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

Quantitative functional magnetic resonance imaging methods make it possible to measure cerebral oxygen metabolism (CMRO 2 ) 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 CMRO 2 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 R 2 *; VSEAN to estimate baseline O 2 extraction fraction (OEF) from a measurement of venous blood R 2 , which in combination with the baseline CBF measurement yields an estimate of baseline CMRO 2 ; and FLAIR-GESSE to measure tissue R 2 ′ to estimate the scaling parameter needed for calculating the change in CMRO 2 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 CMRO 2 in response to a visual stimulus were in good agreement between the traditional hypercapnia calibration and calibration based on R 2 ′ . These results support the potential of gas-free methods for quantitative physiological measurements. Synopsis We tested noninvasive methods to measure absolute oxygen metabolism (CMRO 2 ) in both baseline and activation states without the use of special gases: VSEAN to measure baseline O 2 extraction fraction (OEF), and FLAIR-GESSE to measure R 2 ′ to estimate the scaling parameter M . Primary findings were: CMRO 2 changes to visual stimulation derived from R 2 ′ were similar to estimates based on hypercapnia-derived M ; OEF values were in good agreement with previous PET findings; and, variation of baseline CBF/CMRO 2 coupling across subjects does not follow activation coupling, suggesting different mechanisms may be involved. These results support the potential of gas-free methods for quantitative physiological measurements. Purpose To demonstrate the potential for two non-invasive techniques, VSEAN and FLAIR-GESSE, for absolute measurements of CMRO 2 during both baseline and activation states.