This is an invasive medical equipment which was pioneered by the Wilder Penfield and Herbert Jasper in early 1950s. Even though this gives the same functionalities what EEG gives, in practice the electrodes of this device are surgically connected to the brain to accurately record the signals from cerebral cortex. In result neurosurgeons are more confident about the place where seizure activity occurs and removing diseased tissues. ECoG gives temporal and spatial resolutions approximately 5 ms and 1 cm which are much higher than that of EEG.
Figure: Surgical Electrode Placement
These electrodes consist of sixteen sterile, stainless steel, carbon tip, gold or platinum ball electrodes. The electrodes are then connected to a overlying frame in a ‘Crown’ or ‘Halo’ configuration. Recommended spacing between electrodes is 1 cm where individual electrode is 5 mm in diameter.
Figure: Electrode Configuration
Placement is performed either by ‘Craniotomy’ or ‘Burr holes’. Craniotomy is a surgical procedure where a part of skull is removed in order to expose the brain surface where Burr holes are small holes drilled in to the skull in order to place the electrodes. Since injuries would pave a severe threat on the subject, both insertion and removal of the electrodes are performed in an operating room by a neurosurgeon. Electrodes are placed just below the Dura mater (outer cranial membrane) which gives the flexible functioning which doesn’t cause injuries due to normal brain movements.
Epilepsy which is a collection of seizures is primarily intended on monitoring using ECoG. The connected electrodes are then plugged in to EEG equipment where to monitor the seizures activities in Intensive Care Unit. In a typical usage the subject would be instructed to contract his arm thereby creating an action potential in Cortical Pyramidal Cells. This will be conducted through several layers such as Cerebrospinal Fluid, Pia Mater and Arachnoid Mater to reach the electrodes.
As this proceeds, the recording is done on specific electrodes for the coincident neural activities. The plotted responses are in high Gamma band (66-144 Hz) and they are colour coded for the convenient inspection. The aggregation of all the simultaneous signal (128) is also generated to get the sense of how the neural activities are evolved.
Primarily ECoG is used in the following places.
- During pre-surgical planning, to localize the epileptogenic zones
- To take the blueprint of the cortical functions
- To evaluate the success of the epileptic surgical resectioning
- For the research aspects
Figure: EEG and ECoG comparison
Even though ECoG gives greater flexibility at stimulating and recording signal altering electrodes before a surgery, during a surgery and after a surgery, it has its own limitations which should be dealt with such as limited sampling time, limited field of view and influence of anesthetics, surgery itself, etc. Epilepsy varies widely with the etiology, clinical symptoms and the origin site of the brain. This emphasizes the importance of localizing the epileptogenic zone precisely and accurately than what would be gained using EEG. ECoG can be performed in either of the following forms.
- In the operating room during surgery (Intra-operative ECoG)
- Outside of surgery for pre-surgery planning(Extra-operative ECoG)
This is particularly useful when the resectioning surgery is undergoing. During the surgery, this can be used to monitor the alleged tissue’s epileptic activities and to ensure whether the resectioning procedure removes the alleged tissue completely. The objective of resectioning surgery is to remove the epileptogenic tissues which cause unacceptable neurological consequences. In identifying and localizing the epileptogenic regions, DCES (Direct Cortical Electrical Stimulation) comes into play as it is a valuable tool for functional cortical mapping. It helps to localize the critical zones that must be left over in order to preserve sensory processing, motor coordination and speech.
Figure: Before a resectioning surgery
Before a patient undergoes a resectioning surgery he/she should be identified as a possible candidate by demonstrating the presence of structural lesion using MRI and EEG. Once the presence of lesion is approved ECoG is performed to identify the exact place and extent of lesion and surrounding. As described above the Scalp EEG lacks the precision and accuracy for localize the region and hence ECoG data is assessed on ictal spike activity recorded during a seizure recorded between epileptic events.
ECoG has been found useful in research applications recently where it can be utilized as an accurate recording technique for use in Brain-Computer Interface (BCI). BCI is a neural signal interface which can be used to drive the prosthetic, electronic and communication devices using individual’s brain signals. Brain signals can be recorded invasively and non-invasively. Here ECoG serves as a hybrid since it does not penetrate the blood-brain barrier like other invasive recording devices.
ECoG is basically considered as an invasive instrument as it directly interacts with the brain where the ECoG’s electrodes are typically placed on the Dura surface of brain after a surgery. The minor mistakes on the insertion and removal of the electrodes would cost severe damages on the subjects’ brain structures. Therefore it is highly demanded to keep the subjects as well as the instruments well occupied as to produce the desired outcome of the procedure.
A maintenance strategy worksheet has been proposed by University of Vermont Technical Services Program to carry out Risk-based assessment of a medical device. This determines the frequency of inspection of the device. The below table shows how the rating of each criterion is made.
In ‘Clinical function’ criterion, the device is evaluated for the invasiveness to the subject. The ECoG is considered to be a device which is used to monitor the subject’s brain activities directly. Therefore this category would score 3. ‘Physical risk’ criterion determines the risk associated with the device failure where ECoG will score 3 because device failure would cause misdiagnosis or loss of monitoring. But errors made in electrode placement would cause severe damages to the brain and hence someone could rate this as 4 in this criterion. But for the time being the device is evaluated for the functioning failure and hence rated as 3. ‘Problem avoidance probability’ evaluates the relationship of failure to the historical data of the device. Since ECoG is intended on the precision of the data, the device is recommended to be tested between specific time periods. In each such inspection the results are recorded and compared with the actual results. Historically Common device failures are not very predictable and therefore ECoG scores 2. ‘Incident history’ looks back the history to find out whether there are incidents happened due to device failure. This would score 1 whereas ‘Manufacturer requirements’ criterion scores 2 as the device is regarded as a sensitive device and hence they demand a specific schedule for the inspection. This gives a total score of 12 where an annual (1x) inspection interval is recommended as functional test frequency.
 Fluke Biomedical – University of Vermont Worksheet