Functional MR imaging of pulmonary ventilation using hyperpolarized noble gases

The current status of experimental and clinical applications for functional MR imaging of pulmonary ventilation using hyperpolarized noble gases are reviewed. 3-helium (3He) and 129-xenon (129Xe) can be hyperpolarized by optical pumping techniques such as spin exchange or metastability exchange in sufficient amounts. This process leads to an artificial, non-equilibrium increase of the density of excited nuclei which represents the source of the MR signal. Those hyperpolarized gases are administered mostly via inhalation, and will fill airways and airspaces allowing for ventilation imaging. Recent human studies concentrate on imaging the airways and airspaces with high spatial resolution. Normal ventilation is reflected by an almost complete and homogeneous distribution of the hyperpolarized gas represented by the signal detected. Loss of signal or inhomogeneous signal distribution represent mass effects and ventilatory abnormalities. Even healthy subjects with seasonal allergies without pulmonary symptoms have been observed to exhibit transient ventilation defects. Real-time imaging of ventilation has become feasible for 3He MR imaging and allows for assessment of ventilation-distribution. Furthermore, functional oxygen-sensitive 3He MR imaging opens the field of non-invasive assessment of regional intrapulmonary oxygen concentrations in vivo. Knowing that the diffusion of gas is affected by the geometry and nature of its environment, diffusion measurements are under investigation as a sensitive marker of diseases that involve structural changes of lung parenchyma, such as emphysema and fibrosis. Whereas 3He is not absorbed and is restricted to the airspaces, 129Xe is soluble in blood and lipid-rich tissue. This presents the opportunity for additional dissolved-phase imaging, providing a step towards simultaneous ventilation-perfusion studies.