Picture this

时间:2019-03-08 07:14:23166网络整理admin

By Charles Seife in Atlanta THE COST of taking magnetic resonance images could soon be slashed dramatically. Physicists announced last week that they have developed techniques that allow them to take resonance images using a magnet with just a thousandth of the strength of magnets in typical MRI machines. They say the new technique also images some organs with more detail than ever before. MRI creates images by manipulating the nuclei of atoms. Nuclei have “spin”, a quantum-mechanical property likened to the whirling of a top. In ordinary materials such as body tissue, the nuclear spin axes point in different directions. But when exposed to a strong magnetic field, the nuclei start to align their spin axes. If you then apply a second, oscillating magnetic field, the atoms start to emit radiation. The pattern of radiation can be detected to form an image of the tissue. However, it takes a mighty magnetic field to get enough spins to point in one direction. A typical MRI machine has a magnet with a field strength of 1.5 tesla—about 25 000 times the strength of the Earth’s magnetic field—pushing the cost up to an astronomical $3 million, says Ronald Walsworth, a physicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. Another problem is that some tissues, in particular lung tissue filled with air, don’t emit enough radiation to make a strong image. “Traditionally, the lungs are black holes,” says Walsworth. “The quality of the signal from that segment was low because the density was low.” But now things look set to change. Walsworth and his colleagues have managed to take detailed MRI scans of lungs with only a small magnet. To do this, they aligned the nuclear spins of atoms in xenon gas by shining lasers on the gas. When rats then inhaled the xenon the team could image the gas passing through the lung walls using a small weak magnet costing $30 000. “When you fill the lungs with polarised gas, you get beautiful pictures of the airways,” Walsworth says, adding that the technique will be useful for early diagnosis of lung diseases such as emphysema. Other researchers have imaged the hearts and brains of rats by injecting spin-polarised gas into these organs. Another team, led by Allen Johnson of Duke University in Durham, North Carolina, has used spin-polarised xenon and helium with a strong magnetic field to get even more detailed images of lungs, including a 3D image of the airways. “Using computers, you can fly through the bronchi,” says Johnson. His images even reveal details of the tiny air sacs in the lungs that take oxygen into the bloodstream. Klaus Schlenga, a physicist at the University of California at Berkeley, adds that he has also explored using extremely sensitive detectors called SQUIDS in MRI work. SQUIDS can pick up tiny radiation emissions, so fewer atoms need to be aligned to create strong images. This makes it possible to to create an image using a magnetic field as puny as the Earth’s. “It’s MRI with no magnet,