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

Abstract

We developed a three-dimensional (3D) second-order gradiometer connected to three superconducting quantum interference devices (SQUIDs) for vector measurement of a magnetoencephalogram (MEG) that can detect magnetic-field components perpendicular and tangential to the scalp simultaneously. Each coil is orthogonally wound with Nb–Ti wire on a rectangular solid 3×3×6 cm. To assess discrimination and separation of multiple sources, we carried out a 3D vector measurement of the MEG with a mixed auditory evoked field (AEF) and a somatosensory evoked field (SEF) overlapping in time. The magnetic-field distribution perpendicular to the scalp was not helpful in estimating the location and number of sources, owing to the lack of a dipole pattern. But the magnetic-field distribution tangential to the scalp can provide information about constraint conditions by visual inspection. We estimated multiple sources of the mixed AEF and SEF from the MEG data of the magnetic field tangential to the scalp and confirmed by comparison with magnetic resonance imaging (MRI) of a subject’s head superimposed source locations. We conclude that this 3D second-order gradiometer can provide constraint conditions by visual inspection for calculating the inverse problem with multiple sources.