Estimation of Epileptic Focus by Means of the Dipole Tracing

从用21个电极测量的头皮记录的事件相关电位中，可以估计与事件相关的活动神经元的位置，前提是活动神经元在空间上集中，并且它们的电活动很好地近似于电偶极子。在估计等效偶极子的过程中，需要计算放置在头部任意位置的电流偶极子所产生的头皮电位。在此计算中，假设了一种真实的均匀导体头形。然而，电导率在头部是不均匀的；与其他部位相比，颅骨的电导率非常低。因此，在计算时必须考虑这种影响。有两种方法。一是引入非均匀导电性的现实水头模型，二是对均匀导电性的现实水头模型的结果进行适当的修正。这三年来，我们一直在尝试第二种模式，其中…更多的校正数据是通过乌普萨拉大学医院Hagbarth和Blom在患者身上进行的硬膜下电极植入实验收集的。从9月到10月，本间、中岛和冈本留在乌普萨拉，对三名患者进行了实验。通过目前的实验，我们检查了5名患者，发现由于颅骨的电导率低，估计的偶极子比实际的偶极子深33-35%。因此，将偶极子的估计位置从颅骨中心向外延伸50%，就可以得到一个非常正确的位置。完成了由头皮、颅骨和脑组织（灰质、白质和脑脊液）组成的三层真实头部模型的计算机程序。颅骨的三维形状取自x射线扫描数据。1月至2月，Musha访问了乌普萨拉，为两名患者拍摄了x射线CT扫描数据，以准确绘制这些患者的头骨形状。我们将调整这三层的电导率，使估计的偶极子位置与植入这些患者的硬脑膜下电极产生的人工偶极子位置一致。重复这个过程，我们完成了3层头部模型。当这个模型完成后，DT方法无论从临床还是从基础研究的角度来看都是非常可靠的。少

From the scalp-recorded event-related potentials measured with 21 electrodes, locations of active neurons associated with the event can be estimated provided active neurons are spatially concentrated and their electric activity is well approximated by an electric dipole. In the process of estimating the equivalent dipole, we need calculate the scalp potentials generated by a current dipole which is placed at an arbitrary position within the head. A realistic head shape of uniform conductor is assumed in this calculation. However, conductivity within the head is not uniform ; the skull has a very low conductivity as compared with other parts. Therefore, this effect must be considered in the calculation.There can be two ways for this. One is to introduce a realistic head model with nonuniform conductivity, and the other is to make a proper correction to the result obtained by a realistic head model with uniform conductivity. We have tried the second model these three years, in which the … More correction data were collected through the experiment with implanted subdural electrodes in patients carried on with Hagbarth and Blom at Uppsala University Hospital. In September through October Homma, Nakajima and Okamoto stayed in Uppsala and carried out the experiment in three patients. Through the experiment so far, we have examined five patients and found that the estimated dipole is 33-35% deeper than the real one due to low electric conductivity of the skull. Therefore, a very correct position will be obtained by extending the estimated dipole position outward by 50% from the center of the skull.A computer program for a three-layered realistic head model, which consists of scalp, skull and brain tissue (gray matter, white matter and cerebrospinal fluid), was finished. The 3-dimensional shape of the skull is taken from the X-ray scan data. In January to February Musha visited Uppsala to take X-ray CT scan data of two patients to make exact shapes of the skull of these patients. We are going to adjust electric conductivities of the three layers such that the estimated dipole locations agree with locations of the artificial dipoles generated with subdural electrodes implanted in these patients.Repeating this process we finalize the 3-layered head model. When this model is completed the DT method will be very reliable from a clinical point of view as well as from a fundamental research point of view. Less