Estimation of Epileptic Focus by Means of the Dipole Tracing

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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