Abstract
Measurements of human brain function in children are of increasing interest in cognitive neuroscience. Many techniques for brain mapping used in children, including functional near-infrared spectroscopy (fNIRS), electroencephalography (EEG), magnetoencephalography (MEG) and transcranial magnetic stimulation (TMS), use probes placed on or near the scalp. The distance between the scalp and the brain is a key variable for these techniques because optical, electrical and magnetic signals are attenuated by distance. However, little is known about how scalp-brain distance differs between different cortical regions in children or how it changes with development. We investigated scalp-brain distance in 71 children, from newborn to age 12 years, using structural T1-weighted MRI scans of the whole head. Three-dimensional reconstructions were created from the scalp surface to allow for accurate calculation of brain-scalp distance. Nine brain landmarks in different cortical regions were manually selected in each subject based on the published fNIRS literature. Significant effects were found for age, cortical region and hemisphere. Brain-scalp distances were lowest in young children, and increased with age to up to double the newborn distance. There were also dramatic differences between brain regions, with up to 50% differences between landmarks. In frontal and temporal regions, scalp-brain distances were significantly greater in the right hemisphere than in the left hemisphere. The largest contributors to developmental changes in brain-scalp distance were increases in the corticospinal fluid (CSF) and inner table of the cranium. These results have important implications for functional imaging studies of children: age and brain-region related differences in fNIRS signals could be due to the confounding factor of brain-scalp distance and not true differences in brain activity.
Highlights
Human brain and behavior both undergo remarkable changes during development, and there is intense interest in using functional neuroimaging techniques to better understand neurodevelopment
In functional near-infrared spectroscopy (fNIRS), and in related techniques such as event-related optical signaling [1], low-power near-infrared light is directed through the scalp and intervening tissues into the surface of the brain [2,3]
The optimal placement of the detector and emitter depends on the depth that light must penetrate: if the emitter and detector are close, more light will travel from the emitter to the detector, but none will travel through the brain; if the emitter and detector are distant, little light will reach the detector, resulting in poor signalto-noise ratio
Summary
Human brain and behavior both undergo remarkable changes during development, and there is intense interest in using functional neuroimaging techniques to better understand neurodevelopment. In order to determine the optimal emitter-detector distance, we wanted to establish the distance between the brain and scalp for the different aged children in our study population. While there are published studies of brain-scalp distance in adults [8,9,10] and children [11,12], we could find little information on how brainscalp distance changes during development. To fill this gap, we examined MRI scans from 71 healthy children ranging in age from 1 day to 12 years old. We hypothesized that there would be significant differences between ages, with younger children having reduced brain-scalp distances; and significant differences between brain areas, with greater brain-scalp distances in some regions relative to others
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