|
Number 398, October 21, 1998 by Phillip F. Schewe and Ben Stein
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.)
SUPERLUMINAL TRAVEL REQUIRES NEGATIVE ENERGIES. Einstein's special theory of relativity asserts that no physical object can travel faster than the speed of light. The theory also holds that mass and energy will have different values depending on your frame of reference. The idea that the mass (energy) density in any one frame would always be at least equal to or greater than zero is called the "weak energy condition." Ken Olum of Tufts (kdo@cosmos5.phy.tufts.edu, 617-628-5000, x2753) follows the reverse tack in arguing that superluminal travel is possible in certain warped versions of space/time but that this would entail the existence of negative energy. In this case the concepts of superluminal motion and of negative energy need to be explored. An object with negative mass would be less massive than empty space. We don't know of any such object, but physicists have detected small regions of space characterized by a very slightly negative energy density (the so called Casimir effect; see Updates 122 and 300). If you combine negative energy with positive energy you get nothing, very different from the explosion you get when you combine matter and antimatter. As for superluminal travel---in Olum's model objects and signals do not actually travel faster than light. Rather, the curvature of a spacetime incorporating a negative-energy density is such that one can arrive quickly at distant places using sub-light speeds. (Physical Review Letters, 26 October 1998.)
[an error occurred while processing this directive]
|
