Pediatric MEG data is often complicated by head movement artifacts. This novel protocol offers a comprehensive demonstration of how we reduce head movement while collecting MEG data from young children. Implementing the child-friendly procedures outlined in our protocol is important for improving data quality, minimizing participant attrition rate string longitudinal studies, and ensuring that families have a positive experience of research participation.
This protocol can be applied when assessing young children with other neuroimaging systems, such as fMRI or PET, where reducing head movement during scanning is also critical to data quality. To begin this procedure, provide families with resources to learn about magnetoencephalography, or MEG, prior to visiting the MEG laboratory, such as a storyboard detailing the steps involved in completing the MEG experiment and a MEG information sheet for parents or caregivers. Subsequently, take the child on a tour of the magnetically shielded room, or MSR, housing the pediatric MEG system, which is decorated in space-related wall art to reinforce the space mission theme.
Ask the child to practice lying back with their head in the helmet door. Then, tell the child to lie as still as possible so that the spaceship stays on course and can reach its final destination. For digitization, direct the child to sit on a high chair and fit them with a polyester swimming cap containing five marker coils, which send data to a continuous motion tracking unit.
Adapt loose fitting caps by folding up the sides. Place a transmitter and three receivers around the child's neck. Ask the child to demonstrate their best statue pose and offer frequent positive reinforcement while they remain still.
This serves to minimize head movement during digitization that may compromise the accuracy of the subsequent coregistration between the child's head and the MEG sensors. Use a pen digitizer to record the position of three fiducial points and the five marker coils, as well as the shape of the surface of the head. This data is used to later determine the position of the child's head in relation to the MEG sensors.
At the end of the procedure, remove the cap, transmitter, and three receivers from the child's neck. Next, take the child to the room housing the MEG simulator, a full-sized replica of the MEG system. The MEG simulator is decorated with space-themed stickers and is equipped with a mock helmet door, a bed, a button box, and, for visual displays, a screen situated above the mock door.
Briefly describe the MEG scanning procedures through the narrative of a practice space mission. If the child appears nervous, first demonstrate the experimental procedures with a toy. Fit the child with an astronaut helmet, a polyester swimming cap which has a motion detector attached at the front.
Invite the child to lie in the simulator and watch a video of their choice. Whenever the child's head movement exceeds a predetermined threshold, the motion tracking system will automatically pause the video and wait for the experimenter to manually restart the video and restore the movement baseline. When the child completes this part of the simulator training, provide them with training on the experimental task using a separate set of unique stimuli.
At the end of the task training, offer the child an astronaut training certificate. When the child arrives, confirm that they are not wearing any magnetic material on their clothes or body, as magnetic materials can distort the MEG signal. Then, repeat the digitization procedure outlined in the MEG familiarization session previously.
Subsequently, take the child to the MSR. Two researchers are required for this procedure, one to accompany the child inside the MSR as the assistant researcher, and the other to run MEG data acquisition outside the MSR as the main researcher. The MSR setup typically takes five minutes.
Inside the MSR, ask the child to place their head into the helmet door. Check that the child's head is centrally aligned, such that the crown of the head is as close as possible to the back of the helmet door without touching it. Ensure that the child is comfortable, relaxed, and remains as still as possible for the MEG recording.
While setting up the equipment, keep the child entertained by playing a video of their choice on the screen above the door. Outside the MSR, conduct a pre-experiment baseline marker coil measurement to record the initial head position with respect to the helmet door. Next, conduct a coregistration between the child's head and the sensor array using both the initial marker coil measurement and the digitization head shape data.
These preparatory measurements enable visual inspection of head position inside the door to ensure that the child's head is correctly positioned. If these conditions are not met, reposition the child's head and conduct another coregistration before starting data acquisition. Once satisfied with the head position with respect to the helmet door, start the MEG recording and the experimental task.
Record ongoing head movements with a pediatric MEG software system called real-time head movement. At the end of the experiment, offer the child a gift bag for their participation and remunerate the family for their time and travel costs. In this experiment, data was collected from a three year old boy who passively listened to auditory tones for 15 minutes.
The data was de-noised, then past filtered, baseline corrected, and averaged. Root-mean-square magnetic, or RMS, wave forms were computed from all sensors and the averaged in-scanner head movements were 44.3 millimeters. As demonstrated, real-time head movement compensated for motion-related artifacts resulting in more focal isofield contour maps.
Much distorted RMS magnetic waveforms and more meaningful source reconstruction in the bilateral auditory lobes. Throughout the testing procedure, it is important to build rapport with the child, keep the session fun and engaging, and respond to any signs of anxiety or restlessness in the child. By training children to keep their heads still during pediatric MEG recordings, we have improved the quality of our MEG data.
Advanced analysis techniques can now be applied to these data. The pediatric MEG not only opens a window for what's happening electrically in the brain but also allows us to map certain cognitive functions onto the developing brain.
