On MEG forward modelling using multipolar expansions
On MEG forward modelling using multipolar expansions
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DOI：
10.1088/0031-9155/47/4/301
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发表时间：
2002-02-21
影响因子：
3.5
通讯作者：
Leahy, RM
中科院分区：
文献类型：
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作者：
Jerbi, K;Mosher, JC;Leahy, RM
脑磁图（MEG）是一种基于测量大脑内神经电流源产生的外部磁场的非侵入性功能成像方式。底层源的重建是一个严重不适定的逆问题，通常使用低维参数源模型（如等效电流偶极子（ECD））或高维最小范数成像技术来解决。由于ECD无法正确地表示非震源，并且通过最小范数方法获得的解过于平滑，因此需要一种替代方法。多极展开方法具有参数化方法的优点，同时又能充分描述具有显著空间范围和任意激活模式的源。在本文中，我们首先比较回顾了可用于MEG的球面调和和笛卡尔多极展开方法。给出了任意导体和实际传感器配置的一般情况下以及球对称导体和径向定向传感器的特殊情况下的方程。然后，我们报告了用于研究一阶多极模型（偶极子和四极子）表示空间扩展源的能力的计算机模拟结果，这些模拟是由二维和三维元素偶极子簇模拟的。使用奇异值分解分析聚类的整体场，并使用子空间相关度量将其与以聚类中间为中心的多极子的单位场进行比较。我们的研究结果表明，在提供扩展活动区域的源表示方面，多极源模型优于ECD模型。
Magnetoencephalography (MEG) is a non-invasive functional imaging modality based on the measurement of the external magnetic field produced by neural current sources within the brain. The reconstruction of the underlying sources is a severely ill-posed inverse problem typically tackled using either low-dimensional parametric source models, such as an equivalent current dipole (ECD), or high-dimensional minimum-norm imaging techniques. The inability of the ECD to properly represent non-focal sources and the over-smoothed solutions obtained by minimum-norm methods underline the need for an alternative approach. Multipole expansion methods have the advantages of the parametric approach while at the same time adequately describing sources with significant spatial extent and arbitrary activation patterns. In this paper we first present a comparative review of spherical harmonic and Cartesian multipole expansion methods that can be used in MEG. The equations are given for the general case of arbitrary conductors and realistic sensor configurations and also for the special cases of spherically symmetric conductors and radially oriented sensors, We then report the results of computer simulations used to investigate the ability of a first-order multipole model (dipole and quadrupole) to represent spatially extended sources, which are simulated by 2D and 3D clusters of elemental dipoles. The overall field of a cluster is analysed using singular value decomposition and compared to the unit fields of a multipole, centred in the middle of the cluster, using subspace correlation metrics. Our results demonstrate the superior utility of the multipolar source model over ECD models in providing source representations of extended regions of activity.