LOW-FIELD MRI has been achieved by Harvard-Smithsonian scientists. In a typical magnetic resonance imaging (MRI) device, a powerful magnet polarizes hydrogen nuclei inside water molecules (e.g., in a human body) to a few parts in a million. The radio signal used to image a tumor, say, is broadcast by polarized nuclei that are momentarily tipped over by a radio-frequency pulse resonant with the spins' precession about the applied magnetic field. This magnetic resonance effect can produce high-quality, non-invasive images of liquids in large magnetic fields, but is not effective for gases (e.g., in the lung) because of their low density and hence weak radio signals. In the last few years, however, practical gas-phase MRI has been achieved by greatly increasing the nuclear spin-polarization of certain gas samples using laser techniques. One of these techniques is "spin-exchange optical pumping," a process in which circularly polarized laser light transfers its spin to rubidium vapor, which in turn collisionally spin-polarizes a sample of noble-gas atoms---either helium-3 or xenon-129. This procedure was used a few years ago to produce high-resolution MRI images of lungs filled with chemically inert helium-3 gas (see Physics Today, June 1995). The Harvard-Smithsonian innovation is to carry out laser-polarized helium-3 MRI with low magnetic fields of 20 gauss, compared to the usual 15,000 gauss in conventional MRI machines. The low-field imaging (with helium-3 polarizations of about 20%) provided 1 mm spatial resolution, comparable to high-field MRI using either the conventional hydrogen approach or spin-polarized noble gas. The much-simpler magnet setup for low-field noble-gas MRI offers many advantages: the possibility of using open magnets (instead of the claustrophobia-inducing closed magnets used in many hospital MRI units); low-cost, portable MRI instruments for imaging of lungs and sinuses; compatibility with nearby electronic equipment, enabling MRI for people with pacemakers; and practicality in cramped environments such as space vehicles. Furthermore, low magnetic fields and resonant frequencies will aid the imaging of porous materials and the interior of metals. (C.H. Tseng et al., Physical Rev Lett, 26 Oct; contact Ron Walsworth, 617-495-7274, rwalsworth@cfa.harvard.edu; see figure at Physics News Graphics.)