With temperatures hundreds of degrees below freezing, cryogens are cold enough to freeze human tissue within seconds, and pose serious cold burn and frostbite hazards. As the cryogens are released to keep the magnet cool, they evaporate into odorless, colorless and tasteless gases. Most of the gas is recaptured, but some escapes during the process. These gases are still extremely cold and are normally vented safely out of the building.
The greatest risk of exposure to cryogens is during an emergency shut down of the magnet, called a system quench. During a quench, all of the cryogen evaporates quickly, causing a loss of superconductivity in the magnet. Emergency venting systems direct the escaping cryogens through a quench pipe out of the building.
If there is a problem with the emergency venting system, or the cryogen tank malfunctions, the liquid helium could be released into the MR scanner room. The rapidly expanding helium will cause displacement of oxygen and present an asphyxiation hazard. The force of quenching can be strong enough to destroy MR equipment or the walls of the scanner room.
Unintentional quenches are extremely rare and seldom cause incidents. The most common causes of unintentional quenches are equipment malfunctions, improperly filling the cryogen tank, contaminants inside the cryostat and extreme magnetic or vibrational disturbances.
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