Dipole estimation errors in EEG source localization due to not incorporating anisotropic conductivities of white matter in realistic head models

The electroencephalogram (EEG) is a useful tool in the diagnosis of epilepsy. EEG source localization can provide neurologists with an estimation of the epileptogenic zone. Many EEG source localization approaches assume head models with isotropic conductivity, while in reality the conductivity of white matter is anisotropic. The conductivity along the nerve bundle is higher than the conductivity perpendicular to the nerve bundle. Using diffusion weighted magnetic resonance images (DW-MRI), we can determine the directions the anisotropic diffusion. Using the latter we can derive the anisotropic conductivity tensor. These anisotropic conductivities can be fused with the realistic head model, derived from MR images. Using a grid of dipoles, placed in white and grey matter regions, we can compare the head model with white matter anisotropy with a head model with isotropic conductivity for the white matter compartment. As quantification measures we used the dipole location and orientation error. Results show that the location error was very small in both white and grey matter regions (≪ 5 mm). The dipole orientation error had a mean of 3.8 degrees and 6.1 degrees in grey and white matter regions. This would indicate that the systematical error due to not incorporating anisotropic conductivities of white matter is very small.