Using surface normals to localize subdural intracranial electrodes placed during neurosurgery

Clinical and research neurosurgical studies involving the use of intracranial electroencephalography and cortical stimulation depend critically on accurate localization of electrodes. Neuronavigation tracking probes enable surgeons to record the position of exposed electrodes, but they cannot be used to reliably measure subdural contacts placed under the dura and beyond the extent of the craniotomy. We describe an algorithm to estimate the position of these inaccessible sites using the patient's preoperative structural MRI data. After generating a model of the cortical surface, our algorithm estimates surface normals at the sites of accessible electrodes, establishes the orientation of the electrode strips, and then extrapolates the position of adjacent contacts based on the known inter-electrode distance. We performed validation using a patient dataset consisting of 37 electrodes placed on the cortical surface during surgery.