Three-dimensional visualization of subdural electrodes for presurgical planning.

Epileptic patients with medically intractable localization-related seizure disorders often require implantation of subdural electrodes for the purpose of seizure localization1–4 and mapping of eloquent cortex5,6 as part of their surgical evaluation. Once the electrode locations where seizures arise are identified, neurosurgical planning to define the boundaries of the required surgical resection can pose a challenging problem if the cortical locations of the subdural electrodes are not clearly determined. Accurate knowledge of electrode positions assumes greater significance when the epileptogenic zones border eloquent cortices such as language, motor, or visual areas. Several techniques have been used to visualize electrode placement in vivo, including artifact localization,7,8 3-dimensional reconstruction and x-ray-derived location projection,9,10 automated template magnetic resonance imaging (MRI) transformation and projection,11 x-ray coregistration,9,10,12 curvilinear reformation,13 computed tomography (CT)/MRI coregistration14–16 computer-aided stereotactic model creation,17 as well as digital 2D photography coregistered to 3-dimensional (3D) reconstructed MRI.18,19 More recently, intraoperative fluoroscopy has been used for strip electrode placement.20 These methods are useful to a degree in the visualization of electrode locations, but each lack accuracy and/or resolution in imaging detail because of interpolation and projection limitations. Some of the more recently published procedures rely on 2-dimensional (2D) photographic and x-ray9,10 information regarding electrode position. This requires the estimation of complicated 3D brain geometry with 2D pictorial information and limits localization to those electrodes exposed. Strip electrodes placed via burr holes are not visible with these techniques. Closure of the craniotomy causes brain distortion and cortical compression,21,22 and can physically shift electrodes from their initial position. This brain compression has been depicted in previous studies7,10,23 and affects the cortex at the critical period of patient monitoring. In all previously published techniques, preimplantation MRI has been used to create cortical rendering models.10,12,14,19,24 This is done because MRI images gathered postimplantation are subject to magnetic field susceptibility artifacts caused by the metal electrodes at the surface of the brain. Thus, 3D rendering of electrode-affected images has been limited. Consequently, brain shift and compression caused by electrode grids and craniotomy defects has never been satisfactorily addressed in imaging models. The method described here provides a measurement of electrode positions overlaid on 3D renderings of patient-specific, post-implantation cortical surface reconstructions. This method relies on a combination of postimplantation CT and MR imaging and provides 3D representations of each patient’s uniquely compressed cortical surface anatomy with intracranial electroencephalography (iEEG) electrodes overlaid. Midline cortical surface views are also realizable. In addition, we describe a method for creating illustrative electrode “shadows” that identify cortical patches beneath each electrode and help locate sulcal-straddling electrodes that may potentially pick up iEEG signals from 2 adjacent gyri.