Improved forward EEG calculations using local mesh refinement of realistic head geometries.

A method for semi-automatically constructing realistic surface meshes of 3 head structures--scalp, skull and brain--from a stack of MR images is described. Then an evaluation is given for both spherical and realistic dipolar models, using the boundary element method (BEM). In both cases, locally refined models were considered. Two characteristic mesh parameters were defined: the global and the local mesh densities (in triangles per cm2). In spherical geometries, numerical and analytical solutions were compared, and in the realistic case, all models were compared to a highly refined one, considered as a reference. Both geometries gave comparable results. It was found that for "deep dipoles" located at more than 20-30 mm under the brain surface, meshes with a global density of 0.5 tri/cm2 gave "acceptable" results, whereas for more superficial dipoles (2-3 mm < depth < 20-30 mm), it was necessary to locally refine meshes near the source location up to a local density of about 5-8 tri/cm2, to get comparable results.