Effects of Oxygen on FLAIR

A 30 yo pt brought unconscious, required intubation and O2 support. MRI axial FLAIR brain screening show diffusely increased signal in the subarachnoid spaces in the region of cortical sulci and basal cisterns but Csf examination was normal.

An Artifactual high signal in Csf spaces due to para magnetic effect of inhaled Oxygen.
"Pseudo SAH" 

Patients who are intubated and on inhaled oxygen, once the concentration of dissolved oxygen is high enough in the blood, its level will increase in the CSF as well leading to sub optimal suppression of Csf signal on FLAIR due to para magnetic effect of Oxygen and results in diffuse Csf T2 hyper intensity.

Reference: Practical MR Physics and case file of MR artifacts and pitfalls, Alexander C. Mamourian, MD