Abstracts

Rationale: Determination of the epileptogenic focus can be challenging, especially when electroencephalography (EEG) and/or imaging studies (e.g. MRI) are inconclusive. A significant number of patients who undergo presurgical MEG testing have neocortical epilepsy with normal (non-lesional) MRI. We herein propose a methodology, the focus localization algorithm (FLA), which first identifies potential current sources in the brain that could account for the spontaneous MEG signals that are recorded in the sensor space, and subsequently measures directional information flow in the space of the identified current dipole sources.Methods: MEGs were recorded using a 306 channel Elekta vectorview system of 204 plano-gradiometers and 102 magnetometer channels. MEG data were band-pass filtered between 1-30 Hz using a third order Butterworth digital filter to reduce low-frequency drifts. A multivariate autoregressive (MVAR) model was constructed using the sensor space data and was projected to the source space using Lead Field and Inverse matrix [1]. MRI co-registration and uniform placement of 1000 dipoles within the cortex were used to create the forward and inverse matrices. The Generalized Partial Directed Coherence was estimated from the MVAR model in the source space [2]. The dipole with the maximum information inflow was hypothesized to be within the epileptogenic focus [3].Results: The FLA was applied to short interictal MEGs from 11 patients with neocortical medically intractable epilepsy recorded sequentially within a pre-determined time period at Cleveland Clinic. We were able to successfully localize the epileptic region in 9 out of the 11 patients. Localization was considered successful if the identified focus was within the area of surgical resection and the patients remained seizure-free for 6 months thereafter. We could not localize the focus in 2 patients due to very low signal-to-noise ratio in the recorded MEG data. FLA succeeded in localizing the focus even when clinical MEG was indeterminate due to absence of recorded spikes. Conclusions: Our preliminary results show that proper mathematical analysis of outpatient MEG studies can contribute to the accurate localization of the epileptogenic focus even when conventional methodology fails to do so. Also, importantly, presence of interictal spikes during MEG acquisition, currently a necessary condition in routine MEG studies, may not be required, thus increasing the yield and cost-effectiveness of MEG studies. Investigation into extent of epileptogenic focus using the above described methodology is currently underway. The accurate identification of the epileptogenic focus and its extent from short, interictal-only, noninvasive electromagnetic recordings could pave the way for a paradigm shift in the diagnosis and treatment of epilepsy. [1] G. Michalareas et al., Hum. Brain Mapp, 34(4), 890 913, 2013 [2] L. A. Baccal , Proc, 15th IDSP, 17, 162-166, 2007. [3] B. Krishnan et al., Proc. 29th SBEC, 51, 57-58, 2013.