2.1.5. Imaging cap

A model head was created by 3‐D printing a structural MR image from a 1 year‐old child (Fonov et al., 2009, 2011), as this was the youngest age we aimed to image. An adjustable, multi‐panel cap design and flexible materials were chosen so the cap would fit a range of head sizes and shapes. The main body of the cap was made by molding four heated panels of thermoplastic Aquaplast (NC81339‐1, North Coast Medical) to four surfaces (occipital, left temporal, right temporal, and dorsal) of the model head. The Aquaplast panels were allowed to cool and harden before lining their inner surfaces with soft foam. The occipital panel was attached to each of the two temporal panels by a series of zip ties, while the dorsal panel was kept separate. The chosen thermoplastic material and this multi‐panel design ensured a combination of structural integrity across long‐term usage and flexibility to fit the cap to multiple head shapes and sizes. A uniform grid of holes with 11 mm spacing was drilled across the formed panels, creating first‐through fifth‐ nearest source‐detector separations of 1.1, 2.5, 3.3, 3.9 and 4.6 cm. The holes were then lined with 3‐D printed, cylindrical plastic guides, secured by rubber bands, to hold fibers perpendicular to the head surface. The cap panels were suspended by string from the wooden halos, and then fibers were inserted in their designated positions through the guides. Fibers were held in place by elastic foam rings (punched out of a sheet, 8785 K82, McMaster Carr) that also provided a spring action to press fiber tips against the participant's scalp. Fiber ends projected 5 mm beyond the inner surface of the cap so that they could be combed through hair like the teeth of a hairbrush (Supplemental Figure S1E). Velcro straps and buckles were added along the edges of the cap so that the panels could be fastened in fixed positions relative to each other and pressed close against the head during imaging sessions. Designing the cap with four distinct panels connected by Velcro straps improved scalp‐to‐optode coupling relative to a rigid single‐piece design, since the separated cap panels – with some inherent flexibility offered by their thermoplastic composition and adjustable relative positioning offered by the straps connecting them – could better contour to varying shapes and curvatures of the head surface.