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» MRI Safety

Exposures, limits, safe systems of work. This section explores safety issues in magnetic resonance imaging.

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Quench Danger

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In the event of a magnet quench with cryogen leakage, what should be done to get the patient out? What risk is there to the patient if they remain in the room (3 risks)?

All systems should be fitted with a venting system, which expels the escaping gas to the outside air. If this system fails, cold helium gas may spread throughout the patient area. It will collect at the top of the room, because helium is lighter than air, and work it's way down as the room fills. A white fog will form from the cold helium freezing the water vapour in the air. Helium is odourless, non-flammable and non-toxic, but the patient should be removed at once from the examination room since oxygen depletion provides a risk of asphyxiation, and there is temperature danger from the very cold gas. For this reason, all scan rooms should contain an oxygen monitor that sounds an alarm if the oxygen in the room falls below a certain level. (The O2 level in air is normally 21%. The human ability to respond is already limited in an O2 concentration below 19%.)

If the evolution of the escaping gas and depletion of oxygen has increased the pressure in the room so much as to prevent the opening of the door to the examination room, the window between the scan room and the control area should be broken to release the pressure, at which time evacuation of the patient will be possible. This is only likely several minutes after the quench, however. The risk of injury from flying glass under pressure from inside the room should also be considered.

In such a case the patient should immediately be evaluated for asphyxia, hypothermia and ruptured eardrums.

Modern MRI machines now provide means to allow the door to the magnet room to open in spite of a failure of the venting system.

Further reading on this topic:
Books: Q&A in MRI p58, MHRA Guidelines p89-90, 94

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