Oxygen extraction fraction mapping between normal cognition and mild cognitive impairment in older adults using...

Oxygen extraction fraction (OEF) represents the proportion of oxygen utilized by the brain from the blood and is a sensitive biomarker in understanding the vascular interplay between oxygen supply and demand. Alterations in OEF are increasingly recognized in normal aging and the pathophysiology of many neurological disorders, including Alzheimer's disease and cognitive impairment. Here, we aimed to assess regional OEF MRI in older adults and investigate its relationship with mild cognitive impairment (MCI) and normal aging using a Variational Bayesian quantitative BOLD (qBOLD) framework. The qBOLD model leverages a biophysical relationship between OEF, deoxygenated blood volume (DBV), and the reversible transverse relaxation rate (R₂ ^') to extract parameter values from asymmetric spin echo (ASE) MRI scans. The reliability of these parameter estimates was enhanced by Variational Bayesian inference, which incorporates prior distributions that we optimized in numerical simulation. We then applied this model to a cohort of older adults (N = 70) from the Alzheimer's Disease Research Center (ADRC) at the University of California, Davis, including 58 cognitively normal (CN) and 12 amnestic mild cognitive impaired (MCI) participants. Our findings revealed a trend of elevated OEF across many cortical brain regions in the MCI group. Additionally, we identified regional positive associations between OEF and age in the MCI group, significantly in the left inferior parietal region ( $\beta =0.88\pm 0.23,\hspace{0.17em}T=3.75,\hspace{0.17em}P=0.0004$ ). Furthermore, the relationship between OEF and episodic memory in the right parahippocampal region differed significantly between two diagnostic groups (negative for CN and positive for MCI) ( $\beta =0.09\pm 0.02,\hspace{0.17em}T=3.77,\hspace{0.17em}P=0.0004$ ). This work adopts a novel qBOLD MRI framework for studying OEF changes in cognitive impairment as a potential biomarker for monitoring pathophysiology in aging and dementia.