Effects of inhomogeneities in cerebral modeling for magnetoencephalography

A current dipole, which is located in a homogeneous sphere and orients radially, produces no magnetic fields outside the sphere. This theory of null-field is widely used in understanding the magneto-encephalography. In the present report, effects of inhomogeneities in the head on the surface magnetic fields which are produced by radially oriented dipoles are investigated. It is shown that normal components of magnetic fields perpendicular to the surface of the sphere generated by radial dipoles are determined by volume currents. The flow patterns of volume currents are strongly affected by local inhomogeneities inside the body when the primary current source is located near the boundaries of inhomogeneous regions.