The nuclear spin polarization of the noble gas isotopes 3 He and 129 Xe can be increased using optical pumping methods by four to five orders of magnitude. This extraordinary gain in polarization translates directly into a gain in signal strength for MRI. The new technology of hyperpolarized (HP) gas MRI holds enormous potential for enhancing sensitivity and contrast in pulmonary imaging. This review outlines the physics underlying the optical pumping process, imaging strategies coping with the nonequilibrium polarization, and effects of the alveolar microstructure on relaxation and diffusion of the noble gases. It presents recent progress in HP gas MRI and applications ranging from MR microscopy of airspaces to imaging pulmonary function in patients and suggests potential directions for future developments. MRI has been extremely successful at diagnosing soft tissue disease since its discovery in 1972 (1). However, MRI is not as sensitive in comparison with other biomedical imaging techniques, such as CT, positron-emission tomography, or single-photon emission computed tomography. This is a consequence of a very small signal from a small population difference between nuclear energy states. For a spin-1/2 system, the "nuclear spin polarization", P N , is defined as:where N ϩ and N Ϫ denote populations with magnetic spin quantum numbers ϩ1/2 and Ϫ1/2, respectively. Typically, the thermal energy of the sample at temperature T exceeds the energy difference between the nuclear spin states in a magnetic field B 0 by several orders of magnitude ("hightemperature approximation") and the equilibrium polarization can be written as:where ␥ is the magnetogyric ratio, ប is Planck's constant divided by 2 , and k B is Boltzmann's constant. As an example, P N,0 Ϸ 5 ppm is predicted with Eq.[2] for protons ( 1 H) at body temperature (T ϭ 37°C) and B 0 ϭ 1.5T. In view of the inherent sensitivity problem, increasing the signalto-noise ratio (SNR) has been a field of continuous research since the discovery of NMR. Recently, the use of optically polarized noble gas isotopes 3 He and 129 Xe has attracted increasing interest for use in a variety of promising MR applications. These systems exhibit polarizations exceeding the thermal levels by several orders of magnitude. While the use of such "hyperpolarized" (HP) gases for MRI is a recent development, it is based on a solid foundation of work in atomic physics. The groundwork was laid by Kastler (2) more than 50 years ago by demonstrating transfer of angular momentum from circularly polarized light to the electron and nuclear spins of atoms, a process called "optical pumping" (OP). Since 1991, exploitation of OP as a means of enhancing signal initiated the development of a novel field in NMR (3,4). Research involving HP noble gases has been exceptionally fruitful in biomedical MRI as well as providing applications for investigation of materials (5-8).In the context of proton MRI, the lung is a particularly challenging area to study (9). Even at end expiration, the overall density is ...

Asthma is a disease characterized by chronic inflammation and reversible obstruction of the small airways resulting in impaired pulmonary ventilation. Hyperpolarized 3 He magnetic resonance (MR) lung imaging is a new technology that provides a detailed image of lung ventilation. Hyperpolarized 3 He lung imaging was performed in 10 asthmatics and 10 healthy subjects. Seven asthmatics had ventilation defects distributed throughout the lungs compared with none of the normal subjects. These ventilation defects were more numerous and larger in the two symptomatic asthmatics who had abnormal spirometry. Ventilation defects studied over time demonstrated no change in appearance over 30 -60 minutes. One asthmatic subject was studied twice in a three-week period and had ventilation defects which resolved and appeared in that time. This same subject was studied before and after bronchodilator therapy, and all ventilation defects resolved after therapy. Hyperpolarized 3 He lung imaging can detect the small, reversible ventilation defects that characterize asthma. The ability to visualize lung ventilation offers a direct method of assessing asthmatics and their response to therapy. J. Magn. Reson. Imaging 2001;13: 378 -384.

Hyperpolarized (3)He diffusion MR imaging demonstrated potential for use in evaluating the global and regional severity of emphysema.

Using a new method of xenon laser-polarization that permits the generation of liter quantities of hyperpolarized 129Xe gas, the first 129Xe imaging results from the human chest and the first 129Xe spectroscopy results from the human chest and head have been obtained. With polarization levels of approximately 2%, cross-sectional images of the lung gas-spaces with a voxel volume of 0.9 cm3 (signal-to-noise ratio (SNR), 28) were acquired and three dissolved-phase resonances in spectra from the chest were detected. In spectra from the head, one prominent dissolved-phase resonance, presumably from brain parenchyma, was detected. With anticipated improvements in the 129Xe polarization system, pulse sequences, RF coils, and breathing maneuvers, these results suggest the possibility for 129Xe gas-phase imaging of the lungs with a resolution approaching that of current conventional thoracic proton imaging. Moreover, the results suggest the feasibility of dissolved-phase imaging of both the chest and brain with a resolution similar to that obtained with the gas-phase images.

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