Functional near-infrared spectroscopy (fNIRS) and functional MRI (fMRI) are non-invasive techniques used to relate activity in different brain regions to certain tasks. Respiratory calibration of the blood oxygen level dependent (BOLD) signal, and combined fNIRS–fMRI approaches have been used to quantify physiological subcomponents giving rise to the BOLD signal. A comparison of absolute oxygen metabolism parameters between MRI and NIRS, using spatially resolved (SRS) NIRS and respiratory calibrated MRI, could yield additional insight in the physiology underlying activation.Changes in the BOLD signal, cerebral blood flow (CBF), and oxygen saturation (SO 2 ) were derived from a single MRI sequence during a respiratory challenge in healthy volunteers. These changes were compared to SO 2 obtained by a single probe SRS NIRS setup. In addition, concentration changes in oxygenated (O 2 Hb), deoxygenated (HHb), and total haemoglobin (tHb), obtained by NIRS, were compared to the parameters obtained by MRI.NIRS SO 2 correlated with end-tidal CO 2 (0.83, p<0.0001), the BOLD signal (0.82, p<0.0001), CBF (0.85, p<0.0001), and also MRI SO 2 (0.82, p<0.0001). The BOLD signal correlated with NIRS HHb (−0.76, p<0.0001), O 2 Hb (0.41, p=0.001), and tHb (r=0.32, p=0.01).Good correlations show that changes in cerebral physiology, following a respiratory challenge, go hand in hand with changes in the BOLD signal, CBF, O 2 Hb, HHb, NIRS SO 2 , and MRI SO 2 . Out of all NIRS derived parameters, the SO 2 showed the best correlation with the BOLD signal.
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