Quote:
Originally Posted by VaSteve
Is the magnet always on???
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Quote:
Originally Posted by DonDavis
YES!! Never assume a hospital's MRI is not "at field". 1.5Tesla systems are roughly 15,000 times the gravitational pull of the earth. Older magnets are not shielded and the field can be felt as far away as 10 feet. Newer ones are shielded but the that means no real warning.
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They are on because they are perpetual motion machines. Modern MRI's use superconducting wire for the primary magnet. At 4K, the wire has zero resistance to electricity. So once the coil is cooled to 4K with liquid He, the coil is energized and then the source of electricity is
removed. As long as the coil stays cold, the magnetic field remains. [BTW: I used to work for a division of Intermagnetics, which makes the coil for Philips MRI systems; and am also familiar with 2G wire which works at LN2 temps. I'm still blown away at the manufacturing prowess to make one of the 3T coils.]
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