## Materials and Methods  
### System Architecture  
We built a CW system equipped with 24 LED sources (12 at 700 nm and 12 at 830 nm, arranged in couples, with each 700 nm/830 nm LED couple mounted with an inter-LED distance of 2 mm) and 13 SiPM detectors, alternatively arranged in a checkerboard of 8 cm \(\times\) 8 cm area and first nearest neighbor SiPM-double LED distance of 2 cm (Figure 1a). Since the system is intended to detect oxy- and deoxy-hemoglobin concentrations, the two wavelengths were chosen to straddle the isosbsite point of the two molecular species absorption spectrums located at \(\sim\)800 nm [24]. The system can acquire 325 independent time averaged measurements, that is: 13 measurements of 13 SiPMs dark currents, plus the signals under light, which are 13 SiPMs, multiplied by 24 LEDs (12 LEDs for each of the two wavelengths). In each cycle, each dark current is subtracted from the relative SiPM photocurrents measured at the two wavelengths. The overall measurement refresh rate of the complete system is about 2.1 Hz. By using the equipment in fNIRS mode, it is possible to acquire 156 optical channels (LED pair source; SiPM detector couple). In Figure 1a, the red circles represent the LED holders and the blue squares represent the SiPM holders. The 700 nm/830 nm LED pairs and each SiPM are mounted on a board and encapsulated in a holder of about 2 cm in diameter. A transparent filter (polycarbonate) is mounted on the top on the holders so that SiPMs and LEDs are optically well coupled and electrically insulated as seen in Figure 1b. The geometry of this arrangement is characterized by SDS distances of 2 cm, 4.47 cm, 6 cm, 7.21 cm, 8.24 cm, and 10 cm. Through the chosen placement of LEDs and SiPMs, it is possible to obtain various overlapping measurements at different SDSs in numerous points.  
It has been shown that in fNIRS of the human brain cortex, as SDS increases, the photodetector signal becomes more sensitive to the tissues located at a larger depth and in the middle region between the source and the detector, with SDS values lower than 6 cm. Above such a value, the SNR and the exponential decrease in sensitivity at large inter-optode distances give rise to important limitations [8]. The system described here takes into consideration quite large values of SDS, up to 10 cm. As shown in the following, such values are possible because of the combination of two factors: first, the better SNR achievable thanks to the use of the SiPM as a photodetector; second, the lower scattering of the phantom material compared to the human head tissues (about 28%).  
To test the proposed system, we built a multilayered phantom made of a medium providing high light scattering. The medium is expanded polyethylene (EPE). Starting from the base at the bottom (Figure 2), the phantom is constituted by a 50 cm \(\times\) 40 cm \(\times\) 3 cm EPE layer and a 3 cm thick layer of air in which a Newport Step Motor moves a 1 mm diameter/20 cm long metal bar. On the top, we placed a second 3 cm thick EPE layer on which the black patch with sensors and LEDs is positioned.  
The stepper motor rotates 100 degrees in about 200 s (0.5 deg/s), so that the end of the bar travels a linear distance of about 10 cm in 30 s. Thanks to the motion driven by the stepper motor, the phantom is dynamic and constitutes a tool to test our system with good reproducibility.