Quantification of CMRO2 and CBF using Simultaneous NIRS and fMRI

Introduction : We introduce an accurate technique to estimate the cerebral metabolic rate of oxygen (CMRO2) and cerebral blood flow (CBF) using simultaneously measured near infrared spectroscopy (NIRS) and blood oxygenation level dependent (BOLD) fMRI signals. Owing to simultaneous acquisition of both fMRI and NIRS, separate hypercapnia condition or arterial spin labeling (ASL) acquisition are not necessary to quantify CMRO2 and CBF, which greatly improves the accuracy of the proposed method. The dynamic coupling ratio of CBF changes to CMRO2 changes has been also investigated. Experimental results using finger tapping task showed that the activation pattern of CBF calculated using NIRS-SPM software [1] is more specific to the primary motor cortex than fMRI BOLD and NIRS-HbR signal. Furthermore, the dynamic couple ratio coincides with the existing results from the literature [2]. Theory : A robust estimation of CMRO2 is important to understand the neural-metabolic-hemodynamic relationship. fMRI approach of Eq. (1) estimates CMRO2 from BOLD and CBF [3]. However, there are two main drawbacks in this approach. First, an ASL technique which measures CBF has low signal to noise ratio due to the small amplitude of the flow related MRI signal. Second, the hypercapnic condition is necessary to calibrate the scaling factor (M) between BOLD and HbR concentration. NIRS approach of Eq.(2) determines the CMRO2 from HbR and total-hemoglobin (HbT) [4]. However, there exists many unknown hyperparameters that determine the accuracy of NIRS estimates.