Oxygen-dependent hyperpolarized 129Xe brain MR

Hyperpolarized (HP) Xe-129 MR offers unique advantages for brain functional imaging (fMRI) because of its extremely high sensitivity to different chemical environments and the total absence of background noise in biological tissues. However, its advancement and applications are currently plagued by issues of signal strength. Generally, xenon atoms found in the brain after inhalation are transferred from the lung via the bloodstream. The longitudinal relaxation time (T-1) of HP Xe-129 is inversely proportional to the pulmonary oxygen concentration in the lung because oxygen molecules are paramagnetic. However, the T-1 of Xe-129 is proportional to the pulmonary oxygen concentration in the blood, because the higher pulmonary oxygen concentration will result in a higher concentration of diamagnetic oxyhemoglobin. Accordingly, there should be an optimal pulmonary oxygen concentration for a given quantity of HP Xe-129 in the brain. In this study, the relationship between pulmonary oxygen concentration and HP Xe-129 signal in the brain was analyzed using a theoretical model and measured through in vivo experiments. The results from the theoretical model and experiments in rats are found to be in good agreement with each other. The optimal pulmonary oxygen concentration predicted by the theoretical model was 21%, and the in vivo experiments confirmed the presence of such an optimal ratio by reporting measurements between 25% and 35%. These findings are helpful for improving the Xe-129 signal in the brain and make the most of the limited spin polarization available for brain experiments. Copyright (c) 2016 John Wiley & Sons, Ltd.