Short separation channel location impacts the performance of short channel regression in NIRS

Abstract

Near-Infrared Spectroscopy (NIRS) allows the recovery of cortical oxy-and deoxyhemoglobin changes associated with evoked brain activity. NIRS is a back-reflection measurement making it very sensitive to the superficial layers of the head, i.e. the skin and the skull, where systemic interference occurs. As a result, the NIRS signal is strongly contaminated with systemic interference of superficial origin. A recent approach to overcome this problem has been the use of additional short source-detector separation optodes as regressors. Since these additional measurements are mainly sensitive to superficial layers in adult humans, they can be used to remove the systemic interference present in longer separation measurements, improving the recovery of the cortical hemodynamic response function (HRF). One question that remains to answer is whether or not a short separation measurement is required in close proximity to each long separation NIRS channel. Here, we show that the systemic interference occurring in the superficial layers of the human head is inhomogeneous across the surface of the scalp. As a result, the improvement obtained by using a short separation optode decreases as the relative distance between the short and the long measurement is increased. NIRS data was acquired on 6 human subjects both at rest and during a motor task consisting of finger tapping. The effect of distance between the short and the long channel was first quantified by recovering a synthetic hemodynamic response added over the resting-state data. The effect was also observed in the functional data collected during the finger tapping task. Together, these results suggest that the short separation measurement must be located as close as 1.5 cm from the standard NIRS channel in order to provide an improvement which is of practical use. In this case, the improvement in Contrast-to-Noise Ratio (CNR) compared to a standard General Linear Model (GLM) procedure without using any small separation optode reached 50 % for HbO and 100 % for HbR. Using small separations located farther than 2 cm away resulted in mild or negligible improvements only.

Hand-grasping and finger tapping induced similar functional near-infrared spectroscopy cortical responses

“…Gagnon et al 35 investigated cortical versus superficial oxygenation changes elucidated from a finger tapping task during simultaneous fNIRS and BOLD fMRI. In addition, yet another group looked at cortical activation during apple peeling using fNIRS and fMRI.…”

A review on continuous wave functional near-infrared spectroscopy and imaging instrumentation and methodology

“…In Saager et al (2011) an improved version of the approach was presented using an 8-channel probe configuration and 2 small SD channels (5 mm). Gagnon et al (2011Gagnon et al ( , 2012 investigated how the position of the short SD channel relative to the long SD channel impacts the performance of the method. They concluded that the separation between the channel of interest and the regressor distance should be b 1.5 cm.…”

Method for the discrimination of superficial and deep absorption variations by time domain fNIRS

“…These systemic phenomena typically occur in the superficial tissues (mainly the scalp) and can confound the readings of the instruments, especially when task related physiological responses happen [6,7]. A multi-channel configuration with sensors placed at different distances from the injection point is the solution that is currently being adopted in CW fNIRS to reduce the crosstalk between the superficial systemic signal and the deep cortical signal [8][9][10]. Conversely, in the time domain (TD) approach to fNIRS, picosecond pulsed lasers and fast photodetectors are used to acquire the photon distribution of time-of-flights in the head.…”