The continuous-wave (c.w.) HD-DOT instrument illuminated the head with light-emitting diode (LED) sources at 750 nm and 850 nm (750-03AU and OPE5T85, Roithner Lasertechnik) and utilized APD (Hamamatsu C5460-01) detectors (Fig. 1c, Supplementary Fig. 1). A key feature of the discrete detector channel design was that each detector was digitized by a dedicated 24-bit analog-to-digital converter at 96 kHz (HD-192, MOTU)12 (link). Sources and detectors were coupled with fibre-optic bundles (CeramOptec, 2.5-mm-diameter bundles of 50 mm fibres, 4.2 m in length) to a flexible imaging cap held on the head with hook-and-loop strapping (Supplementary Fig. 3). The fibres were supported by a ‘double-halo’ design comprising two collinear rings that evenly manage the weight of the cap above the subject. The cap comfortably coupled the 188 optical fibres onto the scalp surface by minimizing torque (parallel to the scalp surface) while providing lateral mobility (perpendicular to the scalp). This design allowed the fibres to translate relative to the head surface. Soft pressure from the fibre management and foam springs helped hold the fibres snugly against the scalp surface.
The array had 96 source and 92 detector positions placed in two interlaced rectangular arrays with first-through fourth-nearest neighbour separations as follows: 1.3, 3.0, 3.9 and 4.7 cm. The source positions were organized into six encoding regions (Fig. 1b), each with 16 source positions that were sequentially temporally encoded (Fig. 1e) in time steps of 6.25 ms. The two wavelengths at each source position were modulated at different frequencies (Fig. 1d; even regions, 750 nm at 17.9 kHz and 850 nm at 20.8 kHz; odd regions, 750 nm at 25.0 kHz and 850 nm at 31.3 kHz, all with a 50% duty cycle). With temporal, frequency and spatial encoding, the system worked with a frame rate of 10 Hz. Further details on the system infrastructure, electronics and imaging cap are available in the Supplementary Section I. The processing steps that convert SD-pair light levels into voxelated movies of relative changes in haemodynamics can be broken into five separate phases (Supplementary Fig. 4): anatomical light modelling, light-level measurement pre-processing, image reconstruction, spectroscopy and spatial normalization. These steps are described in detail in the Supplementary Sections III, IV and VII.