# Optics Based Label-Free Techniques and Applications in Brain Monitoring
## 2 FNIRS in Human Brain Imaging
### Towards Wearable Brain Monitoring Using fNIRS  
A possibility to build a wearable fNIRS device and provide long-term monitoring opens tremendous new opportunities for clinical monitoring and diagnostics. In general, a wearable fNIRS system requires considering some important aspects such as portability and wearability [90], configurability [90; 91; 92; 93; 94] and real-time data processing and visualization [94]. In addition, experimental validation, low power consumption, appropriate post-processing is required to handle motion artefacts, such as, principal component analysis [95], independent component analysis [96], Kalman Filter [97], correlation-based signal improvement [98], wavelet [99] and spline interpolation [100]. On the other hand, signal processing algorithms must be relatively light for the sake of battery life. Further, to enable long term clinical monitoring, the stability of the optical contact with the scalp is of high importance [101].  
Table 2 presents some important works on wearable fNIRS along with the configurations and studies/applications. Wearable fNIRS studies have been increasingly conducted. [PERSON] et al. [19] developed a wearable multichannel fNIRS using eight channels having wavelengths of 760 nm and 850 nm. [PERSON] et al. [102] explored the use of a wearable fNIRS to monitor brain activity during a real-world prospective memory task. The system was covered with a black cap to avoid ambient light during outdoor experiment. [PERSON] et al. [91] and [PERSON] et al. [92] developed a wearable fNIRS with modular optodes. To enable brain hemodynamics and metabolism monitoring at different depths, [PERSON] et al. [93] and [PERSON] et al. [94] implemented a multi-distance fNIRS. The WearLight was developed by [PERSON] et al. [90] as a configurable wearable fNIRS. It was a battery-operated system with CWfNIRS that can be configured up to 128 channels with 8 dual-wavelength (750 nm and 850 nm) LEDs. Commercial wearable fNIRS are also available on the market [103].  
For example, a wearable fNIRS was used to evaluate mental health [104]. It reported studies on measuring inter-participant interaction using hyper scanning and practical application of evaluating the mental state of participants in a return-to-work program. During the exploration, a wearable NIRS system measured changes in cerebral blood during a performance of working memory. Further, the mood state was inferred from the NIRS signal and results were compared with a so called profile of mood states (POMS) questionnaire.  
Another study used a wearable fNIRS to provide signals as biofeedback for stress management in daily hassles [105]. It measures cerebral blood flow from the rostrolateral prefrontal cortex (RLPFC) to the frontopolar cortex and heart rate as the biological signals associated with blood flow. The hypothesis was that learning to regulate the activity of the RLPFC/frontopolar cortex would help to change the negative mood and thought patterns. In addition, to enhance stress-coping skills by learning to regulate the access to internal states. They found that the biofeedback had effects on the regional gray matter variation, psychological test scores and salivary cortisol levels were associated with daily hassles.  
A compact wearable system consisting of fNIRS, electrocardiography (ECG) and acceleration sensors was developed for 24 h ambulatory monitoring of cerebral hemodynamics, systemic hemodynamics, ECG and actigraphy [83]. [PERSON] et al. [19] and [PERSON] et al. [102] developed a wearable multichannel fNIRS to monitor brain activity for outdoor use with normal daily activities, such as walking, cycling and meeting other people.