Associations between OSA‐related hypoxemia and left fornix microstructure. (A–E) Scatter plots showing the associations between PCA_hypoxemia and FW‐DTI measures of the left fornix. The y‐axis shows the predicted values from the regression analyses adjusted for age (continuous), sex (men vs. women), body mass index (continuous), Epworth Sleepiness Scale (continuous), Vascular Burden Index (<2 vs. ≥2 points), left fornix volume (divided by total intracranial volume; continuous), and cognitive status (CU vs. MCI). The solid line is the regression line and the gray area represents the 95% confidence interval. (F) Changes in the diffusion ellipsoid shape of the tissue compartment (gray ellipsoid) and the extracellular/FW compartment (blue sphere) associated with higher levels of hypoxemia in participants with MCI. Our results indicate that hypoxemia is associated with an increase in the three diffusivities (MDT, AxDT, and RDT), a constant FAT (which is consistent with a proportional increase in λ1, λ2, and λ3), and a constant FW index (which is consistent with a proportional increase in the tissue compartment and the FW/extracellular comportment). Note that the changes in the shape of the diffusion ellipsoid are shown here for illustrative and conceptual purposes (i.e., these changes are conceptually consistent with our results, but the magnitudes of the changes are not scaled or calculated based on our data). *p < 0.01. AxDT, tissue axial diffusivity; CU, cognitively unimpaired; DTI, diffusion tensor imaging; FW, free water; MCI, mild cognitive impairment; MDT, tissue mean diffusivity; OSA, obstructive sleep apnea; PCA, principal component analysis; RDT, tissue radial diffusivity; λ, eigenvalue.