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

Area of Science:

• Neuroimaging
• Medical Physics
• Physiology

Background:

• Quantitative susceptibility mapping (QSM) is an MRI technique used to assess magnetic susceptibility variations in tissues.
• Measuring oxygen extraction fraction (OEF) and cerebral metabolic rate of oxygen (CMRO2) is crucial for understanding brain oxygenation.
• Previous methods for OEF and CMRO2 quantification often involved hypercapnia, which can be uncomfortable for subjects.

Purpose of the Study:

• To evaluate the use of hyperoxia with QSM for quantifying OEF and CMRO2 in healthy individuals.
• To cross-validate the results obtained using hyperoxia-QSM with those derived from hypercapnia-QSM.

Main Methods:

• Ten healthy subjects underwent 3T MRI scans.
• QSM and pseudo-continuous arterial spin labeling (ASL) were used to acquire data during normoxia and hyperoxia (PetO2 = +300 mmHg).
• OEF and CMRO2 maps were computed and compared with hypercapnia-derived values using correlation and Bland-Altman analysis across 16 vascular territories.

Main Results:

• Hyperoxia-QSM yielded physiologically plausible OEF (0.42 ± 0.04) and CMRO2 (181 ± 34 μmol O2/min/100g) values.
• Comparison with hypercapnia-QSM showed small deviations in OEF (0.015) and CMRO2 (4.9 μmol O2/min/100g).
• Good to excellent correlations were observed for regional OEF and CMRO2 between the two methods, with minimal impact on cerebral blood flow (7.5 ± 5.4% reduction).

Conclusions:

• Hyperoxia in conjunction with QSM provides a reliable method for measuring OEF.
• The hyperoxia approach is more patient-friendly and causes less disruption to cerebral blood flow than hypercapnia.