due to precipitation/dissolution reactions to changes in hydraulic properties affecting flow in porous media. The model accounts for (i) precipitation/dissolution taking place only in the water-filled part of the pore space and further that (ii) the amount of mineral precipitation/dissolution within a pore depends on the local pore volume. The pore bundle concept was used to connect pore-scale changes to macroscopic soil hydraulic properties. Precipitation/dissolution induces changes in the pore radii of water-filled pores and, consequently, in the effective porosity. In a time step of the numerical model, mineral reactions lead to a discontinuous pore-size distribution because only the water- filled pores are affected. The pore-size distribution is converted back to a soil moisture characteristic function to which a new water retention curve is fitted under physically plausible constraints. The model equations were derived for the commonly used van Genuchten/Mualem hydraulic properties. Together with a mixed‐form solution of Richards’ equation for aqueous phase flow, the model was implemented into the geochemical modelling framework PHREEQC, thereby making available PHREEQC’s comprehensive geochemical reactions. Example applications include kinetic halite dissolution and calcite precipitation as a consequence of cation exchange. These applications showed marked changes in the soil’s hydraulic properties due to mineral precipitation/dissolution and the dependency of these changes on water contents. The simulations also revealed the strong influence of the degree of saturation on the development of the saturated hydraulic conductivity through its quadratic dependency on the van Genuchten parameter alpha. Furthermore, it was shown that the unsaturated hydraulic conductivity at fixed reduced water content can even increase during precipitation due to changes in the pore-size distribution