# Optics Based Label-Free Techniques and Applications in Brain Monitoring
## 2 FNIRS in Human Brain Imaging  
FNIRS, or diffuse optical spectroscopy (DOS), is mainly utilized for measuring oxygenation changes in the cerebral cortex that are linked to brain function. The principle of fNIRS is based on absorption and scattering which are wavelength-dependent. Each chromophore (a light-absorbing molecule) has a distinct absorption spectrum determined by its content and energy level structure. These spectra can be used as footprints for detecting individual compounds in tissue [8]. Most commonly the spectrum range between 660 nm and 950 nm is exploited to have sufficiently low light attenuation to enable light \begin{table}
\begin{tabular}{c c c c c c c c c c c c} \hline \hline  & & & & & & & & & & & & & & \\ \hline \multirow{2}{*}{**FNIRS**} & Scattering + absorption & \multirow{2}{*}{\begin{tabular}{c} CMRO\({}_{2}\), \\ HBT \\ \end{tabular} } & \multirow{2}{*}{660-950} & \multirow{2}{*}{\
\begin{tabular}{c} LED, LD, \\ Laser \\ \end{tabular} } & \multirow{2}{*}{PD} & \multirow{2}{*}{A few cm} & \(\
eg 1\) & \multirow{2}{*}{Yes} & \multirow{2}{*}{High} & \multirow{2}{*}{Yes} & \multirow{2}{*}{Low} \\  & & & & & & & & & & & & \\ \hline \multirow{2}{*}{**FNIRS**} & Speckle fluctuation & \multirow{2}{*}{\begin{tabular}{c} BFI, \\ CIRO\({}_{2}\) \\ \end{tabular} } & \multirow{2}{*}{660-950} & \multirow{2}{*}{\begin{tabular}{c} Laser \\ (coherent) \\ \end{tabular} } & \multirow{2}{*}{APD} & \multirow{2}{*}{\
\begin{tabular}{c} up to \(\
eg 15\) \\ cm \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} \(\
eg 1\) \\ cm \\ fast \\ \end{tabular} } & \multirow{2}{*}{Yes} & \multirow{2}{*}{High} & \multirow{2}{*}{Yes} & \multirow{2}{*}{Low} \\  & & & & & & & & & & & \\ \hline \multirow{2}{*}{**FNIRS**} & \multirow{2}{*}{PA effect} & CMRO\({}_{2}\), & \multirow{2}{*}{500-1300} & \multirow{2}{*}{LD, Laser} & \multirow{2}{*}{\
\begin{tabular}{c} up to \(\
eg 1\) \\ array \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} up to \(\
eg 1\) \\ cm in soft \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} Up to \(\
eg 1\) \\ \(\mu\)m \\ \end{tabular} } & \multirow{2}{*}{Yes} & \multirow{2}{*}{Depend on \\ \(\mu\)m & & & & & & & & & & \\ \hline \multirow{2}{*}{**FNIRS**} & Light coherence properties & CBF, & Visible & \multirow{2}{*}{\begin{tabular}{c} Wide \\ +IR \\ \end{tabular} } & band source & PD\({}^{b}\) & \
\begin{tabular}{c} up to \(\
eg 1\) \\ \(\mu\)m \\ \end{tabular} & No &
\begin{tabular}{c} Depend on \\ image size, \\ \(\
eg 1\) \\ \end{tabular} & Yes & Low \\ \hline \multicolumn{1}{c}{} & \multicolumn{6}{c}{Publication [38]} \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Comparison between most common optical techniques that are non-invasive, label-free and can provide information on brain function. to penetrate in the brain. As a light source, most of these setups employ a high-power LED or laser diode (LD) and highly sensitive photodiodes (PD) to detect back-scattered light [39; 40] see Figure 1.  
FNIRS systems still use most commonly continuous-wave (CW) technique, where the illuminating light has a constant frequency and amplitude [41]. CW technique is relatively low cost and can be also miniaturized as a wearable system, however, it cannot provide the absolute value of HbO and HbR [42]. In CW, amplitude of each wavelength is usually modulated at frequencies of the order of few to tens in kHz range, for distinction of the wavelengths and enabling better sensitivity when using, for example lock-in amplification [40]. The time-domain (TD) technique uses a very short illuminating NIR pulse, generally with a pulse length of a few picoseconds, while the photon path-length is based on time-of-flight [43; 44]. The technique is attracting increasing interest because of its recent technological developments, making it now more affordable, robust and smaller in size [45; 46].