Localization of a dipolar source in a skull phantom: realistic versus spherical model.

The dependence of the neuromagnetic source localization accuracy on the volume conductor model was studied by the analysis of measured magnetic fields generated by tangentially oriented dipoles in a realistically shaped skull phantom. When using a homogeneous sphere model in the localization procedure, the errors were found to increase from about 3 mm to about 9 mm when the distance between the dipoles and the inner surface of the skull increased from 1 cm to 3 cm, whereas when using a true, realistic model in the inverse procedure the localization errors were only about 2-3 mm, independent of dipole depth. To account for the realistic geometry of the inner surface of the skull, the Boundary Element Method, based on a surface discretization in terms of about 300 triangles, proved to be sufficient. In addition to these analyses of experimental data, simulations were carried out to study the localization errors in the case of the spherical approximation for a dipole orientation changing from tangential to radial. For the latter orientation, errors of up to a few centimeters were found.