MRI magnets (1.5-3 Tesla) cool superconducting windings with 1500 L liquid helium at -269 C. Fire or a quench event vaporises the helium (760x volume expansion) and can blow the room's pressure envelope. NFPA 99 defines healthcare safety requirements.
MRI Magnet Properties
- Field strength: 1.5T, 3T clinical; 7T research
- Cold winding: NbTi superconductor -269 C
- Helium volume: 1500-2000 L liquid
- Boil-off: 1 L liquid -> 760 L gas
- 5-Gauss line: 4 m dia magnet
Quench Vent
- Diameter: 300-400 mm steel duct
- Route: Direct to roof, minimum bends
- Velocity: Emergency discharge 5-10 s
- Burst disc: Safety rupture
- Oxygen monitor: Room 19.5 percent alarm
Non-Magnetic Sprinklers
- Sprinkler head: Beryllium-copper or bronze
- Pipe: Copper or CPVC
- Fitting: MRI-safe brass
- Hanger: Aluminum or plastic
- 5-Gauss zone: All metal replaced
NFPA 99 Healthcare Category
- Category 1: Life support - redundant
- Category 2: Injury likely - double
- Category 3: Discomfort - single
- Category 4: Minor - normal
- MRI: Typically Category 2
Emergency Procedure
- Magnet quench: Emergency run-down button
- Evacuation: Room must clear in 15 s
- FFP3 + tyvek: Post-fire entry
- Ferromagnetic object: Fire extinguisher BANNED
- Helsinki 2001: Oxygen cylinder accident lesson
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Sources & Further Reading
NFPA 99, NFPA 101, ACR MR Safety, IEC 60601-2-33, BYKHY Annex-11. NFPA 99 Health Care Facilities Code.