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

Abstract

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) was 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.

MRI of brain tissue oxygen tension under hyperbaric conditions

“…Under NBO and HBO, animals showed reduced respiration rate (causing hypercapnia), and thus cerebral blood flow was slightly elevated compared to NB, 32, 38 which could result in reduced effective T 1 . However, given the small blood volume fraction, the effects of these blood contributions on T 1 -derived tpO2 are likely negligible in the brain parenchyma.…”

“…30, 31 This is consistent with our previous findings that venous baseline T 2 * signals in the brain still have room to increase and that stimulus-evoked blood oxygen level dependent functional MRI signals (which arise from changes in hemoglobin saturation) are still substantial under 3ATA HBO. 32 …”

A Hitchhiker's Guide to Functional Magnetic Resonance Imaging

“…BOLD responses are known to vary with different acquisition parameters (Renvall et al, 2014) and to be highly dependent on the specific parameters of neurovascular coupling which are known to vary with age, medication and in certain pathological states (D'Esposito et al, 2003; Bangen et al, 2009; Di et al, 2013; Tsvetanov et al, 2015). In addition, the nature of the BOLD signal has been shown to be affected by a variety of chemical compounds (e.g., caffeine and alcohol) (Levin et al, 1998; Mulderink et al, 2002; Perthen et al, 2008) as well as by respiration (Birn et al, 2008) and oxygen level (Cardenas et al, 2015). …”

Hydrogen gas alleviates oxygen toxicity by reducing hydroxyl radical levels in PC12 cells

“…The chamber was filled with hydrogen and oxygen mixed gas (the volume ratio of Hydrogen and oxygen is 2%:98%). Because of safety, the compression and decompression were conducted at a rate of 0.13ATA per min, slower than those in the previous studies[22]. Pressure in chamber was stabilized for 2h after it reached 5 ATA.…”