Abstracts

Rationale: Interpreting Electrocorticography (ECoG) with advanced electrophysiology and neuroimaging methods requires multimodal information to be integrated accurately. For example, mapping ictal electrographic activity could be enhanced by overlapping cortical thickness information derived from MRI in cases of focal cortical dysplasia. To take advantage of multimodal information, the locations of the ECoG electrodes need to be known during long term monitoring. However, the locations of ECoG electrodes change as the brain shifts in response to surgery and during recovery. This shift in electrode location is seen in post-implant imaging and can impact the spatial interpretation of the location. Though this brain shift is not inherently detrimental, the discrepancy in electrode location can lead to challenges in interpreting electrophysiological data. The amount of shift may potentially impact interpreting ECoG information and is unknown in children where these techniques can have a great impact. Methods: To characterize the changes in electrode locations immediately after implant, differences between pre-implant brain surface and post-implant ECoG locations were assessed. The pre-implant brain surface was derived from a 3D brain model created from a pre-implant T1 MRI sequence. The immediate post-implant electrode locations were determined from a post-implant CT image taken within 24 hours after surgery that was co-registered with the pre-implant T1. The amount of post-implant shift was defined as the distance between a contact and the pre-implant brain surface using a validated toolbox. The shift was considered in the context of patient age, intracranial volume, and lobe for 20 children undergoing intra-cranial monitoring with ECoG grids in preparation for epilepsy resection surgery. Results: A total of 1254 ECoG contact locations were assessed in 20 patients with a mean age of 12.1±5.2 years at surgery (range 3.25 – 20.17). Patients had an average of 62.7 channels and 5.53±3.24 mm shifted from the pre-implant brain surface immediately after implant. This shift significantly increased with increased intracranial volume (p = 0.02; mixed effects model), but not age (p = 0.84). Shift also varied significantly depending of the lobe the contact was over (p < 0.001); where contacts on the temporal lobe had less shift than the parietal (p = 0.03; Tukey multi-comparisons). Conclusions: The shift in ECoG immediately after implantation could lead to a misinterpretation of the contact location by an average of ~5 mm particularly in the frontal and/or parietal lobes. The dependence of the shift on intracranial volume suggests that electrode shift in patients with larger intracranial volume should be taken into consideration. Funding: Supported by the Phoenix Children’s Hospital Innovators in Neuroscience for Kids Foundation, NSF I/UCRC Cooperative Grant for Building Reliable Innovation and Advances in Neurotechnology, and Phoenix Children’s Hospital Foundation Leadership Circle Award. Its contents are the responsibility of the authors and do not necessarily represent the view of the funding agencies.