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1、J ames Brookeman wasnt surprised that the student volunteers taking part in his medical experiments enjoyed themselves. All they had to do was inhale xenon gas, the side effects of which are a strangely low voice and a feeling of eupho- ria. “They giggled quite a while after inhal- ing,”says Brookem
2、an,a biomedical engineer at the University of Virginia in Char- lottesville.“They loved it.” While the volunteers enjoyed themselves, they were helping to change the face of med- ical imaging.By tweaking the magnetic prop- erties of xenon and helium, Brookeman and others are improving magnetic reson
3、ance imaging (MRI) so that it can reveal parts ofthe body previously invisible to the technique. By offering a fresh window into the lungs, these researchers should improve our under- standing of diseases such as asthma and emphysema. “This is the technique of the future for looking at lung function
4、,” says Edwin van Beek,who is working on the tech- nique at the University ofSheffield,UK.And in the long term,the experiments could lead to cheaper MRI devices and new ways of studying brain disease. All in a row Conventional MRI makes use of the fact that hydrogen nuclei act like tiny magnets. Whe
5、n a patient lies inside an MRI machine, some hydrogen nuclei in water atoms in their bodys tissues become polarized they align themselves with the powerful magnetic field the machine produces. The device then delivers pulses of radio waves that temporarily tilt the nuclei, inducing an electrical cur
6、rent in a coil of wire in the machine. Because the nuclei in different tis- sues fall back into line at different rates, MRI machines can distinguish between these tissues to produce images of the brain, for example,or the kidneys. But not all organs can be imaged in this way. The lungs contain litt
7、le water, and so have been off-limits to MRI scans. Other imaging techniques, such as positron-emis- sion tomography (PET), which tracks the movement of radionuclides around the body, can image the lungs. But PET scans expose the patient to radiation,making them impractical tools for studying diseas
8、es,such as asthma, which would require patients to have regular scans. Such problems led Brookeman and others to explore an alternative technique:filling the airways of the lungs with a gas that can respond to MRI. The idea dates back to the early 1990s,and is the brainchild of William Happer and Go
9、rdon Cates, then both at Princeton University in New Jersey.The two physicists were studying the process ofhyper- polarization, which involves using a polar- ized beam of laser light to align the nuclei of news feature NATURE|VOL 424|21 AUGUST 2003| gaseous atoms. Their work wasnt aimed at medicine
10、hyperpolarized helium-3 gas is used as a target in particle-scattering experi- ments but they realized that a hyper- polarized gas should react to radio pulses in a similar way to aligned hydrogen nuclei. In 1994, Cates and others described the first image taken using hyperpolarized xenon-129 gas in
11、 a lung, in this case from a Take a deep breath Want to know whats going on inside your lungs? Conventional imaging techniques are not much use, but by inhaling a magnetized gas you could get a clear picture of your airways. Erica Klarreich investigates. Magnetic resonance imaging offers an insight
12、into the body,but until now it could not show the lungs. Clear view:normal lungs (left) and a simulated asthma attack taken using gas-based imaging. BRIGHAM 1994). The resulting lung image was a spectacular success,and was soon fol- lowed by in vivoimages ofhuman and mouse lungs (see,for example,M.E
13、bert et al.Lancet 347,12971299;1996). In theory,the technique offers significant benefits. A much greater proportion of nuclei are aligned in hyperpolarized gases than in water exposed to the magnetic fields in MRI,so the signals produced using the gas are about 100 times stronger. As a result, a hy
14、perpolarized gas can image an entire lung in the few seconds it takes a patient to inhale, hold their breath, and exhale a marked improvement over the hour or so it typically takes to get an image using conventional MRI.By taking images as a patient breathes in and out, the new technique can also pr
15、o- duce dynamic images ofairflow in and out of the lungs. Polar vision The treatment and diagnosis of asthma could be one area to benefit. In an asthma attack, branches of the tree-like structure of the lungs airways either narrow or shut down. This June, Brookeman and his team described how they us
16、ed hyperpolarized helium to detect which portions of the lungs were cut off during an asthma attack areas where gas flow was reduced or eliminated showed up darker in the images (S. Samee et al. J. Allergy Clin. Immunol. 111, 12051211; 2003). “Parts of the lung totally closed; other parts remained nor- mal,” says Brookeman. “Before, it wasnt clear whether the narrowing happened everywhere or not.” Hyperpolarized gas MRI should also be valuable for testin
