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Magnetic resonance imaging (MRI) machines use superconducting magnets to polarize hydrogen nuclei within the body, which are then made to emit telltale radio signals. Strong as they are, the magnets used in MRI scanners are not particularly effective. Even the most capable polarize only a tiny fraction of the hydrogen nuclei present. Thus hydrogen-poor materials, such airy lung tissue, provide little signal and are virtually invisible in the resultant images. But over the past decade physicists and physicians have been experimenting with another approach: polarizing nuclei outside the body before administering them to the patient. One strategy uses light to polarize an alkali vapor, which can then pass its polarization on to the nuclei of an appropriate noble gas. Detailed scans of the lung can be obtained after the patient inhales the specially prepared gas. Variations of this technique also allow one to measure the rate at which such a gas moves into the blood and to determine, for example, how different tissues in the brain take up the dissolved gas.
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