Study on Neuromagnetic Imaging and Modeling of MEG Activities

在过去的十年中，通过脑磁图地形图技术在理解人脑的功能组织方面取得了实质性的进展。这些研究大多涉及源的估计和定位，这些源被建模为位于均匀电导率的半无限大体积导体中的单电流偶极子。然而，大脑中的来源是复杂的，头部作为体积导体由不同的材料组成，具有不同的电导率。通过计算机模拟，将源模拟为位于非均匀体导体中的单个或多个偶极子，研究了这些不均匀性对脑磁图形貌的影响。计算机模拟表明了声源定位估计中的一些重要方面。(1)在特殊情况下，脑磁图可观察到脑磁图的磁通反转现象，其中非均一区的电导随病理…而改变。更符合逻辑的条件。磁通反转可能是由于头部不同不均匀区域之间边界面上水流流态的改变而引起的。(2)由于不均匀的影响，径向取向的偶极子在球外产生磁场。由不均匀效应产生的垂直于球体表面的磁场的法向分量由体积电流确定。研究了受试者睡眠时脑磁图活动的来源，得到了以下结果：(3)K-复合体的来源可以用位于头部较深部位的两个电流偶极子来模拟。(4)睡眠阶段3和4的Delta活动具有多个不同频率的来源。其中一些来源可以通过脑磁图测量检测到，但通过脑电测量很难检测到。脑磁图技术有一个潜在的优势，可以提供有关大脑功能的重要信息，而这些信息是使用脑电测量无法获得的。较少

During the past decade, substantial advances in the understanding of the functional organization of the human brain have been made through the technique of MEG topographic mapping. Most of these investigations were concerned with the estimation and localization of sources which were modeled as single current dipoles positioned in a semi infinite volume conductor with homogeneous conductivities. However, the sources in the brain are complex, and the head as a volume conductor consisits of different materials with different electrical conductivities. The influence of these inhomogeneities on the MEG topography is studied by a computer simulation, modeling the sources as single or multiple dipoles located in inhomogeneous volume conductors. The computer simulation suggests some important aspects in estimation of source localization. (1) Flux reversal phenomena can be observed in MEG to pographic patterns in special cases where conductivities of the inhomogeneous regions change with pathol … More ogical conditions. The flux reversal can be caused by the change of flow patterns of currents which flow on the surface of boundary between different inhomogeneous regions in the head. (2) Radially oriented dipoles produced magnetic fields outside the sphere due to the effects of inhomogeneities. Normal components of magnetic fields perpendicular to the surface of the sphere generated by the effects of inhomogeneities were determined by volume current. The sources of MEG activities in human subject during sleep were also studied, The following results were obtained ; (3) Sources of K-complexes can be modeled by two current dipoles positioned in the deeper parts of the head. (4) Delta activities in sleep stages 3 and 4 have many sources with different frequencies. Some of these sources can be detected by MEG measurements, but are difficult to detect with EEG measurements. The MEG technique has a potential advantage to provide important information concerning brain function which cannot be obtained using the EEG measurements. Less