Separation of sources of neuromagnetic examinations.
Phantom experiments and computer simulations in combination with the Wiener-Helstrom filter reconstruction technique demonstrate the ability of the improvement of source separation in distributed current arrangements. A spatial region of interest (ROI), in which the current distribution is expected, is defined and discretized into three-dimensional voxel elements. Inside the ROI the number and spatial extension of the current sources are determined by reconstructing the impressed current density distribution separately from volume conductor effects. The volume currents can be determined depending on the actual head shape and the location of the ROI using finite difference methods before the reconstruction of the impressed current density. As a result the reconstruction procedure is very fast. Without a priori information about the number of active generators and the spatial extension inside the ROI, the method improves the separation of simultaneously active sources and distributed sources in comparison with the reconstruction of the total current density. However, with increasing depth of the sources (deeper than 5-6 cm) the intrinsic smoothing properties of the inverse magnetic problem prevent the separation. The resolution is better in the case of distributed sources in comparison with multiple point-like generators.