title: Comparison of three-shell and simplified volume conductor models in magnetoencephalography.
abstract: Experimental MEG source imaging studies have typically been carried out with either a spherically symmetric head model or a single-shell boundary-element (BEM) model that is shaped according to the inner skull surface. The concepts and comparisons behind these simplified models have led to misunderstandings regarding the role of skull and scalp in MEG. In this work, we assess the forward-model errors due to different skull/scalp approximations and due to differences and errors in model geometries. We built five anatomical models of a volunteer using a set of T1-weighted MR scans and three common toolboxes. Three of the models represented typical models in experimental MEG, one was manually constructed, and one contained a major segmentation error at the skull base. For these anatomical models, we built forward models using four simplified approaches and a three-shell BEM approach that has been used as reference in previous studies. Our reference model contained in addition the skull fine-structure (spongy bone). We computed signal topographies for cortically constrained sources in the left hemisphere and compared the topographies using relative error and correlation metrics. The results show that the spongy bone has a minimal effect on MEG topographies, and thus the skull approximation of the three-shell model is justified. The three-shell model performed best, followed by the corrected-sphere and single-shell models, whereas the local-spheres and single-sphere models were clearly worse. The three-shell model was the most robust against the introduced segmentation error. In contrast to earlier claims, there was no noteworthy difference in the computation times between the realistically-shaped and sphere-based models, and the manual effort of building a three-shell model and a simplified model is comparable. We thus recommend the realistically-shaped three-shell model for experimental MEG work. In cases where this is not possible, we recommend a realistically-shaped corrected-sphere or single-shell model.
authors.affiliation: Aalto University, Department of Biomedical Engineering and Computational Science, P.O. Box 12200, FI-00076 Aalto, Finland; MRC Cognition and Brain Sciences Unit, 15 Chaucer Road, Cambridge CB2 7EF, UK. Electronic address: matti.stenroos@aalto.fi.
authors.forename: Matti
authors.initials: M
authors.lastname: Stenroos
authors.affiliation: Ilmenau University of Technology, Institute of Biomedical Engineering and Informatics, P.O. Box 100565, D-98684 Ilmenau, Germany.
authors.forename: Alexander
authors.initials: A
authors.lastname: Hunold
authors.affiliation: Ilmenau University of Technology, Institute of Biomedical Engineering and Informatics, P.O. Box 100565, D-98684 Ilmenau, Germany.
authors.forename: Jens
authors.initials: J
authors.lastname: Haueisen
mesh_terms: D000465:Algorithms; D000818:Animals; D001921:Brain / Q000502:physiology*; D001931:Brain Mapping / Q000379:methods*; D003198:Computer Simulation; D003217:Conductometry / Q000379:methods*; D004553:Electric Conductivity; D006801:Humans; D015225:Magnetoencephalography / Q000379:methods*; D008959:Models, Neurological; D009415:Nerve Net / Q000502:physiology*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
keywords: Boundary element method; Forward model; Magnetoencephalography; Source imaging; Spherical model