Magnetic resonance imaging using the MR signal from hyperpolarized noble gases Xe-129 and He-3 may become an important new diagnostic technique. Alex Pines (adapting the hyperpolarization technique pioneered by William Happer) presented MR spectroscopy studies using hyperpolarized Xe-129. The current authors recognized that the enormous enhancement in the detectability of Xe-129, promised by hyperpolarization, would solve the daunting SNR problems impeding their attempts to use Xe-129 as an in vivo MR probe, especially in order to study the action of general anesthetics. It was hoped that hyperpolarized Xe-129 MRI would yield resolutions equivalent to that achievable with conventional (H2O)-H-1 MRI, and that xenon's solubility in lipids would facilitate investigations of lipid-rich tissues that had as yet been hard to image. The publication of hyperpolarized Xe-129 images of excised mouse lungs heralded the emergence of hyperpolarized noble-gas MRI. Using hyperpolarized He-3, researchers have obtained images of the lung gas space of guinea pigs and of humans. Lung gas images from patients with pulmonary disease have recently been reported. 3He is easier to hyperpolarize than Xe-129, and it yields a stronger MR signal, but its extremely low solubility in blood precludes its use for the imaging of tissue. Xenon, however, readily dissolves in blood, and the T-1 of dissolved Xe-129 is long enough for sufficient polarization to be carried by the circulation to distal tissues. Hyperpolarized Xe-129 dissolved-phase tissue spectra from the thorax and head of rodents and humans have been obtained, as have chemical shift Xe-129 images from the head of rats. Lung gas Xe-129 images of rodents, and more recently of humans, have been reported. Hyperpolarized Xe-129 MRI (HypX-MRI) may elucidate the link between the structure of the lung and its function. The technique may also be useful in identifying ventilation-perfusion mismatch in patients with pulmonary embolism, in staging and tracking the success of therapeutic approaches in patients with chronic obstructive airway diseases, and in identifying candidates for lung transplantation or reduction surgery. The high lipophilicity of xenon may allow MR investigations of the integrity and function of excitable lipid membranes. Eventually, HypX-MRI may permit better imaging of the lipid-rich structures of the brain. Cortical brain function is one perfusion-dependent phenomena that may be explored with hyperpolarized Xe-129 MR. This leads to the exciting possibility of conducting hyperpolarized Xe-129 functional MRI (HypX-fMRI) studies.
