Functional MRI during hyperbaric oxygen: Effects of oxygen on neurovascular coupling and BOLD fMRI signals.

Hyperbaric oxygen (HBO) therapy is used to treat a number of ailments. Improved understanding of how HBO affects neuronal activity, cerebral blood flow (CBF) and blood-oxygenation-level dependent (BOLD) changes could shed light on the role of oxygen in neurovascular coupling and help guide HBO treatments. The goal of this study was to test two hypotheses: i) activation-induced CBF fMRI response is not dependent on hemoglobin deoxygenation, and ii) activation-induced BOLD fMRI is markedly attenuated under HBO. CBF and BOLD fMRI of forepaw stimulation in anesthetized rats under HBO at 3 atmospheres absolute (ATA) were compared with normobaric air. Robust BOLD and CBF fMRI were detected under HBO. Inflow effects and spin-density changes did not contribute significantly to the BOLD fMRI signal under HBO. Analysis of the T2(⁎)-weighted signal at normobaric air and 1, 2 and 3ATA oxygen in the tissue and the superior sagittal sinus showed a strong dependence on increasing inhaled [O2]. Spontaneous electrophysiological activity and evoked local-field potentials were reduced under HBO. The differences between normobaric air and HBO in basal and evoked electrical activity could not fully account for the strong BOLD responses under HBO. We concluded that activation-induced CBF regulation in the brain does not operate through an oxygen-sensing mechanism and that stimulus-evoked BOLD responses and the venous T2(⁎)-weighted signals still have room to increase under 3ATA HBO. To our knowledge, this is the first fMRI study under HBO, providing insights into the effects of HBO on neural activity, neurovascular coupling, tissue oxygenation, and the BOLD signal.