Baseline oxygenation in the brain: Correlation between respiratory-calibration and susceptibility methods

Area of Science:

• Neuroimaging
• Biophysics
• Medical Physics

Background:

• Noninvasive brain oxygen extraction fraction (OEF) imaging is crucial for understanding brain physiology.
• Quantitative O2 imaging (QUO2) and quantitative susceptibility mapping (QSM) are novel MRI methods for OEF assessment.
• Direct comparison and validation of these OEF imaging techniques against physiological signals are needed.

Purpose of the Study:

• To compare OEF values derived from QUO2 and QSM.
• To investigate the relationship between baseline OEF and functional brain activity (BOLD and CBF changes) during a visual task.
• To validate novel MRI-based OEF measurement techniques.

Main Methods:

• Simultaneous BOLD and arterial spin labeling (ASL) MRI at 7T were acquired in 11 healthy subjects under various gas manipulations (hypercapnia, hyperoxia, carbogen) for QUO2 analysis.
• Gradient echo phase imaging was used for QSM reconstruction of OEF along cerebral veins.
• BOLD-ASL scans were also acquired during a visual stimulus task.

Main Results:

• Mean baseline OEF values were (43.5±14)% for 2-gas QUO2, (42.3±17)% for 3-gas QUO2, and (29.4±3)% for QSM.
• A significant correlation was found between 3-gas QUO2 and QSM OEF values (P=0.03).
• Percent cerebral blood flow (CBF) change during the visual task correlated with OEF measured by both 3-gas QUO2 (P<0.04) and QSM (P=0.035).

Conclusions:

• QUO2 and QSM are viable noninvasive MRI methods for assessing baseline brain OEF.
• While QUO2 tended to yield higher OEF values than QSM, both methods demonstrated significant correlations with physiological changes during functional activation.
• These findings support the use of QUO2 and QSM for measuring true variations in brain physiology across subjects.