ON THE INFLUENCE OF VOLUME CURRENTS AND EXTENDED SOURCES ON NEUROMAGNETIC FIELDS - A SIMULATION STUDY

The influence of volume currents on the magnetic field is an important question in magnetoencephalography since the spherical volume conduct or is still widely used for source localization. In theory, the magnet ic field of a radial dipole in a homogeneous sphere is zero. In realis tic models of the head, the field is suppressed when compared with a t angential dipole. To determine the influence of the volume currents, t his suppression ratio (magnetic field of the radial dipole divided by the field of the tangential dipole) needs to be quantified. Large-scal e finite element method models of the human head and the rabbit head w ere constructed and the suppression ratio was computed. The computed s uppression value of 0.28 in the rabbit head was similar to the previou sly measured experimental value. In the human head, an average suppres sion ratio of 0.19 +/- 0.07 was found for different regions and depths in the gray matter. It was found that the computed magnetic field of radial sources varied significantly with the conductivities of the sur rounding tissues where the dipole was located. We also modeled the mag netic field of an epileptic interictal spike in a finite element model of the rabbit head with a single dipole and with extended sources of varying length (1-8 mm). The extended source models developed were bas ed on invasive measurements of an interictal spike within the rabbit b rain. The field patterns of the small (1-2 mm) extended sources were s imilar to a single dipolar source and begin to deviate significantly f rom a dipolar field for the larger extended sources (6-8 mm).