Abstracts

Rationale: Electrocorticographic (ECoG) grids are routinely placed subdurally on the cortex in people with epilepsy to delineate cortical areas involved in seizures or function. These grids provide exceptional opportunities for neuroscientific research, because they detect signals from the brain at high spatial and temporal resolution. Relating these signals to the underlying anatomy requires co-registering the location of the electrodes to the location of the anatomy. Existing methods that support this co-registration use pre-operative magnetic resonance imaging (MRI), which captures the cortical anatomy, and post-operative computerized tomography (CT), which captures the electrode locations. However, when ECoG grids are placed during a surgery, no CT that includes the grid is available. Other existing methods make use of intraoperative photographs, but they require that the electrodes can be visually related to the underlying anatomy, which is difficult or impossible with high-resolution grids or when electrodes are invisible. These limitations of existing localization techniques limit the potential value of the interpretation of ECoG signals collected during surgeries. To address this problem, we developed a method to co-register ECoG grids using only a pre-operative MRI, a clinical neuro-navigation device (such as BrainLab Vector Vision), and fiducials. Methods: The patient undergoes imaging with fiducial markers on the day of surgery. During surgery, the patient's anatomy is registered to the pre-operative images in the navigation system using laser-based surface registration. The fiducial points are then traversed with the navigator tip and the coordinates for each point are acquired and saved. The same pre-operative MRI is read into CURRY software and the fiducial points are detected. A rigid transformation matrix is estimated from these and the acquired points on the neuronavigator. Results: To validate our new technique, we collected data from four subjects that had post-operative CTs also available, and compared the resulting electrode coordinates to results using an existing co-registration technique that required a post-operative CT. We found that the electrode locations determined using the two methods were in generally close proximity (average error: 8.13 mm) even in the limited dataset. Conclusions: We developed and validated a method for co-registering ECoG electrodes to the underlying cortical anatomy when a post-operative CT is not available, such as during single-stage, intra-operative studies. Thus our technique should prove useful for research in intra-operative scenarios. More generally, it may also reduce the need for post-operative CTs and thus help to reduce radiographic imaging exposures.