Quantification of the cortical contribution to the NIRS signal over the motor cortex using concurrent NIRS-fMRI measurements

Near-Infrared Spectroscopy (NIRS) measures the functional hemodynamic response occurring at the surface of the cortex. Large pial veins are located above the surface of the cerebral cortex. Following activation, these veins exhibit oxygenation changes but their volume likely stays constant. The back-reflection geometry of the NIRS measurement renders the signal very sensitive to these superficial pial veins. As such, the measured NIRS signal contains contributions from both the cortical region as well as the pial vasculature. In this work, the cortical contribution to the NIRS signal was investigated using (1) Monte Carlo simulations over a realistic geometry constructed from anatomical and vascular MRI and (2) multimodal NIRS-BOLD recordings during motor stimulation. A good agreement was found between the simulations and the modeling analysis of in vivo measurements. Our results suggest that the cortical contribution to the deoxyhemoglobin signal change (Delta HbR) is equal to 16-22% of the cortical contribution to the total hemoglobin signal change (Delta HbT). Similarly, the cortical contribution of the oxyhemoglobin signal change (,Delta HbO) is equal to 73-79% of the cortical contribution to the Delta HbT signal. These results suggest that Delta HbT is far less sensitive to pial vein contamination and therefore, it is likely that the Delta HbT signal provides better spatial specificity and should be used instead of Delta HbO or Delta HbR to map cerebral activity with NIRS. While different stimuli will result in different pial vein contributions, our finger tapping results do reveal the importance of considering the pial contribution. (C) 2011 Elsevier Inc. All rights reserved.