## 4 Applications
### Tomography  
Another field where time-gating is relevant is diffuse optical tomography (DOT). Indeed, in the last years several clinical and diagnostics applications [8; 131] adopted DOT due to its capability to improve the imaging of deep inhomogeneities of the medium (e.g., tumor, local changes in O\({}_{2}\)Hb/HHb due to brain activation, etc.). The technological improvements in terms of both detectors (matrix of small detectors or large area ones) and detection electronics allows one to have faster measurements, thus going in the direction of real-time tracking of buried objects [132; 133; 134; 135]. Additionally, the use of multiple-view acquisitions coupled to the tomographic reconstruction enables: (i) to overcome the limitation in spatial resolution typical of the diffuse optics; (ii) to improve the quantification of the absorption and scattering coefficient of the inclusion [136; 137; 138]. All those advantages, coupled with the enhanced penetration depth given by the use of the time-gating technique, could be exploited to acquire in real time images which may also provide the quantification of the main chromophore in the tissues (e.g., O\({}_{2}\)Hb and HHb for brain imaging or also collagen, lipid, water for breast imaging). This possibility  
Figure 12: Retrieved absorption coefficient for measurements done with a single fiber [130]. opens up a wide range of potential disruptive applications in the imaging field both as stand-alone technique, or in conjunction with other imaging techniques. For example, DOT with a small \(\rho\) could permit the continuous and non-invasive monitoring of deep vessel permeability (even buried three or more cm in depth). On the other hand, if coupled to other techniques (e.g., ultrasound [139; 140; 141; 142]), DOT could represent a powerful diagnostics tool for oncology thanks to the simultaneous information on position and size of a suspect lesion (using ultrasound technique) and the quantification of tissue constituents (given by DOT), potentially giving hints about the lesion malignancy. Along this direction, different papers have been published in the last years. Firstly, as soon as the implementation of the time-gating technique was possible, it was demonstrated the improvement given by the short \(\rho\) approach in terms of maximum depth sensitivity [143]. As schematically depicted in Figure 13, the improvement given by the use of the TG acquisition is clear for both a short (5 mm) and large (15 mm) \(\rho\). It is evident that the use of TG technique is fundamental when \(\rho\) = 5 mm is used to see a deep inclusion. However, due to the memory effect [111; 112], the theoretical gain in depth sensitivity is not attained when using a very small \(\rho\). Indeed, using \(\rho\) = 15 mm with the TG technique it is possible to detect the inclusion up to a depth of 31 mm with a good localization and an improvement of around 50% with respect to the non-gated acquisitions. The use of the TG technique was shown to improve the spatial resolution, as foreseen by theory. Indeed, in Ref [144] the authors demonstrated that using a small \(\rho\) (e.g., 10 mm where the photons' confinement is improved) it was possible to distinguish two inclusions (separated by 3 cm) at a depth of about 15 mm (see Figure 14). Going deeper, the inclusions will not be detectable due to the broadening of the banana shape thus causing artifacts to arise in the middle. Those conclusions were supported also by simulations thus setting a limit to the achievable spatial resolution with the current detector technology. Figure 13: 2D maps of an inclusion (\(\Delta\mu_{a}\) = 0.5 cm\({}^{-1}\)) buried at different depths (rows) inside a homogeneous medium (optical properties: \(\mu_{a}\) = 0.1 cm\({}^{-1}\) and \(\mu_{s}\)’ = 10 cm\({}^{-1}\)) obtained using non-gated and gated approaches for both 5 and 15 mm \(\rho\). The red circle represents the true position of the inclusion. Dimensions of the plane are in mm. For more depths and the geometry of the maps, see Ref. [143].