# Optics Based Label-Free Techniques and Applications in Brain Monitoring
## 2 FNIRS in Human Brain Imaging
### Current Clinical Applications of fNIRS  
FNIRS is currently developed for various clinical applications, but still in most cases in proof of principle (PoP) phase. In oncology, fNIRS has a potential usage, e.g., in preoperative localization of tumors [64] and in cancer therapy monitoring. [PERSON] et al. [65] studied the vascular status and pathophysiological changes that occur during tumor vascularization in an orthotopic brain tumor model. They monitored HbO, HbR and water concentration changes within the tumor region and found a direct correlation between tumor size, intratumoral microvessel density and tumor oxygenation. The relative decrease in tumor oxygenation with growth indicated that, although blood vessels infiltrate the tumor region and proliferate in it, a hypoxic trend is clearly present. Furthermore, a preclinical study on a murine model of head and neck cancer suggests that monitoring of tumor oxygenation status could be used to predict treatment outcome in solid tumors [66]. Recently, fNIRS has been utilized before neurosurgical operation to map language areas in patients suffering from tumors such as glioma [67; 68] and to assess oxygen status in patients with glioblastoma [68; 69]. Presurgical evaluation of language functions is of great importance in brain tumor patients to spare these areas during surgery and to reduce the risk of postsurgical language deficits [67; 68]. FNIRS offers a valuable and easy tool for the identification of essential language functions, especially in children. [PERSON] et al. [68] demonstrated that expressive and receptive language functions can be identified separately using fNIRS in presurgical glioma patients. Further, [PERSON] et al. [67] showed that fNIRS permits identification of language networks in children at rest, i.e., without them needing to perform a task. They acquired fNIRS data at rest and during an expressive language task using a FD NIRS device (Imagent, ISS Inc) equipped with 8 detectors and 64 sources (32 at 690 nm and 32 at 830 nm). FNIRS fibers covered the bilateral frontal and temporal areas to measure brain activity in the anterior and posterior language-related regions, [PERSON]'s and [PERSON]'s areas and the Brodmann area. Results obtained in a resting state were compared to those of a more conventional task-based fNIRS measurement. This comparison indicated very good concordance between both approaches for language localization and hemispheric language dominance. [PERSON] et al. [70] utilized 44 channels (695 nm and 830 nm) together with EEG to localize an epileptogenic zone before the presurgical evaluation of epileptic patients. Use of fNIRS solves problems in ictal single-photon emission computed tomography (SPECT), Ictal PET and EEG/fMRI, by avoiding the risk of putting patients with epilepsy in the MRI chamber. A similar study was conducted by [PERSON] et al. [71], who used fNIRS-EEG to quantify nonlinear hemodynamic responses of human refractory focal epilepsy. They recommended routinely checking for nonlinearities in functional imaging of patients presenting with frequent spikes. [PERSON] et al. [72] proposed fNIRS application within the clinical psychology area because it has several important advantages over currently used methods such as fMRI and PET. In terms of cost, fNIRS is much more affordable and is suitable for all patients, while fMRI must consider the specific condition of the patients. [PERSON] et al. [73] and [PERSON] et al. [74] conducted a study to use fNIRS to monitor the frontal lobe and prefrontal cortical respectively in complex walking tasks among patients with [PERSON]'s disease and healthy older adults. They observed a different pattern of frontal activation while walking between two groups, which may have important implication for the rehabilitation of gait in patients with [PERSON]'s disease. [PERSON] et al. [75] conducted a pilot study using fNIRS-BCI (brain-computer interface) for gait rehabilitation.