By employing laser-polarized noble gases in next-generation MRI technology, great improvements have been made over conventional MRI. While conventional MRI images water-filled solid tissue well, it does a poor job of imaging body cavities. These cavities, such as the interiors of the lungs or colon, appear as dark voids in conventional MRI. However, when an organ such as a lung is filled with laser-polarized noble gas and scanned via MRI, the gas' physical properties help generate highly detailed, three-dimensional images that show organ function in real-time. MRI employing laser-polarized gas may make it possible, for example, for a physician to precisely identify diseased portions of a lung before performing delicate surgery. Plus, while conventional MRI images are static, images produced with hyperpolarized noble gas can be dynamic. Other applications also hold promise such as injecting the gas into porous rock underground to someday help generate images that will point the way to mineral and petroleum deposits. Moreover, in physics and material science, polarized gases could play a central role in experiments delving into quantum mechanics, subatomic particle structure and other areas.
Polarized noble gas production occurs in a pumping cell, or glass cylinder, measuring from 2 x 4 inches to 4 x 10 inches. The cell is evacuated and a small amount of rubidium or potassium vapor is introduced, followed by the noble gas to be polarized: 3He, 21Ne or 129Xe. Circularly polarized laser light then optically pumps the alkali metal vapor and irradiates the pumping cell, kept at 60-80 degrees Centigrade, for up to 30 minutes. Inside the cell, the spin-polarized rubidium or potassium atoms collide with the noble gas atoms, with the former transferring their polarization to the latter - the spin exchange process. To separate polarized noble gas from rubidium or potassium vapor, the pumping cell holding the mixture is cooled until the rubidium or potassium solidifies, then the noble gas is extracted and stored in a magnetic vial to maintain polarization.
When scientists began researching hyperpolarized noble gases some years ago, Coherent's dye and Ti:Sapphire lasers were used to optically pump the alkali metal vapor that initiates spin exchange because of their broad wavelength flexibility and narrow linewidth. With the introduction of high-power diode lasers, researchers have another tool at their disposal. Diode laser systems offer high output power, small size, high stability, ease-of-use and low maintenance requirements. And while diode laser emission bandwidth is broader than that of dye and Ti:Sapphire lasers, the diode laser's high output power can make up for any loss in pumping efficiency.