Estimation of multiple sources using a three-dimensional vector measurement of a magnetoencephalogram

We developed a three-dimensional (3D) second-order gradiometer connected to three superconducting quantum interference devices (SQUIDs) for vector mea surement of a magnetoencephalogram (MEG) that can detect magnetic-field com ponents perpendicular and tangential to the scalp simultaneously. Each coil is orthogonally wound with Nb-Ti wire on a rectangular solid 3x3X6 cm. To assess discrimination and separation of multiple sources, we carried out a 3D vector measurement of the MEO with a mixed auditory evoked field (AEF) a nd a somatosensory evoked field (SEF) overlapping in time. The magnetic-fie ld distribution perpendicular to the scalp was not helpful in estimating th e location and number of sources, owing to the lack of a dipole pattern. Bu t the magnetic-field distribution tangential to the scalp can provide infor mation about constraint conditions by visual inspection. We estimated multi ple sources of the mixed AEF and SEF from the MEG data of the magnetic fiel d tangential to the scalp and confirmed by comparison with magnetic resonan ce imaging (MRI) of a subject's head superimposed source locations. We conc lude that this 3D second-order gradiometer can provide constraint condition s by visual inspection for calculating the inverse problem with multiple so urces. (C) 1998 American Institute of Physics. [S0021-8979(98)39111-2].