MEDICAL IMAGING: Polarized-gas MRI shows lungs in high resolution

A new magnetic-resonance-imaging (MRI) method that uses polarized helium is giving clinicians and biologists a better way to view tissues that are difficult or impossible to image with the conventional proton-based MRI technique. Researchers recently acquired the first high-resolution images of a living human lung with the spin-polarized gas approach (see photo). The method was developed by scientists from the Princeton University physics department (Princeton, NJ) and the Duke University Center for In Vivo Microscopy (Durham, NC), led by William Happer of Princeton.

Magnetic-resonance imaging, developed in the 1980s, relies on magnetization of hydrogen nuclei (protons) to image living tissue. Magnetic-resonance techniques detect the magnetization, which is analyzed to create an image of the tissue. While conventional MRI has been a valuable tool for studying tissues that contain significant amounts of water and therefore hydrogen nuclei, tissues that contain little water, such as the lungs, are difficult to image effectively. Other diagnostic methods such as x-rays or ventilation scans with radioactive isotopes (SPECT) use sufficient ionizing radiation for the dosage to be a consideration when they are used. X-ray techniques do not image the airways directly, and the spatial resolution of SPECT is only about 10 mm.

Polarized-gas MRI

The new MRI method uses spin-polarized helium gas rather than protons as the detectable material. The patient inhales the hyperpolarized gas to fill the lungs, which are imaged with a conventional MRI detector assembly. In a normal state, the magnetic moments associated with spin have almost no net polarization, so the diagnostic gas must be optically pumped to induce spin alignment.

A single-neutron isotope of helium (3He), derived from the decay of tritium, is placed in a gas cell with rubidium. Heating the mixture to about 180°C vaporizes the rubidium, and a 120-W, continuous-wave diode-laser array from Opto Power Corp. (Tucson, AZ) pumps the gas mixture with circularly polarized light at 795 nm, preferentially aligning the electron spin of the rubidium atoms. This spin polarization is collisionally transferred to 3He, after which the rubidium atoms are free to be optically polarized. After a sufficient amount of hyperpolarized 3He is generated, the gases are cooled to room temperature, and rubidium condenses out of the mixture.

The compact, portable diode-laser array requires less than 300 W of input power, for a net conversion efficiency of about 35%. But there are trade-offs. The linewidth of the diode-laser array is 2 nm, exceeding the absorption line of rubidium. Although the rubidium vapor is optically dense and the high partial pressure of helium in the gas cell broadens the rubidium line, only about two-thirds of the available laser power can be absorbed. The high conversion efficiency, low cost, and compactness of the system outweigh this loss, however.

Says Hunter Middleton of the Princeton group, "The use of these diode lasers makes this technique economically feasible. Every two months or so we increase our laser power by a factor of two, and the costs seem to stay about the same." Citing system portability, simplified power requirements, and ease of operation, Middleton adds, "Without diode lasers this would never have become a useful medical technique. There`s no way this could ever have been done in a hospital environment."

The quantity of gas produced is primarily driven by optical power; with an earlier 20-W array, less than half a liter of hyperpolarized 3He was generated in 10 hours of pumping. This was sufficient for early tests on guinea pigs, but couldn`t efficiently provide the 1 to 4 l of gas necessary for human clinical tests. The 120-W array can polarize about one liter of gas in four hours, and the Princeton group will soon acquire a 250-W array for the next-generation system.

In late September, the group acquired the first hyperpolarized-helium MRI image of the lung of a human subject. They obtained data slices approximately 2 cm thick, with two-dimensional in-plane spatial resolution of about 1.25 ¥ 2.5 mm. In comparison, radioactive xenon SPECT scans only achieve resolutions on the order of 10 mm. "That`s why MRI is such a wonderful technique," says Middleton. "It offers high resolution while using materials that are nontoxic, nonradioactive, and noninvasive."