CORTICAL IMAGING OF INTERICTAL EPILEPTIFORM ACTIVITY USING AN INHOMOGENEOUS SPHERICAL HEAD MODEL

X Zhang¹, W van Drongelen², K Hecox², VL Towle³, DM Frim⁴, A McGee², J Lian¹, B He^1
1Department of Bioengineering, University of Illinois at Chicago
 Departments of ²Pediatrics, ³Neurology, and ⁴Surgery, University of Chicago

Abstract: A cortical imaging technique (CIT) was used to estimate cortical potentials from scalp EEG recordings in two pediatric epilepsy patients. For both patients, the CIT analysis was performed during pre-operative interictal spikes using an inhomogeneous 3-concentric-sphere head model. The localized areas of activity were found to be overlying with the epileptogenic zones, as confirmed by neurosurgical resections in the patients. The present study suggests that CIT may become a useful technique for noninvasive localization of intracranial generators of interictal epileptiform activity from pre-operative scalp EEG recordings.

INTRODUCTION

It is important to precisely localize the epileptogenic foci for neurosurgical planning in medically refractory epilepsy patients. To achieve this goal, electrocorticography (ECoG) has been employed for the inspection of the epileptogenic zone on the cerebral cortex. However, invasiveness, risk of morbidity, limited availability, and high costs of this technique limit its use in the clinical routine.

Source localization and imaging techniques have been applied to localize brain electrical sources [1-8]. Of interest is the recently developed cortical imaging technique (CIT) [6], which is an EEG spatial enhancement modality without ad hoc assumption on the number of source dipoles. From SEP data recorded on the scalp and intracranially, it has been shown that consistent spatial patterns exist between post-operatively recorded cortical data and cortical potentials estimated from the scalp data obtained pre-operatively [7]. A unique feature of CIT approach is its applicability to both localized and distributed brain electrical sources. In the present study, we examine the feasibility of noninvasively localizing epileptiform activity from pre-operative EEG measurement by means of the CIT. The localizations are compared with the surgical resection areas, which were determined after a clinical study of the patients including intracranial recordings from the cortical surface.

METHOD

Data Acquisition

Data of two pediatric patients with intractable seizures were included in the present study using a protocol approved by IRB/UC. The clinical studies included long-term video EEG monitoring at the Pediatric Epilepsy Center at The University of Chicago Children’s Hospital. The EEG data were obtained from 24 scalp electrodes, placed according to the 10-10 system, with sampling rate of 400 Hz and band-pass filtering at 1-100 Hz (BMSI 6000, Nicolet, WI). Interictal spikes were identified via visual inspection. Baseline-correction was based on the EEG data from 300 ms to 100 ms before the largest negative peak of the interictal discharge.

Data Analysis

The CIT algorithm used in the present study has been detailed in [6]. In brief, the head volume conductor was approximated using an inhomogeneous 3-concentric-sphere model, in which the radii of the brain, the skull, and the scalp spheres were taken as 0.87, 0.92, and 1.0, respectively. The normalized conductivity of the scalp and the brain was taken as 1.0, and that of the skull as 1/80. The lead field matrix A, which is calculated by the boundary element method (BEM) [6], can directly connect the cortical potential U with the scalp potential F as following:

F  =  A U                                              (1)

The inverse problem of the CIT is to seek the unknown U from the measured F. To overcome the ill-posed nature of the inverse problem, zero-order Tikhonov regularization was applied [9]:

U  = (A^T A + λ I)^-1 A^T F                       (2)

where λ is the regularization parameter used to suppress the effect of noise, and was determined by the L-curve approach in the present study [10].

RESULTS

Patient #1 is a 7 year-old female with intractable seizure. Seven pre-operative interictal spikes were chosen from her long-term EEG monitoring, and subjected to the CIT analysis. Fig. 1(a) shows a typical segment of EEG waveforms with an interictal spike, whose peaks are labeled by black dots. Fig. 1(b) shows the CIT results on the spherical model (right view and top view respectively). The locations of the two major activities observed in the spherical model were further projected to a standard realistic geometry brain model, as shown in Fig. 1(c). For all the 7 interictal spikes, the CIT analysis consistently revealed two localized areas of negativity in the right parietal lobe (N1) and right parietal-temporal area (N2). These results are consistent with the intracranial ECoG recordings, and are also confirmed by surgical resection, as illustrated in Fig. 3(a).

Patient #2 is an 8 year-old female with intractable seizure. Nine pre-operative interictal spikes were chosen for CIT analysis. Fig. 2 shows two typical results obtained during two interictal spikes. In both examples, two localized areas of negativity in the left parietal lobe (N1) and the left temporal lobe (N2 or N3) can be observed. Note that N2 and N3 have different locations on the left temporal lobe. Among all 9 interictal spikes, the CIT results show N1 activity in all cases, while N2 and N3 activities are shown in 5 and 3 cases, respectively. During neural surgery, three resections were performed on this patient’s left temporal lobe, as illustrated in Fig. 3(b). The locations of the resections are consistent with the three activities estimated from the present CIT analysis.

Figure 1. Patient #1: (a) Interictal Spikes. (b) CIT analysis results (N: Nasion, L: left, R: right). (c) Activities projected to realistic geometry brain model.

Figure 2. Patient #2: (a) Interictal Spikes. (b) CIT analysis results (N: Nasion, L: left, R: right). (c) Activities projected to realistic geometry brain model.

DISCUSSION

In this pilot study, the cortical potentials were estimated from pre-operative EEG recordings during interictal spikes in two epilepsy patients. Without a priori knowledge, the estimated cortical potential maps successfully revealed localized areas of activity overlapping with epileptogenic zones as identified from clinical findings and confirmed by surgical resections. The promising results of the present study suggest that the CIT may become a useful tool aiding non-invasive localization of epileptogenic zones from pre-operative EEG monitoring. More accurate estimation is anticipated by employing more scalp electrodes, and by using a realistic geometry head model constructed from individual’s MR images.

Figure 3. (a) Patient #1 resections on right temporal lobe. (b) Patient #2 resection on left temporal lobe.

ACKNOWLEDGEMENT

This work was supported in part by NSF CAREER Award BES-9875344 and a Falk grant.

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