## 3 Results and Discussion  
### Characterization of the Setup (Phantom Measurements)  
Figure 1b represents the IRF of the proposed setup for the two channels. For both wavelengths the dynamic range was about 5.5 decades, while the computed FWHM was about 120 and 180 ps, respectively, for 710 and 820 nm. Some reflections were visible at 5.5 ns or 4.8 ns after the main peak (depending on the wavelength). Those reflections were most probably due to a second order of Fresnel reflections at the tip of the fibers. When conceiving the setup, the fibers' length was chosen in order to delay reflections at least some ns from the peak. Indeed, reflections did not represent a problem for the measurements, since they were quite late and several decades (about 4.5) under the main peak. Other small reflections presented at a shorter time were not significant since their amplitude was small enough to not distort the curve or saturate the dynamic range of the gated measurements. Figure 2 reports the contrast and CNR computed at both the 5 and 30 mm source-detector distance (first and second row respectively) in the case of the fast-gating technique being applied (\"gated\", blue curve) or not (\"non-gated\", red lines). For all features, the time-window used for the analysis was opened 3 ns after the peak of the IRF (i.e., 3 ns after the injection of the light in the sample). When a 5 mm source-detector distance was used, the fast-gating technique was fundamental to retrieve information about a buried perturbation. Indeed, the contrast recorded at 5 mm source-detector distance in the non-gated mode was low and very noisy, while, for the gated acquisitions, the contrast was much higher and its standard deviation (computed on the several repetitions) was nearly negligible. The improvement given by the fast-gating technique was clearly noticeable also in the CNR graph. Indeed, the curve relative to the gated acquisition was always higher than 10, even at the deepest position of the inclusion. On the other hand, the CNR for the non-gated acquisition was always less than 10 and, in most cases, also smaller than 1, meaning that the contrast was more sensitive to the noise of the measurements (e.g., photonic noise) than to the presence of an inclusion. As predicted by the theory [9; 26], the improvement given by the fast-gating technique was fundamental when using a small interfiber distance. Indeed, when the distance between the launching and detection fiber was shortened, a huge increase of the scarcely diffused photons (the so-called \"early photons\") was experienced. If a non-gated detector was used, this peak of \"early photons\" caused the saturation of its dynamic range, thus limiting the number of late photons detected and lowering the capability to discriminate a perturbation in depth. The improvements given by the use of a fast-gated acquisition was less evident if a larger SDD was used, as it was clearly noticeable from curves reported in the last row of Figure 2. Indeed, the contrast obtained exploiting the fast-gating technique was slightly higher than what was achievable in the \"non-gated\" situation. The CNR graph clearly shows that in the gated mode there was a constant improvement of about a factor of 4. This was mostly because, when using the fast-gating technique, we recorded photons enabling the FG-SPAD module with a delay of 2.25 ns with respect to what was done in the non-gated mode, thus rejecting the first part of the time-resolved curve. The delay was chosen in order to exploit the dynamic range of the detector with the latest possible delay. Since we collected the same number of photons (nearly \(10^{6}\) per acquisition) for both gated and not gated acquisition, it was obvious that in the former case there was a boost in the number of late photons detected, thus increasing the number of counts recorded in the region of the time-window used for analysis.