## 1 Introduction
Examples of TD-DO in the field of biomedical optics are (but not limited to) the monitoring of brain hemodynamics [12] or of muscular oxidative metabolism [17]. Other applications of TD-DO ranges from the non-invasive deep spectroscopy of biological tissues like the breast [18] or the bone [19] to the basic characterization of key absorbers such as lipid [20] or collagen [21]. TD-DO was also successfully applied in the non-destructive assessment of qualities of fruit [22] and wood [23], and in the optical characterization of pharmaceutical products [24] and semiconductor powders [25] or even on small animals [26]. The TD technique improves DO capabilities in terms of depth resolution and sensitivity, as will be widely explained in Section 2. For example, it permits to enhance the signal arising from the brain  
Figure 1: Using time-gating it is possible to extract few late photons (red) carrying information from deeper regions. Conversely, a classical continuous wave (CW) measurement (blue and red) mix all the photons, diluting depth sensitivity. cortex, avoiding confounding physiological effects due to the extra-cerebral compartment [16]. This feature is carried out thanks to the possibility to evaluate the DTOF in particular temporal instants. The method of selecting the photons on the basis of their arrival time is called \"time-gating\" and is typically accomplished as a post-processing analysis on the acquired signal. The DTOF is sliced into multiple temporal windows associated with the photon arrival time [27]. Then, different approaches to remove the early (superficial) photons from the whole signal [28; 29; 30] can be applied. More recently, a \"hardware\" gating was proposed to select photons on the basis of their arrival time right at the detection stage itself [31]. For this purpose, detectors operated in a time-gated (TG) modality instead of a free-running regime are needed. Apart from traditional photomultiplier tubes (PMTs) [32] and silicon photomultipliers (SiPMs) [33], which at present cannot be time-gated, the other detectors typically employed in TD-DO, such as intensified charge coupled device (ICCD) [28; 34], microchannel plates (MCP) [35], streak cameras [36] and single-photon avalanche diodes (SPADs), [37] can be used also in the TG modality [31]. In almost all these detectors, performances are limited by early photons impinging onto the detector, thus increasing dramatically the background noise or even damaging the detector. This problem can be overcome by employing SPADs [31], which will be widely discussed in Section 3. Furthermore, these detectors allow for new frontiers of measurements such as the small source-detector distance approach [38], which opens the way to better performances and novel applications. In addition, microelectronics detectors like SPADs and SiPMs permit to reduce the dimensions of the detection stage, in particular combined with new generation of time-to-digital converters (TDCs) [39] rather than the more traditional TD acquisition electronics based on time-correlated single photon counting (TCSPC) [40] boards, enabling the development of portable TD-DO systems. The scalability, modularity and the possibility of components miniaturization, with a resulting drop of the production costs, together with the other advantages already listed, are interesting characteristics of TG-DO. In the future, these will allow for the realization of wearable devices for continuous monitoring of the patient at home, in the settings of the personalized medicine of a rehabilitation program, for example. This technology could be applied for athletes training programs, to monitor and adjust their exercise on the basis of the muscular performances [41; 42; 43].