Application of 3-D MEG measurement for estimating multiple sources

The authors have developed a three-dimensional (3-D) second-order gradiometer connected to three SQUIDs for vector measurement of 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, the authors carried out both simulation study and 3-D vector measurement of MEG with mixed AEF (auditory evoked field) and SEF (somatosensory evoked field) overlapping in time. The magnetic field distribution perpendicular to the scalp was not helpful for 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 new constraint conditions by visual inspection. The authors estimated multiple sources of mixed AEF and SEF from the MEG data of the magnetic field tangential to the scalp and confirmed by comparison with MRI of a subject´s head superimposed source locations