# Optics Based Label-Free Techniques and Applications in Brain Monitoring
## 2 FNIRS in Human Brain Imaging
A third relatively common technique is the frequency-domain (FD) method, which is based on modulating the intensity of the illuminating light, at frequencies of the order of tens to hundreds of MHz, and detecting both the attenuation and phase delay [47]. Theoretically, TD and FD approaches can provide absolute values for hemodynamic changes because tissue absorption and reduced scattering coefficients can be determined directly [48]. Multichannel NIRS is often referred to optical brain topography [11; 42; 49] or tomography, when utilizing overlapping and multi-distance remarks [50]. For instance, [PERSON] [51] developed high-density diffuse optical tomography (HD-DOT) to map functional activation of the visual cortex in healthy term-born infants. The functional images show a high contrast-to-noise ratio obtained in seven neonates. These results illustrate the potential for HD-DOT and provide a foundation for investigations of brain function in more vulnerable newborns, such as preterm infants. [PERSON] et al. [52] presented an approach to optimize the 3-dimensional brain functional activation image with high resolution in order to enhance the spatial information of cortical activity. The proposed algorithm acquired significant results for 3D functional maps with high resolution, in comparison with that of 2D functional t-maps. Different NIR spectrometers and their key features and parameters have been reviewed in detail, for example, in the following publications ([PERSON] et al., [PERSON] et al.) [11; 53] and handbooks ([PERSON], [PERSON] et al.) [8; 10]. Several commercial NIRS systems for brain monitoring are available [53] and some of them are mentioned in the following. For example, the Hamamatsu company has produced a compact and lightweight NIRS device (NIRO-200 NX), aiming at clinical usage. It uses 3 wavelengths (735 nm, 810 nm and 850 nm) [54]. The OBELAB company developed NIRSIT system for prefrontal cortical sampling [55]. It is probably the first fNIRS system that can provide high spatial resolution (millimeter-level) while also providing high temporal resolution (125 ms/8 Hz). The Biopac company has a wide range of fNIRS  
Figure 1: The basic principle of the functional near-infrared spectroscopy (fNIRS) technique. Penetrations depth is dependent on the distance between source and detector [9]. devices, such as fNIR100, using wavelengths of 730 nm and 850 nm and, as receivers, silicon PDs (SiPD) with integrated transimpedance preamplifier [56]. NIRSPORT2 is the product of NIRx company which is completely wearable and portable. It can be combined also with EEG and fMRI for simultaneous recordings. It uses SiPD or avalanche PD (APD) in the receiver and 760 nm and 850 nm LEDs in the source [57]. The Rogue research group designed for clinical applications 'BrainSight' which uses laser diodes (705 nm and 830 nm) and Si APD and SiPD detectors [58]. The Hitachi ETG-4100 consists of 695 nm and 830 nm laser diodes and provides 2D topographic images [59]. The TECHEN company has developed a real-time DSP-based system, namely CW6. It can monitor in real-time the entire head using up to 32 lasers and 32 detectors. Utilizing the CW6 32x32 system, it is possible to acquire data for 32x32 or 1024 individual raw signal measurements at an improved spatial resolution [60]. Another NIRS producer is Artinis with a wide range of NIRS devices [61]. It also produces fNIRS-EEG and transcranial direct electrical stimulation (tDCS) packages. Starstim fNIRS is an fNIRS-EEG-tDCS product which includes up to 24 channels for fNIRS and 32 channels for both tDCS and EEG. The wavelengths for fNIRS are 760 nm and 850 nm [62]. Imagent, the ISS company's fNIRS product, is an MRI compatible device which has up to 512 channels. It utilizes the FD technique and up to 64 LDs emitting at 690 nm and 830 nm and up to 32 photomultipliers tubes (PMTs) as detectors [63].