MRI of brain tissue oxygen tension under hyperbaric conditions

Abstract

The brain depends on a continuous supply of oxygen to maintain its structural and functional integrity. This study measured T1 from MRI under normobaric air, normobaric oxygen, hyperbaric air, and hyperbaric oxygen (HBO) conditions as a marker of tissue pO2 since dissolved molecular oxygen acts as an endogenous contrast agent. Brain tissue T1 decreased corresponding to increased pO2 with increasing inhaled oxygen concentrations, and tissue oxygenation was estimated from the T1 changes between different inhaled oxygen levels. Tissue pO2 difference maps between different oxygen conditions showed heterogeneous pO2 changes in the brain. MRI-derived tissue pO2 was markedly lower than the arterial pO2 but was slightly higher than venous pO2. Additionally, for comparison with published extracellular tissue pO2 data obtained using oxygen electrodes and other invasive techniques, a model was used to estimate extracellular and intracellular pO2 from the MRI-derived mean tissue pO2. This required multiple assumptions, and so the effects of the assumptions and parameters used in modeling brain pO2 were evaluated. MRI-derived pO2 values were strongly dependent on assumptions about the extra- and intracellular compartments but were relatively less sensitive to variations in the relaxivity constant of oxygen and contribution from oxygen in the cerebral blood compartment. This approach may prove useful in evaluating tissue oxygenation in disease states such as stroke.

A simplified empirical model to estimate oxygen relaxivity at different magnetic fields

“…Since Equation 5 contains four parameters, the fitting process was repeated for all combinations of fewer parameters (eg removing C Temp ) and the Akaike information criterion (AIC) was calculated for each version of the model—the best‐fit model according to the AIC is the model that explains the greatest amount of variation using the fewest possible independent variables 32 . The AIC score, R 2 , and MSE results of the different models tested are listed in Supplementary Table S3. The model with all four parameters scored the highest according to the AIC, and was therefore used in this manuscript.…”

“…Many researchers have investigated using the paramagnetic relaxivity effect of oxygen on longitudinal relaxation as a means of inferring oxygenation levels for applications ranging from cancer therapy to seawater analysis 1–5 . For example, measurements of the longitudinal relaxation rate R 1 (1/ T 1 ) have been used to infer oxygen levels in vitreous fluid as a noninvasive alternative to the highly invasive oxygen electrodes used to measure retinal hypoxia, 6–8 bladder urine 9 and urine in the renal pelvis to create a noninvasive detection of renal dysfunction, 5 and cerebrospinal fluid, 9,10 and this relationship between pO 2 and R 1 is also the basis for oxygen‐enhanced MRI techniques 11–13 .…”

Cerebrospinal fluid oxygen optimisation for rescue of metabolically challenged in vitro cortical brain tissue

“…CNS seizures have been observed at lower inspired levels of between 2–3 atmospheres ( Bitterman, 2004 ). Inspired oxygen at 3 atmospheres equates to a brain pO 2 of around 300 mmHg ( Muir et al, 2016 ), which converts to 13 mg/L (at 37 degrees). Clearly, there is some variability and uncertainty around the toxic oxygen threshold for seizures.…”

High spatiotemporal vessel-specific hemodynamic mapping with multi-echo single-vessel fMRI

“…As reported by Vazquez et al., 54 the arterial oxygen saturation levels from 50 µm arterioles can be increased upon sensory stimulation and directly contribute to the increased BOLD signal, which can potentially cancel out the CBV-induced reduction of the TE-dependent signal. It is also interesting to note that the hyperbaric oxygen treatment to increase the pO 2 in the blood of the rat brain can potentially lead to T1 shortening, 101,102 which may contribute to the fMRI signal changes at shorter TEs in arterioles. Also, at longer TEs, the extravascular BOLD effect from venules could influence the signal changes at arteriole and peri-arteriole ROIs due to partial volume effects.…”