T16. Imaging propagation of ictal spike-and-wave activity in the cerebral cortex over milliseconds with fast neural electrical impedance tomography

Abstract

Introduction Electrical Impedance Tomography (EIT) is an emerging medical imaging modality in which the internal impedance of a subject can be imaged using boundary voltage measurements from non-penetrating surface electrodes. It can be used to image neuronal depolarisation over milliseconds as impedance falls when ion channels open during neuronal depolarization. EIT of fast electrical activity due to neuronal depolarisation in the rat cerebral cortex has previously been undertaken with a resolution of 2 ms and μ m using epicortical electrode arrays during evoked potentials and interictal spikes. The purpose of the current work was to use EIT to image ictal spike-wave discharges (SWDs) in the rat cortex and examine the possibility of determining the propagation pattern of this activity through cortical layers. Methods Adult female Sprague-Dawley rats (300–450 g) were anaesthetised with isoflurane; anaesthesia was maintained after surgery with isoflurane and intravenous fentanyl. A large craniotomy was performed on one hemisphere before implanting a 15 × 9 mm257-electrode array, fabricated from stainless steel and silicone rubber, on the cortical surface. Epileptiform events were induced by electrically stimulating the sensorimotor cortex with a 5-s train of 1–2 mA biphasic, charge-balanced square-wave pulses at 100 Hz. For EIT recordings, a 50 μ A sine-wave was injected at 1.7 kHz through a different electrode pair for each of ∼ 30 seizures. EIT images were produced with trigger markers at the peak amplitude of SWDs in the ECoG, averaging over time within one seizure, and over different current pair applications for successive seizures, demodulation of the EIT signal with a ± 500 Hz bandwidth and reconstruction of the processed voltages to produce images with a resolution of 300 μ m and Results There was a maximum impedance change of −0.31 ± 0.06% during the spike phase of averaged SWDs, the early focus of which was spatially confined to the infragranular layers of the primary somatosensory cortex (p Conclusion Fast neural EIT is a novel method which permits visualisation of the trajectory of propagation of ictal activity over milliseconds through the cerebral cortex. This has potential for improving understanding of the pathophysiology of such paroxysmal events and tracking the trajectory of ictal activity throughout the brain in 3D. This could provide a complementary clinical tool to help localise the epileptogenic zone in treatment-resistant focal epilepsies during presurgical evaluation.