Original article MRI using hyperpolarized noble gases

The aim of this study was to review the physical basis of MRI using hyperpolarized noble gases as well as the present status of preclinical and clinical applications. Non-radioactive noble gases with a nuclear spin 1/2 (He-3, Xe-129) can be hyper- polarized by optical pumping. Polarization is trans- ferred from circularly polarized laser light to the no- ble-gas atoms via alkali-metal vapors (spin exchange) or metastable atoms (metastability exchange). Hy- perpolarization results in a non-equilibrium polariza- tion five orders of magnitude higher than the Boltz- mann equilibrium compensating for the several 1000 times lower density of noble gases as compared with liquid state hydrogen concentrations in tissue and al- lows for short imaging times. Hyperpolarization can be stored sufficiently long (3 h to 6 days) to allow for transport and application. Magnetic resonance sys- tems require a broadband radio-frequency system - which is generally available for MR spectroscopy - and dedicated coils. The hyperpolarized gases are ad- ministered as inhalative "contrast agents" allowing for imaging of the airways and airspaces. Besides the known anesthetic effect of xenon, no adverse effects are observed in volunteers or patients. Pulse sequenc- es are optimized to effectively use the non-renewable hyperpolarization before it decays or is destroyed, us- ing fast low-flip-angles strategies to allow for dynam- ic/breath-hold imaging of highly diffusible (He) or soluble (Xe) gases with in vivo T1-times well below 1 min. Since helium is not absorbed in considerable amounts, its application is restricted to the lung. Xe- 129 is also under investigation for imaging of white matter disease and functional studies of cerebral per- fusion. Magnetic resonance imaging using hyperpo- larized gases is emerging as a technical challenge and opportunity for the MR community. Preliminary experience suggests potential for functional imaging of pulmonary ventilation and cerebral perfusion.