T15. Imaging fast neuronal depolarization in 3D during seizures with electrical impedance tomography and scalp and intracranial depth electrodes

Abstract

Electrical Impedance Tomography (EIT) is an imaging technique where internal conductivity changes in an object are reconstructed from measurements with arrays of external electrodes. It has the potential to improve epileptogenic zone localisation as seizure activity changes local impedance in two ways. ‘Fast’ changes, ∼1% over millseconds, are due to the opening of ion channels during synchronised neuronal depolarisation, and ‘slow’ changes, up to 10% over seconds, are due to cell swelling. During SEPs in anaethetised rats, EIT could image evoked activity in S1 with a resolution of 1 ms and <200 μm using 30 electrode epicortical arrays. EIT could offer improved resolution over the current technique of inspection of potentials with depth electrodes (SEEG) in patients undergoing presurgical seizure evaluation. EIT has the advantage that it is not sensitive to dipole orientation and coverage within a volume enclosed by electrodes is much better than for potential recording. The purpose of this study was to undertake a computer modelling study to compare fast neural EIT with SEEG and EEG inverse source modelling in subjects with epilepsy and intracranial electrodes. The location and shape accuracy of reconstructed changes were assessed in 3 subjects with 48–72 depth electrode contacts and 32 scalp electrodes with EIT (1.7 kHz, 50 μA intracranial, 250 μA scalp), EEG inverse source modelling (ISM), and spike detection on depth electrodes (SEEG), using FEM meshes of ∼9 M tetrahedral elements generated from CT-MRI. EIT and inverse source reconstructions were performed on ∼30,000 hexahedral element meshes with zeroth order Tikhonov regularisation and noise-based image weighting. Seizure onset was simulated by 5 mm radius realistic perturbations of 1% and 10% or dipoles which produced 2 mV when 5 mm away from an electrode (5 within and 5 outside the volume enclosed by depth electrodes in each subject). EIT produced the best localisation. It was far superior to ISM and also provided accurate imaging for dipole sources not apparent on SEEG because of distance from the nearest electrode or tangential orientation. For perturbations placed within the depth electrodes, localisation accuracy was 5.2 ± 1.8 mm (1% change) <5 mm (10% change) for EIT and 46.2 ± 25.8 mm for ISM with radially oriented sources. For SEEG, 7/15 radially oriented sources did not reach a visualisable threshold set as 250 μV which occurred when they were >11 mm distant from the nearest electrode. For the sources in the opposite hemisphere, it was 29.6 ± 38.7 mm (1% EIT), 26.1 ± 36.2 (10% EIT), 54.0 ± 26.2 mm (ISM) and 0/15(SEEG). In this modelling study, fast and slow neural EIT with depth electrode offers a new method for localisation of seizure onset and propagation. It employs tiny injected currents which do not damage the brain or alter cerebral function. It offers a potentially valuable additional method for presurgical epilepsy evaluation. A human clinical trial is in progress.