Description

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.

Several applications of xenon-129 MR imaging are under development, including diffusion-weighted and relaxation-weighted imaging. These techniques take advantage of the fact that the rate of loss of xenon-129 polarization is significantly influenced by the local blood flow and concentration of molecular oxygen, as well as the restriction of xenon-129 diffusion by small airway space dimensions. These data can be used to create maps of the lung reflecting regional ventilation/perfusion and micro-airway sizes. Other data that can be obtained with xenon-129 MRI include the volumes of ventilated and unventilated lungs which can subsequently be analyzed to determine the homogeneity of gas distribution within the airspaces. These data can be used to study the structural and functional changes taking place in the lungs associated with pulmonary diseases like CF and asthma. It might provide a diagnostic tool that is able to detect pulmonary diseases more sensitively than the current gold standard measurements of spirometry and plethysmography, and thus prevent irreparable and irreversible damage to the lungs in the early stages of disease.