# Optics Based Label-Free Techniques and Applications in Brain Monitoring
## 3 DCS Technology Used Currently in Human Brain Imaging  
DCS technique is based on temporal speckle fluctuations of the diffused light, reflecting blood flow in the microvasculature. In several medical applications, particularly in neurology, DCS is becoming an essential tool. To measure the blood flow index (BFI), DCS needs optical properties of tissue: reduced scattering and absorption coefficients (\(\mu_{s}^{\prime}\) and \(\mu_{a}\)). The principle of DCS technique is well described in several papers, for instance by [PERSON] and [PERSON] [107]. The correlation diffusion equation is based on the well-known radiation transport equation (RTE) with some applied approximation. Basically, DCS gains the target hemodynamics by measurement of the temporal electric field autocorrelation function (G1). This important quantity relates to the percentage of moving scatterers over all scatterers (\(\alpha\)), effective diffusion coefficient (DB), \(\mu_{s}^{\prime}\) and \(\mu_{a}\). So, a typical DCS system should be able to solve the correlation diffusion equation [21]. In Figure 2, the main components of a DCS setup are shown.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Reference** & **Remark** & **Study/Application** & **Phase** \\ \hline [PERSON] et al. [19] & \begin{tabular}{c} 2-wavelength LED, \\ frequency-encoded illumination, \\ time-multiplexing detector, fixed \\ number of channels \\ \end{tabular} & \begin{tabular}{c} Self-paced left-hand gripping \\ during outdoor bicycle riding, \\ indoor pedaling on a training \\ bicycle and sitting still \\ \end{tabular} & PoC \\ \hline [PERSON] et al. [102] & 2-wavelength LED, 16 channels & \begin{tabular}{c} Social daily life activities with \\ cognitive tasks \\ \end{tabular} & PoC \\ \hline [PERSON] et al. [91] & \begin{tabular}{c} 2-wavelength LED, time division, \\ modular up to 128 channels \\ \end{tabular} & \begin{tabular}{c} classic motor cortex stimulation \\ experiments: movement of \\ dominant hands and \\ thumb-to-finger extension task \\ \end{tabular} & PoC \\ \hline [PERSON] et al. [92] & \begin{tabular}{c} 2-wavelength LED, time division, \\ modular up to 128 individual \\ modules source or detector \\ \end{tabular} & \begin{tabular}{c} human brain activity in the left \\ prefrontal area of an adult male \\ participant during a verbal \\ fluency task \\ \end{tabular} & PoC \\ \hline [PERSON] et al. [93] & 8-wavelength LED, time division, \\ modular, multidistance & \begin{tabular}{c} visual stimulation paradigm \\ designed to elicit a functional \\ response in the primary visual \\ cortex \\ \end{tabular} & PoC \\ \hline [PERSON] et al. [94] & \begin{tabular}{c} 4-wavelength LED, time division, \\ modular, multidistance \\ \end{tabular} & \begin{tabular}{c} Arterial occlusion and \\ Task-evoked brain activity \\ \end{tabular} & PoC \\ \hline [PERSON] et al. [90] & \begin{tabular}{c} 2-wavelength LED, continuous \\ wave, configurable up to 128 \\ channels \\ \end{tabular} & \begin{tabular}{c} Arterial occlusion and Prefrontal \\ Cortex with various breathing task \\ \end{tabular} & PoC \\ \hline [PERSON] [104] & Continuous wave, 22 channels & Mental health & PoC in clinical use \\ \hline [PERSON] et al. [105] & 1 channel, 810 nm & \begin{tabular}{c} Stress management in daily \\ hassles \\ \end{tabular} & PoC \\ \hline [PERSON] et al. [106] & \
\begin{tabular}{c} 2-wavelength laser diode, \\ continuous wave, with ECG and \\ acceleration sensor \\ \end{tabular} &
\begin{tabular}{c} Normal daily activities, e.g., \\ walking, playing \\ \end{tabular} & PoC in clinical use \\ \hline \end{tabular}
\end{table}
Table 2: List of selected wearable fNIRS studies.