Relative influence of model assumptions and measurement procedures in the analysis of the MEG

The relative influences of several model parameters and measurement setups on the MEG are studied quantitatively. The influences of the number of grid points of the flux transformer and the accuracy of the measurements are analysed using as criterion a relative difference measure (RDM). In a similar way, using the RDM, the influence of various models is evaluated. The volume conductor, i.e. the head, is described by three different compartment models: the first model consists of concentric spheres, the second of eccentric spheres optimally fitting the individual compartments of the head, and the third consists of realistically shaped compartments. The evaluation of the influence of the model of the source on the MEG is studied by taking either one single current dipole or a set of two current dipoles. The RDM described in the paper is shown to be a valuable measure in the quantitative analysis of MEGs

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

The 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