Influence of anisotropic conductivity on EEG source reconstruction: investigations in a rabbit model.

The aim of our work was to quantify the influence of white matter anisotropic conductivity information on electroencephalography (EEG) source reconstruction. We performed this quantification in a rabbit head using both simulations and source localization based on invasive measurements. In vivo anisotropic (tensorial) conductivity information was obtained from magnetic resonance diffusion tensor imaging and included into a high-resolution finite-element model. When neglecting anisotropy in the simulations, we found a shift in source location of up to 1.3 mm with a mean value of 0.3 mm. The averaged orientational deviation was 10 degree and the mean magnitude error of the dipole was 29%. Source localization of the first cortical components after median and tibial nerve stimulation resulted in anatomically verified dipole positions with no significant anisotropy effect. Our results indicate that the expected average source localization error due to anisotropic white matter conductivity is within the principal accuracy limits of current inverse procedures. However, larger localization errors might occur in certain cases. In contrast, dipole orientation and dipole strength are influenced significantly by the anisotropy. We conclude that the inclusion of tissue anisotropy information improves source estimation procedures.

Skull Defects in Finite Element Head Models for Source Reconstruction from Magnetoencephalography Signals.

Magnetoencephalography (MEG) signals are influenced by skull defects. However, there is a lack of evidence of this influence during source reconstruction. Our objectives are to characterize errors in source reconstruction from MEG signals due to ignoring skull defects and to assess the ability of an exact finite element head model to eliminate such errors. A detailed finite element model of the head of a rabbit used in a physical experiment was constructed from magnetic resonance and co-registered computer tomography imaging that differentiated nine tissue types. Sources of the MEG measurements above intact skull and above skull defects respectively were reconstructed using a finite element model with the intact skull and one incorporating the skull defects. The forward simulation of the MEG signals reproduced the experimentally observed characteristic magnitude and topography changes due to skull defects. Sources reconstructed from measured MEG signals above intact skull matched the known physical locations and orientations. Ignoring skull defects in the head model during reconstruction displaced sources under a skull defect away from that defect. Sources next to a defect were reoriented. When skull defects, with their physical conductivity, were incorporated in the head model, the location and orientation errors were mostly eliminated. The conductivity of the skull defect material non-uniformly modulated the influence on MEG signals. We propose concrete guidelines for taking into account conducting skull defects during MEG coil placement and modeling. Exact finite element head models can improve localization of brain function, specifically after surgery.

Evaluation of the distortion of EEG signals caused by a hole in the skull mimicking the fontanel in the skull of human neonates.

OBJECTIVE: Interpretation of Electroencephalography (EEG) signals from newborns is in some cases difficult because the fontanels and open sutures produce inhomogeneity in skull conductivity. We experimentally determined how EEG is influenced by a hole mimicking the anterior fontanel since distortion of EEG signals is important in neurological examinations during the perinatal period. METHODS: Experiments were carried out on 10 anesthetized farm swine. The fontanel was mimicked by a hole (12 x 12 mm) in the skull. The hole was filled with 3 types of medium differing in conductivity (air, 0 S/m; sucrose-agar, 0.017 S/m; saline-agar, 1.28 S/m). Three positions of the snout were stimulated with a concentric bipolar electrode to activate cortical areas near the middle, the edge, and the outside of the hole. The somatic-evoked potential (SEP) was recorded by a 4 x 4 electrode array with a 4mm grid spacing. It was placed on the 4 quadrants of a 28 x 28 mm measurement area on a saline-soaked filter paper over the skull, which served as artificial scalp. RESULTS: The SEP over the hole was clearly stronger when the hole was filled with sucrose- or saline-agar as compared to air, although paradoxically the leakage current was stronger for the sucrose- than saline-agar. The current leaking from the hole was strongly related to position of the active tissue. It was nearly negligible for sources 6-10 mm away from the border of the hole. The distortion was different for 3 components of the SEP elicited by each stimulus, probably indicating effects of source distance relative to the hole. CONCLUSIONS: EEG is strongly distorted by the presence of a hole/fontanel with the distortion specifically dependent on both conductivity of the hole and source location. SIGNIFICANCE: The distortion of the EEG is in contrast to the lack of distortion of magnetoencephalography (MEG) signals shown by previous studies. In studying brain development with EEG, the infant's head and sources should be modeled accurately in order to relate the signals to the underlying activity. MEG may be particularly advantageous over EEG for studying brain functions in infants since it is relatively insensitive to skull defects.