Quantification of brain oxygen extraction fraction using QSM and a hyperoxic challenge

Purpose: To use hyperoxia in combination with QSM to quantify microvascular oxygen extraction fraction (OEF) and cerebral metabolic rate of oxygen (CMRO₂) in healthy subjects and to cross-validate results with those from hypercapnia QSM-OEF. Methods: Ten healthy subjects were scanned on a 3T MRI scanner. At baseline normoxia and during hyperoxia (PetO₂ = +300 mmHg), QSM data were acquired using a multi-echo gradient-echo (GRE) sequence, and cerebral blood flow data were acquired using a pseudocontinuous arterial spin labeling sequence. The OEF and CMRO₂ maps were computed and compared with those from hypercapnia QSM-OEF, acquired in the same subjects, using correlation and Bland-Altman analysis in 16 vascular territories. Results: Hyperoxia QSM-OEF produced physiologically reasonable OEF and CMRO₂ values in all subjects (gray-matter region of interest average OEF = 0.42 ± 0.04, average CMRO₂ = 181 ± 34 μmol O₂/min/100 g). When compared with hypercapnia QSM-OEF, Bland-Altman plots revealed small deviations (mean OEF difference = 0.015, mean CMRO₂ difference = 4.9 μmol O₂/min/100 g, P '.05). Good and excellent correlations of regional OEF and CMRO₂ were found for the two methods. In addition, hyperoxia had minimal impact on cerebral blood flow (average gray-matter cerebral blood flow was reduced by 7.5 ± 5.4%). Conclusions: Hyperoxia in combination with QSM is a robust approach to measure OEF. Compared with hypercapnia, hyperoxia is more comfortable and has minimal impact on cerebral blood flow.