We describe a new method of neuronavigated electrode placement for focal tDCS that consider the head anatomy of individual participant. We want to understand whether neuronavigated placement of tDCS electrodes result in superior placement accuracy compared to traditional placement methods and whether these superior accuracy improves the behavioral and neural effect of the stimulation. One of the greatest challenges in tDCS experiment is to ensure accurate placement of electrodes on the scalp of each study participant in a way that current delivery is most effective.
Therefore, researchers must ensure accurate placement of electrodes and verify potential displacement. during fMRI. First, our approach optimizes electrode positions on the scalp using realistic current simulations of current flow for each study participant.
Second, we use neuronavigated electrode placement to ensure that electrodes are attached at individually optimized positions on the scalp. And third, we confirm the accuracy of electrode placement relative to the intended target regions before and after each fMRI session. By improving the placement accuracy of optimize electrode positions on the scalp, we ensure the effect of tDCS are maximized for each study participants.
In the future, we plan to use newer navigated tDCS to systematically investigate newer mechanisms by which stimulation improves human brain function. This research will provide fundamental insights into how tDCS modulate brain functions in both health and disease. It will also help to maximize tDCS effects based on the characteristics of participants and patients in the future.
To begin, assemble all the required equipment for neuronavigation. Open the neuronavigation software and choose New Empty Project. Load the participant's T1 weighted image and save project.
To initiate the 3D head reconstruction, navigate to the reconstruction section and click on New Reconstruction and Skin to open a new window, then press, Compute Skin. Configure five landmarks including nasion, left and right nostrils and left and right preauricular pits. Configure the electrode positions for the right dorsal lateral prefrontal cortex with the X, Y, and Z coordinates of the anode in the target section.
Click on add new, and name the anode. Position the participant comfortably in a chair facing the tracking camera and instruct them to put on the gobbles with the subject tracker attached. Then go to the sessions tab and from the new dropdown menu, select online session.
Choose the Polaris tab to verify the visibility of the subject tracker and the pointer in the camera's field of view, ensuring both the devices are recognized. Select the registration section to register at the five predefined landmarks. Place the pointer perpendicularly to the skull with the sensors facing the camera, and press the foot pedal to confirm the position.
Navigate to the validation section and validate the landmarks after placing the pointer on the registered landmarks. Next in the refinement landmarks panel, click the add button for each position. Position the pointer on the surface of the head and press the pedal to register the additional landmarks.
Then select the perform section and move the pointer to the approximate location of the dorsal lateral prefrontal cortex. Mark the electrode positions when the pointer aligns with the center of the green cross hair on the screen. Move the participant's hair away from the corresponding scalp area and mark the electrode positions using a skin marker or pen.
Apply a small amount of topical anesthetic cream to the marked positions to reduce physical sensations during tDCS fMRI. Insert the power plug into the same power strip as the scanner to use the multi-channel DC stimulator in normal mode. Then connect it to the outer box using the outer box cable and adapter.
Next, apply one millimeter of conductive gel evenly on the surface of all electrodes in the focal tDCS setup, and use the electrode fill aid to standardize the gel thickness. Turn on the DC stimulator, followed by the panel PC.Double click the DC stimulator MC icon and select the relevant sequence setup, then calibrate the stimulator to perform the calibration without the participant connected. Position the participant comfortably near the DC stimulator outside the MRI scanner room, and measure the widest part of the participant's head, which is the circumference from the forehead to the occipital bone.
Then place the electrodes, the spacer to ensure equal spacing of the cathodes around the center anode and attach them to the marked scalp positions. Use an optimal sized EEG cap without plastic inserts to secure the electrodes. Connect the outer box to the inner box of the DC stimulator to perform an impedance check.
To conduct the impedance check, press the respective button on the stimulator interface. Then disconnect the inner box from the outer box and insert the outer box into the waveguide of the scanner. Next, ask the participant to sit on the MRI examination table and reconnect the inner box to the outer box inserted in the wave guide.
Then position the participant supine on the MRI examination table with their head in the open head coil. Lead the electrode cables through the lower part of the head coil before attaching the upper part and locking it in place. Position the inner box next to the participant on the MRI examination table and move them into the scanner bore.
Leave the scanner room and inform the participant about the upcoming procedures via the communication interface. On the scanner panel PC navigate to the main menu followed by examination and patient registration. After filling in the required fields, go to program choice and select the planned imaging protocol.
Click on patient orientation and select the Head First Supine position. From the region of examination and laterality dropdown menus choose brain and click examination to proceed. Inform the participant that two 10-minute resting state fMRI scans will be conducted and instruct them to maintain their gaze on a fixation cross for the entire scanning period.
To start the tDCS, press the initiate stimulation button on the panel PC.Click the Release start trigger button to begin the stimulation with a 10-second ramp up before starting the functional imaging sequences. After the functional scans and stimulation period, acquire a second PET scan while the electrodes are still attached to the participant's head. Upon completion of the MRI session, disconnect the electrode cables from the outer box and turn off the DC stimulator.
Move the participant out of the scanner bore and remove the cap and electrodes from the participant's head. A significant deviation between intended and actual electrode positions was found both before fMRI and after fMRI. Electrode displacement between pre and post FMRI sessions was minimal with an average drift of minus 0.08 millimeters.
No significant region specific differences in electrode placement accuracy were found between right occipital temporal cortex, left temporal parietal cortex, and right dorsal lateral prefrontal cortex in the intended actual pre fMRI and actual post fMRI electrode positioning stages.
