On the magnetic field distribution generated by a dipolar current source situated in a realistically shaped compartment model of the head

Abstract

SummaryThe magnetic field distribution around the head is simulated using a realistically shaped compartment model of the head. The model is based on magnetic resonance images. The 3 compartments describe the brain, the skull and the scalp. The source is represented by a current dipole situated in the visual cortex. The magnetic field distribution due to the source and that due to the volume currents are calculated separately. The simulations are carried out in order to ascertain which matrix of grid points is suitable as a measuring grid. The possibilities studied are grid points situated in a plane, in a surface which follows the contours of the head and in a sphere. This sphere is taken concentric to the sphere which is the best possible fit for the head. Taking into account the relative contribution of the volume currents and the possible accuracy in the positioning of the magnetic field detector, it can be concluded that the best choice is to measure the normal component of the magnetic field at points which are situated in the spherical surface. The results of this study also show that the magnetic field distribution based on a realistically shaped compartment model differs from that based on a compartment model consisting of concentric spheres. In the spherical model of the head no contribution of the volume currents to the component of the field normal to the sphere can be expected. The difference between the results obtained with these two volume conductor models increases with source depth.Key words: magnetoencephalography; visual cortex; simulation; volume conductor shape; recording surfacesThe accuracy of source localization within the brain based on magnetoencephalographic (MEG) measurements around the head depends on the adequacy of the models used to represent both the source and the volume conductor (i.e., the head). The most commonly used model for a source is a single-current dipole or a current dipole layer. Up to now mathematical models of the head have been confined to spherical models (Cohen and Cuffin 1983), although the influence of the shape of an isolated human skull on the magnetic field was measured by Weinberg et al. (1984) and Barth et al. (1986).In some cases the adequacy of a model used for the localization of a source may be confirmed, for instance, by X-ray tomographs in studies of focal epilepsy (Barth et al. 1984). However, in most other cases such a confirmation cannot be obtained. Although not conclusive, a model inspires confidence when source localization, using this model, leads to the same results when the localization is based on the measured EEG distribution as in the case where the localization is based on the measured MEG distribution.For sources in the visual cortex the estimated source location based on the visual evoked potentials did not coincide with the location based on the visual evoked magnetic fields (Stok et al. 1984). The model used for the computations consisted of 4 concentric spheres, where the spheres were fitted in the section of the head near the vi...

Monte Carlo simulation studies of EEG and MEG localization accuracy

“…First, to specifically evaluate the inverse procedure, we assumed that there were no errors in the EEG and MEG forward solutions. Currently, the MEG forward solution is more accurate than the EEG forward solution, owing to the fact that the MEG forward solution requires only the inner skull surface and does not depend on the conductivities of the various tissue types in the brain [Hamalainen and Sarvas, 1989;Meijs et al, 1987Meijs et al, , 1989. If errors in the head model are included, which is likely to be the case with the currently available head models, the EEG accuracy will worsen relative to the MEG accuracy.…”

“…The computation of the MEG forward solution has been shown to only require the inner skull boundary to achieve an accurate solution [Hamalainen and Sarvas, 1989;Meijs et al, 1987Meijs et al, , 1989. The EEG forward solution computation requires the specification of boundaries between brain and skull, skull and scalp, scalp and air, and the relative conductivities of each of those regions.…”

“…Specifically, there are fundamental differences between the forward solution accuracy required by EEG and MEG, with MEG requiring a simpler model [Hamalainen and Sarvas, 1989;Meijs et al, 1987Meijs et al, , 1989. Therefore, one would expect better accuracy for experimental MEG data using the more accurate MEG head model.…”

A novel integrated MEG and EEG analysis method for dipolar sources

“…Realistically shaped EEG and MEG head models under the piece-wise homogeneous approximation can be solved by using the BEM (Meijs et al, 1987;Hamalainen and Sarvas, 1989;Ferguson et al, 1994;Schlitt et al, 1995;Mosher et al, 1999). With the BEM, each compartment of the head is assumed to be isotropic, with a constant conductivity value.…”

Neuromagnetic source analysis using magnetic resonance images for the construction of source and volume conductor model

“…The numerical efforts required for dealing with this type of realistic volume conductor model are relatively low since only the surface of the volume conductor has to be considered and efficient algorithms (boundary element method) are available (Meijs et al 1987;H~imal~iinen and Sarvas 1989;.…”