## 4 Conclusions  
In conclusion, we have presented a compact eight-channel time-domain system for diffuse optical tomography, exploiting the latest advances in photonics technologies and the spatial sampling capability of the time-domain approach. In a first simple configuration, with eight SiPMs that are directly embedded on the probe in a circular arrangement centered on a single injection source at 689 nm, the system is capable to acquire full tomographic data at an acquisition rate of 1 Hz. A basic reconstruction procedure based on analytical solution for the Born approximation is applied to the diffusion equation [41; 42], and a purely absorption change was adopted. A first study on simulations with realistic noise added demonstrated that the proposed scheme can recover buried absorption perturbations with \(\Delta\mu_{a}\ \geq 0.017\) mm\({}^{-1}\), down to a depth \(z=15\) mm, and as far as the inhomogeneity is beneath the detectors' circle. Figure 9: **(a)** 3D tomographic reconstruction of \(\Delta\mu_{a}\) for subject one, averaged only during the activation time course and indicating three selected regions: ROI_1 with solid lines and focused on the activation, ROI_2 with dashed lines and positioned far from the activation but at the same depth of ROI_1, and ROI_3 with dotted lines and considering the superficial 5 mm deep volume; **(b)** temporal time courses, averaged each 5 s, of the absorption properties of the three different regions for all three subjects. For each region, each \(\Delta\mu_{a}\) time value is calculated as the spatial average over the respective region. These findings were confirmed by experimental measurements on a solid switchable phantom. Tomographic images that are acquired at 1 Hz frame rate can detect the absorption inclusion (equivalent \(\Delta\mu_{a}\geq 0.010\) mm\({}^{-1}\) over 1 cm\({}^{3}\)) at a depth of 15 mm as far as the perturbation does not overcome the detectors' ring. In vivo absorption tomographies on volunteers performing a standard motor-cortex activation protocol (20 s finger tapping, 5 repetitions) showed a consistent brain activation response in two subjects out of three. In the positive cases, different task-related temporal evolutions of the absorption coefficient can be tracked in the cortex area as compared to the skull-scalp. The system still suffers from some limitations. The first limitation is the single source, which implies a poor spatial coverage, leading to artifacts, particularly in the superficial layer, and weak spatial resolution. To cope with those problems, an increased number of source points can be easily inserted by adding an optical switch. However, the distribution of sources and detectors has to be carefully studied to have a sufficient equalization of recorded photon counts for each detector. Another limitation is represented by the single wavelength, since the system is not capable to disentangle deoxy-(HHb) from oxygenated-hemoglobin (HbO\({}_{2}\)) variations. Also in this case the insertion of a switch coupled to lasers at different wavelengths can be a simple and effective solution to recover the concentration of both HHb and HbO\({}_{2}\). However, the increase in the number of sources requires a time multiplexing of the DTOF curves, thus forcing it to use a shorter acquisition time for each injection point in order to keep the 1 Hz acquisition rate. This will result in a decrease of the overall signal and also of the signal to noise ratio. For this reason, a trade-off between the number of sources and the minimum signal that is acceptable has to be considered. In vivo results are not fully satisfactory, although not so different from the expected variability in these kinds of tests. Possibly, these results are consistent with the limiting sensitivity of \(\Delta\mu_{a}\geq 0.017\) mm\({}^{-1}\) at 15 mm depth observed in simulations. The duplication of wavelengths and the increase in the number of injection sources are feasible, adopting e.g., fiber switches.