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

脑磁图(MEG)测量垂直于头皮的磁场被广泛使用。当大脑皮质的许多不同区域活动时，存在分离多个来源的问题，这些来源在时间上重叠。我们研制了一台连接三个超导量子干涉器件(SQUID)的三维(3D)二阶梯度仪，用于MEG的矢量测量，可以同时检测垂直于头皮和与头皮相切的磁场分量。为了评估多源的分辨和分离，我们进行了混合听觉诱发场(AEF)和体感诱发场(SEF)在时间上重叠的脑磁图的3D矢量测量。由于缺乏偶极模式，垂直于头皮的磁场分布无助于估计源的位置和数目。但与头皮相切的磁场分布可以提供有关约束条件的信息。我们从与头皮相切的磁场的脑磁图数据中估计了混合AEF和SEF的多个来源，使用开发的反计算算法。这一结果通过与受试者头部叠加信号源的磁共振成像(MRI)的对比得到了证实。我们得出结论，这种三维矢量磁场测量可以提供关于约束条件的信息。当我们计算多个源的反问题时，可以从切向磁场中得到。到目前为止，我们只使用了垂直于头皮的磁场。

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.