Intracranial EEG (I-EEG), sometimes called a "sub-dural EEG" (SD-EEG), is where the electrodes for an electroencephalograph (EEG) are placed near the surface of the brain. [ [http://www.aboutkidshealth.ca/Epilepsy/Intracranial-EEG.aspx?articleID=7016&categoryID=EP-nh3-03a10 AboutKidsHealth: Epilepsy: Intracranial EEG ] ] In some cases, such as epileptic studies, deeper brain activity cannot be recorded accurately or not at all by EEG. This is when a deeper EEG is required, due to the shielding effect of the scalp and skull. The EEG electrodes are placed under the surface of the scalp and skull directly on the brain surface. This is also called "electrocorticography" (ECoG). Electrodes can even be placed into specific brain areas, such as the hippocampus. The EEG signal then processes in the same manner as with a surface EEG, but with higher recording rates because of higher frequencies and little to no interference from outside sources - as with surface EEG.
Recent Advances in ECoG technology
The electrocorticogram is still considered to be the “gold” standard for defining epileptogenic zones; however, this procedure is risky and highly invasive. Recent studies have explored the development of a noninvasive cortical imaging technique for presurgical planning that can provide the same information and resolution of the invasive ECoG.
In one novel approach, Bin He et al [cite journal | author = Lei Ding, Christopher Wilke, Bobby Xu, Xiaoliang Xu, Wim van Drongelen, Michael Kohrman, Bin He | year = 2007 | title = EEG source imaging: correlating source locations and extents with electrocorticography and surgical resections in epilepsy patients | journal = Journal of Clinical Neurophysiology | volume = 24 | pages = 130–136| id = | url = | doi = 10.1097/WNP.0b013e318038fd52 ] seek to integrate the information provided by a structural MRI and scalp EEG to provide a noninvasive alternative to ECoG. This study investigated a high-resolution subspace source localization approach, FINE (first principle vectors) to image the locations and estimate the extents of current sources from the scalp EEG. A thresholding technique was applied to the resulting tomography of subspace correlation values in order to identify epileptogenic sources. This method was tested in three pediatric patients with intractable epilepsy, with encouraging clinical results. Each patient was evaluated using structural MRI, long-term video EEG monitoring with scalp electrodes, and subsequently with subdural electrodes. The ECoG data was then recorded from implanted subdural electrode grids placed directly on the surface of the cortex. MRI and computed tomography images were also obtained for each subject.]The epileptogenic zones identified from preoperative EEG data were validated by observations from postoperative ECoG data in all three patients. These preliminary results suggest that it is possible to direct surgical planning and locate epileptogenic zones noninvasively using the described imaging and integrating methods. EEG findings were further validated by the surgical outcomes of all three patients. After surgical resectioning, two patients are seizure-free and the third has experienced a significant reduction in seizures. Due to its clinical success, FINE offers a promising alternative to preoperative ECoG, providing information about both the location and extent of epileptogenic sources through a noninvasive imaging procedure.
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