Collecting fMRI and fNIRS signal simultaneously from the same subjects is extremely important because it allow us to align the fNIRS signal with that of fMRI, which is the gold standard in neuroimaging. fNIRS is a methodology that is still developing and it cannot reach deeper brain structures. So fMRI recordings give us a better understanding of the fNIRS signal.
Previous attempts to record fNIRS and fMRI signals from the same participant's brain required two recording sessions. Unfortunately, these two sessions mean that the participant has seen and heard the same stimuli repeatedly, which can change the way the brain responds. Although in the protocol, fMRI and fNIRS signals are only aligned in typical adults, we hope that the current protocol can also be extended to test clinical and developmental populations.
This will be particularly important, for example, when collecting data from individuals who may have neuropsychological deficits since we will be able to interpret the fNIRS signal with the highest confidence. We believe these simultaneous recordings of fNIRS and fMRI will be particularly important in understanding the neural networks in the brain for special populations and for children. There's no doubt that collecting fMRI and fNIRS signals simultaneously is challenging, but we work through all the steps and we are confident that our protocol will enable other labs to collect the same type of data.
One of the main challenges will likely be to ensure that the participant is comfortable in the scanner, especially when wearing a whole head fNIRS cap. Demonstrating the procedure will be Virginia Chambers, the lab manager from our laboratory. Once the participant has signed the necessary consent forms and received instructions for the tasks, direct them to sit on a chair in the control room.
Wrap a soft measuring tape around the forehead. Move to the widest part at the back and return to the starting point. Choose the fNIRS capsize closest to the measured circumference.
Firmly grasp and slide the base of the short distance detector probes around the part of the grommet going through the mesh of the fNIRS cap. Ensure the short distance detector cable face the back of the cap for a clear face area. Next, ask the participant to slide the cap downwards from the crown of their head in a manner like putting on a winter hat and tighten the chin strap to a comfortable extent.
Fasten the back straps. Verify that the cap is securely attached and the optode sockets are firm against the head. Then place green stickers to mark key fiducial locations in accordance with the 10/20 system positions.
Use a measuring tape to align scalp points symmetrically with points on the cap. Ensure equal distance from the CZ point to the pre-auricular, inion, and nasion point. Instruct the participant to remain stationary to create a 3D model of their head.
Then launch the structure application on a tablet or iPad. Ensure the high resolution color, infrared auto exposure, and improved tracker features are switched off. Now position the participant centrally, ensuring their entire head fits within the 3D square on the screen, and their shoulders are minimally visible in the frame.
Walk around the subject to scan the head shape and optodes with the 3D scanner. Once the entire scan is completed, press the button on the right side of the screen to initiate the 3D rendering. Verify the clarity and detail level of the rendering to ensure accurate placement of the optodes and green fiducial stickers.
For safety and privacy, save the 3D scan onto HIPAA protected server. Once the 3D model is complete, remove the green stickers and prompt the participant to insert earplugs in their ears. In the scanning room, ensure the 20 channel head coil is placed in the scanner.
Place a foam pillow inside the bottom of the MRI head coil to support the upper part of the participant's head. Instruct the participant to sit comfortably on the scanner table. While holding the optode grommet steady with one hand, use an MRI-safe applicator to clear hair from the middle of the grommet.
Then apply firm pressure to secure the optode into the grommet. Instruct the participant to lie down slowly and carefully. Adjust the optical fiber bundles to ensure the participant's head rests comfortably within the head coil.
Depending on the position of the cables from the wave guide, raise the scanner table. Place a soft cushion beneath the legs of the participant to guarantee comfort. Wrap the respiratory belt around the waist of the participant.
Instruct the participant to wear noise canceling headphones, making sure it does not interfere with the fNIRS probe placement. Place the pulse oximeter on the subject's left index finger. In case a button box is being used for the experimental tasks, instruct the subject to hold it with the dominant hand.
Provide clear instructions on how to use the button box. Place the squeeze ball or button alarm on the subject's non-dominant hand and instruct the participant how to use it. Test the alarm by asking the participant to press it.
Slide the participant a few inches into the scanner bore to align the head. Position the top part of the head coil. Next, insert the microphone and mirror in the corresponding coil inserts.
Slide the participant slowly into the scanner bore while holding the optical fibers. On the scanner's computer, choose the pertinent structural and functional sequences requisite for the study. Verify the localizer for proper positioning of the head within the scanner bore, ensuring it provides full coverage from the top of the head to the cerebellum.
Confirm with the participant that the computer screen is visible via the head coil mirror and run the initial structural scan. Subject-level fNIRS data showed increased activation in bilateral visual cortex areas during the flashing checkerboard blocks compared to the inter-trial periods. Time traces of brain activity showed an increase in HBO signal during the presentation of the flashing checkerboard and a decrease during inter-trial periods.
Visualization of the HBO data during the flashing checkerboard period showed bilateral activation in visual cortex areas at the individual subject level. Subject-level fMRI data show greater bold signal response in the primary and secondary visual cortex during the flashing checkerboard periods relative to the inter-trial periods At the subcortical level, increased activation is observed in the lateral geniculate nucleus of the thalamus. One of the key challenges of our protocol is to ensure that the participant can rest comfortably in the scanner, and this is gonna be primarily determined by the type of optode that comes with the fNIRS system.
The protocol can be adapted to collect data from specific regions of interest and across a variety of experimental paradigms. We also provide specific suggestions about how to use the protocol with clinical and developmental populations.
