2.1. Instrumentation

The DCS system consisted of a long coherence length (>5 m) continuous-wave laser operating at 785 nm with a maximum power output of 100 mW (DL-785-100-3S, CrystaLaser, Reno, NV). The laser was coupled to a multimode fiber (NA = 0.22, core = 400 µm, 4.7 mm outer diameter; Fiberoptics Technology, Connecticut, US) which directed the light to the head. The diffusively reflected light was collected by three 2.5-m long single-mode fibers (SMF-28-J9, NA = 0.14, core = 8.2 µm, single-mode cutoff wavelength = 1260 nm, Thorlabs, New Jersey, US). Each detection fiber was wrapped into a 15-cm coil to convert higher order modes into non-propagating modes. Light from the detectors was received by a four-channel single photon counting module (SPCM-AQR-15-FC, PerkinElmer Canada Inc, Quebec, CA), which fed TTL pulses into an edge-detecting counter on a PCIe6612 counter/timer data acquisition board (National Instrument, Austin, Texas). Photon counts were recorded and processed using in-house developed software (LabVIEW, National Instrument and MATLAB). For each detector, the software recorded the total photon count and generated intensity autocorrelation curves at 40 delay times ranging from 1 to 40 µs.

The TR-NIRS system utilized a picosecond diode laser emitting at 802 nm (LDH-P-C-810, PicoQuant, Germany). The output and pulse repetition rate of the laser were set to 1.4 mW and 80 MHz, respectively, through a computer-controlled laser driver (PDL 828, PicoQuant). The light was coupled into a multimode fiber (NA = 0.22, core = 400 µm, 4.7 mm outer diameter; Fiberoptics Technology, US) and directed towards the head. Diffusively reflected light was collected by a fiber bundle (core = 3600 µm, NA = 0.55, Fiberoptics Technology, United States). The collected photons were then sent to a hybrid photomultiplier tube (PMA Hybrid 50, PicoQuant, Germany), coupled to a HydraHarp 400 (PicoQuant, Germany) time-correlated single photon counting module to generate a distribution of times of flight of photons (DTOF). The temporal dispersion caused by the system was corrected for by measuring the instrument IRF (FWHM = 0.489 ± 0.107 ns), and the system was allowed to warm up for one hour before an experiment to minimize instrument temporal drift [19].