Our protocol provides important steps that ensure data quality, particularly the quality of each data, which is key to the success of a simultaneous EEG-fMRI study, but is rarely emphasized. This protocol is straightforward and uses only readily available medical products, therefore it is very easy to follow. This protocol leads to better quality and readability of the EEG acquired simultaneously with fMRI, which is particularly useful for identifying subtle EEG changes in epilepsy studies.
This protocol is also useful for identifying subtle EEG events in single trial ERPs of neurocognitive studies. To begin, connect the MRI compatible EEG and bipolar amplifiers to the fully charged battery packs and to the recording computer. Ensure that the workspace of the recording software is set up correctly.
Load a 10 milliliter syringe with the abrasive conductive gel for application of the EEG cap. Measure the head circumference of the subject by wrapping a flexible, non-stretchable measuring tape around the head, over the super orbital ridges in the occiput. Select an appropriately sized cap that is one centimeter larger than the head circumference, and make sure that it is comfortable for the subject.
After placing the cap at the approximate position over the subject's head, measure the lengths of the inion-nasion arc, and mark the intersection of the inion-nasion arc and the periauricular arc. Slide the cap over the head so that the position of electrode CZ is adjusted to this intersection. Ensure that the cap is not rotated horizontally by manually checking weather electrodes FZ, PZ, OZ, reference, and ground are positioned over the inion-nasion arc.
Expose the skin underneath each electrode by displacing the hair to the side using the back of a cotton swab. Rub the skin beneath each electrode by quickly spinning a cotton swab containing 70%alcohol solution placed through the opening of the electrode. Apply a small amount of the abrasive conductive gel in the opening, and abrade the skin by quickly spinning a cotton swab.
Monitor the impedance of the electrode, and repeat the abrasion until the impedance drops to at least below 20 kiloohms. Once the impedance is satisfactory, fill the opening with the same gel, making sure not to apply excessive gel in the opening to avoid bridging between electrodes. Before placing the ECG electrode at the back, ask the subject to sit upright without flexing the neck.
Place the ECG electrode two to three centimeters left from the median furrow, making sure that the wire is straight at the back, but with allowance along the curve of the neck to avoid displacement when the subject lays down on the MRI table. Rub the skin underneath the ECG electrode with an alcohol swab. Attach the ECG electrode to the skin using a double-sided adhesive ring, and monitor the impedance by repeating the abrasion of skin with a cotton swab.
Fold the dry alcohol cotton swab into four and place it on the ECG electrode, then tape the swab to the skin using a surgical adhesive tape. Tape the ECG electrode wire to the skin up to the shoulder. Place a set of pre-braided carbon wires consisting of six loops over the cap so that the wires are in parallel with the bundle of electrodes on the top of the head.
Use surgical tape to secure the loops around the electrodes so that the loops cover the head with each loop covering almost an equal area. Wrap the subject's head with an elastic bandage over the EEG cap and the carbon loops, making sure that the bandage covers all the electrodes and is not too tight. Ask whether the subject feels uncomfortable pressure on the head while applying the bandage.
For resting state acquisition, instruct the subject to apply MRI compatible earbuds. Use a headset or earphones for task-based acquisition, making sure that the subject can hear through both sides of the headset or earphones. Place an MRI-compatible flat memory foam pillow in the lower half of the head coil before asking the subject to lie down and place the head in the coil.
Then place the electrode and carbon wire bundles straight through the top opening of the head coil. Add memory foam pillows to the top of the head, forehead, and temporal area, filling up all spaces left within the head coil, while not compressing the subject's head too tightly. Place the upper half of the head coil and close the coil, making sure that the pillows are not squeezing the head.
Place a half-cylinder shape memory foam pillow at the back of the neck so that the ECG electrode wire is sandwiched well between the pillow and the neck. After placing all the memory foam pillows, test that the subject can still hear through both sides of the headsets or earphones. Place the mirror and instruct the subject to adjust the mirror once the subject's head is at the isocenter of the MRI bore.
After connecting the EEG and ECG electrodes to the EEG amplifier, and the carbon wire loops to the bipolar amplifier placed at the back of the MRI bore, connect the amplifiers to the recording computer using optic fibers. Then switch on the amplifiers and check that the impedance of all electrodes is still low. Arrange all wires between the outlet of the top opening of the head coil and the amplifiers so that they are placed straight and at the center of the MRI bore.
Place one carbon wire loop around the ribbon cable, going from the EEG and ECG electrodes connector box to the amplifier. Immobilize the wires by sandwiching all of them with MR-safe and non-ferromagnetic sandbags between the outlet of the top opening of the head coil and the amplifiers. Place sandbags on the amplifiers and position the amplifiers outside the bore of the magnet, using cables supplied by the manufacturer.
Communicate with the subject from the console room to confirm that the subject can hear the operator and instruct the subject not to move during data acquisition. Start the EEG recording before starting fMRI acquisition. Check whether markers from the scanner and volume trigger are periodically displayed in the online EEG recording.
Representative EEG signals obtained from subjects who participated in the neurocognitive study and epilepsy study using this protocol are shown here. The EEG signals acquired from both studies were similar before processing. The resultant EEG signals from both studies were of analyzable quality without visible contamination of ECG artifacts, as seen here.
Epileptic activities were clearly seen on the EEG during the epilepsy study. On the EEG acquired during the neurocognitive study, blinking, eye movement, and muscle artifacts were seen, especially in the frontal leads FP1 and FP2 after artifact removal. No artifact originating from machinery vibrations was seen on post-processed EEG signals acquired during both studies.
Furthermore, the EEG electrodes did not cause visible artifacts on the MR images acquired simultaneously. When attempting this protocol, immobilize the head of the subject, the wires, and the cables to adequately diminish mechanical vibrations. This protocol is currently used for our neuroscience project that requires ERP single trial detection.
It is paving the way to using simultaneous EEG-MRI in non-invasive brain computer interface research.
