# Assessment of fNIRS Signal Processing Pipelines: Towards Clinical Applications
###### Abstract  
Functional Near-Infrared Spectroscopy (fNIRS) captures activations and inhibitions of cortical areas and implements a viable approach to neuromonitoring in clinical research. Compared to more advanced methods, continuous wave fNIRS (CW-fNIRS) is currently used in clinics for its simplicity in mapping the whole sub-cranial cortex. Conversely, it often lacks hardware reduction of confounding factors, stressing the importance of a correct signal processing. The proposed pipeline includes movement artifact reduction (MAR), bandpass filtering (BPF), and principal component analysis (PCA). Eight MAR algorithms were compared among 23 young adult volunteers under motor-grasping task. Single-subject examples are shown followed by the percentage in energy reduction (ERD%) statistics by single steps and cumulative values. The block average of the hemodynamic response function was compared with generalized linear model fitting. Maps of significant activation/inhibition were illustrated. The mean ERD% of pre-processed signals concerning the initial raw signal energy reached 4%. A tested multichannel MAR variant showed overcorrection on 4-fold more expansive windows. All of the MAR algorithms found similar activations in the contralateral motor area. In conclusion, single channel MAR algorithms are suggested followed by BPF and PCA. The importance of whole cortex mapping for fNIRS integration in clinical applications was also confirmed by our results. continuous wave functional near-infrared spectroscopy; rehabilitation monitoring; brain activation mapping; motor tasks; functional near-infrared signal processing; movement artifact removal; hemispheric hemodynamic response; clinical fNIRS translation +
Footnote †: journal: Medical  
many intrinsic or dye-enhanced optical responses available to investigators [2; 3; 4; 5]. The feature which allows HRF to be accessible to fNIRS and other non-invasive transcranial measurements is the critical variation of perfusion to even modest increments in the local oxygen consumption. This is probably needed to compensate for transport delays from vessels to cells and is driven by powerful neuro-vascular coupling mechanisms [5]. Briefly, neuro-vascular coupling actively enhances the regional cerebral blood flow and volume. A peak in oxygenated hemoglobin concentration \([HbO_{2}]\) having a time delay of 4-5 s after neuronal activation is shown. Venous outflow increases, causing a drop in deoxygenated (alias, reduced) hemoglobin concentration \([HbR]\). Next, the hemodynamic equilibrium recovers its baseline within 10-12 s. Accordingly, HRF-related signals are powerful, yet indirect markers of neural activity with a fair spatial resolution (order of 10 mm) but limited time resolution (order of seconds, compared to the 1 ms of neural action potentials). Nonetheless, functional imaging has gained a prominent position in studying brain anatomic-functional organization, functional and practical connectivity, and their damage associated with neural pathologies [6]. The first alternative approach to fNIRS for monitoring regional blood volume changes was based on blood labeling radiotracers in Single Photon Emission Computed Tomography and Positron Emission Tomography [7]. Despite the limitation in both space and time resolution of nuclear imaging, these pioneering studies paved the way to the actual application of functional imaging. The advent of functional MRI (fMRI) [8] offered better space and time resolution, as well as complete non-invasiveness and improved signal-to-noise (SNR) ratio. Indeed, fMRI is currently the gold standard in the non-invasive investigation of neural activity, even if the unique quantification capabilities of nuclear medicine should not be forgotten. fMRI is based on the blood-oxygen-level-dependent (BOLD) contrast, which reflects up to 4-5% of the overall anatomical contrast. The BOLD signal is due to the washout, which augments the local magnetic field homogeneity and, ultimately, the signal level due to its paramagnetic properties.