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  3. It takes only a tiny magnetic field to see clear through a person’s head, a new study shows. A method called ultra-low field mag

It takes only a tiny magnetic field to see clear through a person’s head, a new study shows. A method called ultra-low field mag

admin2009-06-24  56
问题     It takes only a tiny magnetic field to see clear through a person’s head, a new study shows. A method called ultra-low field magnetic resonance imaging (MRI) has captured its first, blurry shots of a human brain, revealing activity as well as structure.
    MRI scanners image the human body by detecting how hydrogen atoms respond to magnetic fields. They typically require fields of a few tesla—about 10,000 to 100,000 times stronger than the Earth’s magnetic field. The powerful magnets necessary make scanners pricey and also dangerous for people with metal implants.
    The new device hits a sample with a 30 millitesla magnetic field, about 100 times weaker than is normally used in MRI. The device then uses a 46 microtesla magnetic field—about the same as the Earth’s magnetic field—to capture images of the sample.
    The first target for the device was the head of lead researcher Vadim Zotev of Los Alamos National Laboratory in New Mexico, U.S.
    "The cost of MRI can be reduced dramatically", Zotev says. The new set-up uses several ultra-sensitive sensors called superconducting quantum interference devices (SQUIDs), which have to be kept at very low temperatures. "The most expensive part of our system is the liquid helium cryostat, which costs about $20,000", Zotev adds.
    Ultra-low field MRI scanning was first performed with a single SQUID in 2004 by a group led by John Clarke at University of California, Berkeley, U.S., but this only allowed objects about the size of an apple to be scanned. The new device uses seven SQUIDs and can scan much larger objects.
    MRI machines in the clinic today require a patient to be slotted into a long, cylindrical tube. Ultra-low field MRI machines can be much more open. "Microtesla MRI is more suitable for surgical environment than high-field MRI", Zotev says. "Some medical equipment can be conveniently placed inside [the scanner]", including surgical robots, Zotev says.
    Today’s MRI machines can also be problematic for people with metal implants, since intense magnetic fields can move or heat them causing damage to surrounding tissue.
    Experiments show that ultra-low field MRI can image materials even when metal is placed near the magnets.
    However, ultra-low field MRI hasn’t been tested on animals or people with metal implants yet. "It would be wrong to claim that it is absolutely safe", Zotev says.
    Since the new device also doubles as magnetoencephalography (MEG) machine, by picking up the feeble magnetic fields from electrical activity in the brain, it could perhaps let surgeons more easily identify areas of the brain with abnormal activity, such as in epilepsy.
    "This is the main advantage of the new set-up", Clarke says. "It’s a nice step forward".
选项 A、10,000 to 100,000 times stronger than the Earth’s magnetic field.
B、100 times weaker than the Earth’s magnetic field.
C、100 times weaker than is normally used in MRI.
D、the same as the Earth’s magnetic field.
答案D
解析
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