MRI USING HYPERPOLARIZED NOBLE-GASES

The aim of this study was to review the physical basis of MRI using hy perpolarized 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 hyperpolarized by optical pumping. Pola rization is transferred from circularly polarized laser light to the n oble-gas atoms via alkali-metal vapors (spin exchange) or metastable a toms (metastability exchange). Hyperpolarization results in a non-equi librium polarization five orders of magnitude higher than the Boltzman n equilibrium compensating for the several 1000 times lower density of noble gases as compared with liquid state hydrogen concentrations in tissue and allows for short imaging times. Hyperpolarization can be st ored sufficiently long (3 h to 6 days) to allow for transport and appl ication. Magnetic resonance systems require a broadband radio-frequenc y system which is generally available for MR spectroscopy and dedicate d coils. The hyperpolarized gases are administered as inhalative ''con trast agents'' allowing for imaging of the airways and airspaces. Besi des the known anesthetic effect of xenon, no adverse effects are obser ved in volunteers or patients. Pulse sequences are optimized to effect ively use the non-renewable hyperpolarization before it decays or is d estroyed, using fast low-flip-angles strategies to allow for dynamic/b reath-hold imaging of highly diffusible (He) or soluble (Xe) gases wit h in vivo T1-times well below 1 min. Since helium is not absorbed in c onsiderable amounts, its application is restricted to the lung. Xe-129 is also under investigation for imaging of white matter disease and f unctional studies of cerebral perfusion. Magnetic resonance imaging us ing hyperpolarized gases is emerging as a technical challenge and oppo rtunity for the MR community. Preliminary experience suggests potentia l for functional imaging of pulmonary ventilation and cerebral perfusi on.