Probabilistic comparison of gray and white matter coverage between depth and surface intracranial electrodes in epilepsy: a patient-specific modeling and empirical study

Objective The objective of this study is to quantify the coverage of gray and white matter during intracranial electroencephalography in a cohort of epilepsy patients with surface and depth electrodes. Methods We included 65 patients with strip electrodes (n=12), strip and grid electrodes (n=24), strip, grid, and depth electrodes (n=7), or depth electrodes only (n=22) from the University of Utah spanning 2010-2020. Patient-specific imaging was used to generate probabilistic gray and white matter maps and atlas segmentations. The gray and white matter coverage was quantified based on spherical volumes centered on electrode centroids, with radii ranging from 1-15 mm, along with detailed finite element models of local electric fields Results Gray matter coverage was highly dependent on the chosen radius of influence (RoI). Using a 2.5 mm RoI, depth electrodes covered more gray matter than surface electrodes; however, surface electrodes covered more gray matter at RoI larger than 4 mm. White matter coverage was greatest for depth electrodes at all RoIs, which is noteworthy for studies involving stimulation mapping. Depth electrodes were able to record significantly more gray matter from the amygdala and hippocampus than subdural electrodes. Significance This study provides the first probabilistic analysis to quantify gray and white matter coverage for multiple categories of intracranial recording configurations. Depth electrodes may offer increased per contact coverage of gray matter over other recording strategies if the desired signals are local to the contact, while subdural grids and strips can sample more gray matter if the desired signals are more diffuse.