# Optics Based Label-Free Techniques and Applications in Brain Monitoring
## 3 DCS Technology Used Currently in Human Brain Imaging
### Wearable DCS Devices And the Commercial Ones  
A wearable technique to monitor absolute blood flow is presented in the patent [120]. This patent determines absolute blood flow using DCS technique and optical data is produced in faster manner even more than pulsatile frequency of a cardiac cycle. Afterwards, pulsatile blood flow measurement, as well as absolute blood flow measurement are done. Absolute blood flow is valuable for the diagnosis and treatment of patients with a wide range of medical disorders like traumatic brain injury [120]. Plus, a research team recently developed a fiber-less diffuse speckle contrast flowmeter (DSCF), which provides a low-cost device for monitoring of blood flow variations in relative deep tissues (up to \(\sim\)8 mm depth). This depth is usually sufficient for detecting CBF in newborn infants. The setup employs a CCD chip with no lens to detect any spatial fluctuation of laser speckles due to RBCs movement [22; 23; 121].  
As mentioned before, FD-NIRS measures the phase and frequency of modulated light [24; 25] and TD-NIRS detects the time of flight of short light pulses in the tissue [26; 27]. For commercial products, [PERSON] et al. [122] introduced \"MetaOx\" which works based on FD-NIRS and DCS instruments. For using NIRS-DCS pair simultaneously, it is necessary to resolve the problem of two light source interference and their detection. The presented setup includes 8 light wavelengths (670, 690, 705, 730, 760, 780, 810 and 830 nm) for FD-NIRS and one 850 nm long coherent laser for DCS. In the receivers' part, four PMTs and four APD are employed. Such a setup provides significant features: the usage of PMTs makes higher modulation depth possible and using 4 APDs for DCS allows multiple separation measurements resulting in a better signal to noise ratio (SNR).  
For the future vision of wearable DCS, making probes compact, mounting the detector and source closer (lower than 1 cm), are surely necessary. According to the banana-shaped paradigm of light propagation in the tissue, when the separation is very low, the deeper traveled photons will be lost. The solution is using a time-gated DCS method based on the possibility of the switching off and on the detector (time gating) to receive the deeper traveled photons. These photon's traveling time differs from the photons traveled only in the surface. It allows using a detector very close to the coherent light source, resulting in more compact probes for brain wearable devices [123]. In addition, the main challenge for conventional DCS is the need for the tissue's optical properties. By using TD-DCS mode, it is possible to find the optical properties and BFI at the same time. [PERSON] et al. [20] fabricated a prototype of TD-DCS and used an 852 nm laser combined with an electrically pumped taper diode amplifier to increase the maximum optical power while retaining the coherence length. The employed receiver is a new single photon APD (SPAPD) which can detect the photons with high sensitivity and time resolution. Development of TD-DCS, instead of using the NIRS-DCS pair can be an alternative for simplification of the next generation of DCS brain monitoring devices.