STUDY OF FUNCTIONAL IMAGING OF THE BRAIN BASED ON A 3-D VECTOR MEASUREMENT OF A MAGNETOENCEPHALOGRAM.

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Magnetoencephalogram (MEG) measurement of the magnetic field perpendicular to the scalp is widely used. There are problems of separating multiple sources overlapping in time when many distinct areas of the cortex are active. We have developed a three dimensional (3D) second-order gradiometer connected to three superconducting quantum interference devices (SQUIDs) for vector measurement of the MEG that can detect magnetic-field components perpendicular and tangential to the scalp simultaneously. To asses 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. We estimated multiple sources of the mixed AEF and SEF from the MEG data of the magnetic-field tangential to the scalp, using a inverse calculation algorithm developed. The result was confirmed by comparison with magnetic resonance imaging (MRI) of a subject's head superimposed source.We conclude that this 3D vector magnetic-field measurement can provide information on constraint conditions. which can obtained from the tangential magnetic-field when we calculate the inverse problem with multiple sources. Up to the present time, we have used only the magnetic-field perpendicular to the scalp.