Spatial patterns of magnetic fields produced by multiple dipoles in an inhomogeneous volume conductor

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Abstract
This study is focused on the influence of inhomogeneities in the head upon surface magnetic fields. The effects of combinations of dipoles and an inhomogeneous region on spatial distributions of magnetoencephalograms (MEGs) are investigated. The head is assumed to be a spherical volume conductor, in which there is a small sphere in the frontal region. That is assumed to be a brain lesion such as a brain tumor. Magnetic field components perpendicular to the surface of the sphere are calculated, and spatial distributions of MEGs and EEGs are simulated. Two or four current dipoles are used as electrical sources. Two dipoles are positioned in parallel and in the opposite directions. Four dipoles are positioned to form a closed loop. These types of electrical sources generate very small electric fields on the sphere, whereas the closed current loop and opposing dipoles act as equivalent magnetic dipoles which generate high magnetic fields. When a single dipole is located near a boundary of inhomogeneous region, surface magnetic fields are strongly influenced by volume currents which flow on the inhomogeneous boundary. In contrast, when the opposing or circular current dipoles exist near a boundary of an inhomogeneous region, magnetic fields are not almost affected by the inhomogeneous boundary.