QUantitative Imaging of eXtraction of oxygen and TIssue consumption (QUIXOTIC) using venular-targeted velocity-selective spin labeling.

ABSTRACT

While oxygen extraction fraction (OEF) and cerebral metabolic rate of oxygen (CMRO(2)) are fundamental parameters of brain health and function, a robust MRI-based mapping of OEF and CMRO(2) amenable to functional MRI (fMRI) has not been established. To address this issue, a novel method called QUantitative Imaging of eXtraction of Oxygen and TIssue Consumption, or QUIXOTIC, is introduced. The key innovation in QUIXOTIC is the use of velocity-selective spin labeling to isolate MR signal exclusively from postcapillary venular blood on a voxel-by-voxel basis. Measuring the T(2) of this venular-targeted blood allows calibration to venular oxygen saturation (Y(v)) via theoretical and experimental T(2) versus blood oxygen saturation relationships. Y(v) is converted to OEF, and baseline CMRO(2) is subsequently estimated from OEF and additional cerebral blood flow and hematocrit measurements. Theory behind the QUIXOTIC technique is presented, and implications of cutoff velocity (V(CUTOFF)) and outflow time parameters are discussed. Cortical gray matter values obtained with QUIXOTIC in 10 healthy volunteers are Y(v) = 0.73 ± 0.02, OEF = 0.26 ± 0.02, and CMRO(2) = 125 ± 15 µmol/100 g min. Results are compared to global measures obtained with the T(2) relaxation under spin tagging (TRUST) technique. The preliminary data presented suggest that QUIXOTIC will be useful for mapping Y(v), OEF, and CMRO(2), in both clinical and functional MRI settings.