Hyperpolarized noble gas magnetic resonance (MR) lung imaging is a relatively new imaging method that allows depiction of both lung function and morphology. Hyperpolarized gases are a new class of MR contrast agent which, when inhaled, provide high temporal and spatial resolution MR images of the lung airspaces. Since no ionizing radiation is involved, hyperpolarized gas MR imaging is ideal for the evaluation of lung diseases especially in children. With hyperpolarized gases, the nuclear spins of the gas atoms are brought into alignment outside of the MR scanner via a process called optical pumping; this yields high polarizations and permits visualization of the lung airspaces with MR imaging (despite the low physical density of the gas in the lung). Two non-radioactive (i.e. stable) isotopes of noble gases helium-3 and xenon-129 can be hyperpolarized. Until recently, higher polarizations could be achieved with helium-3 than with xenon-129, so in humans, helium-3 was more commonly used for hyperpolarized gas MR imaging of the lungs. Recently, the technology has been developed to provide large quantities of highly polarized xenon-129. Helium-3 gas is also extremely expensive and since there are limited reserves of the gas, difficult to procure for research. Unlike helium-3, since xenon-129 is naturally present in the atmosphere, it is less expensive and easier to procure for imaging.