Turning off the Magnet
Is the magnetic field turned on and off between patients?
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Regardless of the type of scanner, it is generally wise to assume the main magnetic field is always ON.
Permanent magnet scanners are permanently "on" by definition.
Resistive electromagnet scanners, in theory, can be turned on and off. However, it may take 30-60 minutes for their magnetic fields to stabilize after being off and hence they are generally left continuously on during daily operations. To save electricity, they are usually turned off or have their power levels reduced after hours.
Superconducting scanners remain on at all times as long as superconducting temperatures are maintained. The main field may be ramped down for servicing in a controlled manner with minimal loss of cryogens in about 30 minutes and restored in a few hours, but this is performed only occasionally. In the event of an emergency, all superconducting scanners have a "quench" button that releases liquid helium into the atmosphere and turns their fields off within a few seconds.
Permanent magnet scanners are permanently "on" by definition.
Resistive electromagnet scanners, in theory, can be turned on and off. However, it may take 30-60 minutes for their magnetic fields to stabilize after being off and hence they are generally left continuously on during daily operations. To save electricity, they are usually turned off or have their power levels reduced after hours.
Superconducting scanners remain on at all times as long as superconducting temperatures are maintained. The main field may be ramped down for servicing in a controlled manner with minimal loss of cryogens in about 30 minutes and restored in a few hours, but this is performed only occasionally. In the event of an emergency, all superconducting scanners have a "quench" button that releases liquid helium into the atmosphere and turns their fields off within a few seconds.
Related Questions
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There seem to be many different MRI magnets and designs. Can you explain?
Does the scanner have an emergency "stop" button?