# Clinical Brain Monitoring with Time Domain NIRS: A Review and Future Perspectives
## 4 Discussion
This open issue still needs to be evaluated but we can report that new methodological developments might help in this (see next section). All of these different clinical applications require us to take advantages of the possibility of TD-NIRS to measure absolute values of brain tissue saturation and haemoglobin concentration, to compare either patients with controls, injured and less injured hemispheres, or the same patient over time. In all situations, TD-NIRS showed itself relatively easy to use and functionally robust. Even though TD-NIRS has the reputation of requiring bulky instruments, most of the instruments are transportable and useable at the patient bedside, without disrupting patient care. Indeed, even though a large number of these clinical applications can be done using CW-NIRS systems, the inability of such instruments to interrogate the absorption and scattering properties of the brain tissue will affect their accuracy. This has been for example demonstrated in [93], where a modification of the scattering properties of TBI patients were reported, or in [91] where modification of the optical path length between the affected and unaffected hemispheres of stroke patients were noted. The use of TD-NIRS to acquire insight in anatomical differences of patients is, thus, an interesting path to explore and could justify their use even further. Moreover, the optical properties information extracted from TD-NIRS system could also be used to strengthen the application of the CW-NIRS, by providing more accurate information of path length, as has been seen in the applications in psychiatry. Lastly, it is worth mentioning that even though TD-NIRS can provide absolute information about HHb and HbO\({}_{2}\) concentrations, the extraction of the absolute properties and then of the concentration of the chromophore are also subjected to assumptions that also limit the accuracy of TD-NIRS. Indeed, the fitting approach is usually based on the solution of the DE in simple homogeneous geometries which do not reflect the complexity of the head architecture [119]. Moreover, one major assumption that needs to be made to calculate the chromophore concentration from the absorption coefficient, is the number of chromophores considered. Indeed, it is usually assumed than HHb, HbO\({}_{2}\) and water are the only significant absorbers, with the water content most often fixed to a certain value (i.e., usually 80% for adults) [41]. However, it is likely that this water content is highly subject-dependent, notably due to the variability of the thickness of skin, skull and CSF and that other chromophores might need to be considered [120]. New methodological and hardware developments are addressing these issues, and these will be detailed in the next section. We could also see that by using complementary techniques, like DCE-TD-NIRS or TD-NIRS and DCS, it was possible to assess brain perfusion. In particular, the accuracy of the assessment of brain perfusion with the DCE technique, although achievable with a CW-NIRS system, is greatly improved by using TD-NIRS instrumentation, notably because it helps to distinguish between the extra- and intra-cerebal tissues. Having the possibility to continuously assess the CBF together with the brain oxygenation, at the bedside, is a true benefit because it can give information about both oxygen delivery and demand, facilitating the identification of the underlying issue. The novel development in that area (see next section) will probably contribute greatly to the adoption of TD-NIRS in the clinic. Finally, TD-NIRS proved to be able to be easily integrated with other modalities, and that even simultaneous imaging was possible with MRI, MEG or EEG. As the use of multi-modality imaging in modern medicine is growing, because it helps to get a clearer picture of a patient condition, this capability is also a great strength for TD-NIRS. In the context of brain imaging, this possibility can be used to give a complete picture of the link between anatomical and functional disorders.