The potential for gas-free measurements of absolute oxygen metabolism during both baseline and activation states in the human brain

Abstract

Quantitative functional magnetic resonance imaging methods make it possible to measure cerebral oxygen metabolism (CMRO₂) in the human brain. Current methods require the subject to breathe special gas mixtures (hypercapnia and hyperoxia). We tested a noninvasive suite of methods to measure absolute CMRO₂ in both baseline and dynamic activation states without the use of special gases: arterial spin labeling (ASL) to measure baseline and activation cerebral blood flow (CBF), with concurrent measurement of the blood oxygenation level dependent (BOLD) signal as a dynamic change in tissue R₂*; VSEAN to estimate baseline O₂ extraction fraction (OEF) from a measurement of venous blood R₂, which in combination with the baseline CBF measurement yields an estimate of baseline CMRO₂; and FLAIR-GESSE to measure tissue R₂' to estimate the scaling parameter needed for calculating the change in CMRO₂ in response to a stimulus with the calibrated BOLD method. Here we describe results for a study sample of 17 subjects (8 female, mean age = 25.3 years, range 21-31 years). The primary findings were that OEF values measured with the VSEAN method were in good agreement with previous PET findings, while estimates of the dynamic change in CMRO₂ in response to a visual stimulus were in good agreement between the traditional hypercapnia calibration and calibration based on R₂'. These results support the potential of gas-free methods for quantitative physiological measurements.

Keywords: Blood oxygenation level dependent; Cerebral blood flow; Cerebral metabolism of oxygen; Functional MRI; Hypercapnia; Oxygen extraction fraction.