## 4 Discussion
The large number of wavelengths enabled by SCL also give the possibility to go beyond the traditional contrasts, like the hemodynamic one, in order to get information about metabolism for example, by extracting information about more chromophores, like oxCCO [7; 24]. We can also note that beyond the intrinsic optical signal, a contrast agent, like ICG, can also be used in order to get information on tissue perfusion. This has notably been extensively used to monitor cerebral perfusion, and the SCL has also proved to be a suitable light source for these applications, even used in extremely complicated clinical settings like ICUs [65]. From a historical point of view, we can note that the development of commercial fibre also largely helped with the adoption of that technology. Indeed, out of the 35 different systems used in the reviewed paper, only 4 (less than 12%) are home-made. They correspond to the system developed before 2010. Past that date, all systems reviewed used a commercial source. We can also see the increase number of papers since the adoption of the commercial sources, with a number of papers published per year roughly doubling from that date. One of the main advantages reported by the authors when they adopted commercial SCL was their relative compactness, compared to the initial development of SC generation. This can be seen in Figure 2, with systems able to be mounted on trolley compatible with the clinical environment. However, the use of these lasers in actual clinical studies remains marginal. In our recent review [141], which has explored the use of TD-NIRS for clinical applications, less than 4% of the 52 papers reviewed were using a system based on a SCL source. Indeed, despite their advantages, a few drawbacks still hindered the adoption of SCL as a standard source. The first one is the cost which, despite a significant decrease in the recent years, remains high (i.e., more than 20,000 pounds for a SCL with several watts of power, enabling a final power of at least 1 mW per wavelength). The second main disadvantage is the warmup time, and the overall stability of these sources. Indeed, the stability of the output can be a concern, but the use of a reference arm can solve this issue [23]. This of course comes at the cost of a more complicated and bulkier instrument. Regarding the warmup time, typically, 1 h is required so the instrument reach thermal equilibrium, which can be too long for certain clinical applications. Some recent TD-NIRS systems based on pulsed laser diode have reported a warmup time of only 15 min, which is way more advantageous in a clinical setting [142]. Finally, even though the novel commercial SCL are more compact than in the early days, their size is still significantly bigger than traditional pulsed laser diode, which limit the reduction of the overall instrument size. Therefore, this can also slow down the adoption of these systems, as space are often very limited in busy clinical environment like ICUs. These drawbacks are less of an issue for the preclinical settings as the environment is way less constrained. Therefore, the great potential of applications based on SCL outweighs its current main drawbacks, and several instruments based on SCL have been developed for preclinical imaging. The interested reader can refer to the recent review by [PERSON] and colleagues, which explore more particularly the use of DOT in preclinical settings [143]. It worth noting that these systems are non-contact, and that this particular detection approach has also been explored for human imaging. Here, having an extremely narrow pulse, like the one provided by SCL, is of great interest as one can take advantage of the null-source approach [77]. Even if these approaches are less common, they can have potential great applications for perioperative measurements, or when the patient is difficult to access, and represent a promising avenue. The above-mentioned drawbacks are also less problematic in a typical lab setting.