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MRI Safety — Zones, Projectiles, Quench, MR-Conditional Devices
TEXTEquipment · 7 min read
The MRI suite is the most dangerous room in the hospital for the unprepared anesthetist. A 1.5 or 3T magnet is always on, attracts ferromagnetic objects at lethal velocity, and can asphyxiate the room in a quench.
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3 min read6 sectionsThe four zones — and where screening happens
Zone 2 is the interface — patients screened here for implants, foreign bodies, devices, prior surgery (especially cardiac, neurosurgical, ocular).
Zone 3 is restricted to screened persons; this is where the control room sits and where personal items (phones, pagers, badges, scissors) MUST be removed.
Zone 4 is the magnet room itself — the bore.
The static magnetic field is ALWAYS on, even when the scanner isn't actively imaging.
Most projectile fatalities have occurred when an unscreened person carried a ferromagnetic object into Zone 4.
Projectile hazard + the screening checklist
- oxygen tanks (the 2001 Colombini case — a child killed by a flying tank)
- IV poles
- stethoscopes
- scissors
- hemostats
- pens
- keys
- hairpins
- watches
- badges
- pagers
- mobile phones
Screen EVERY person entering Zone 4, every time — including familiar staff.
- cardiac pacemaker/ICD (MR-conditional only with specific protocol)
- aneurysm clips (older ferromagnetic types are absolute contraindication)
- cochlear implants
- neurostimulators
- retained foreign bodies (welders, gunshot — get an orbit X-ray for any metal-in-eye history)
- insulin pumps
- port-a-caths (most modern ones are MR-conditional, check the model)
MR-safe vs MR-conditional vs MR-unsafe
MR-SAFE (green square): no known hazards in any MR environment — e.g., plastic, non-conductive, non-metallic items.
MR-CONDITIONAL (yellow triangle): safe ONLY under specified conditions — field strength (1.5T vs 3T), specific absorption rate (SAR) limit, gradient slew rate, body position, programming mode.
The condition list is device-specific and must be followed exactly.
MR-UNSAFE (red circle): contraindicated in any MR environment.
Many older pacemakers, ferromagnetic aneurysm clips, and most anesthesia machines + monitors are MR-unsafe — must stay outside Zone 4 with MR-conditional equipment used inside.
Cryogen quench emergency
1,000 L of liquid helium expands to ~750,000 L of cold gas in seconds.Normal: vented through the quench pipe to outside.
Failure mode: vent blockage or pipe rupture helium fills the magnet room DISPLACES OXYGEN (asphyxiation risk) + extreme cold (frostbite) + window pressurization (door may become impossible to open from increased room pressure).
- evacuate everyone immediately
- do NOT re-enter until ventilated and oxygen-monitored
- call MR engineering
After quench the magnetic field collapses — projectiles become safe to remove, but the room is still oxygen-displaced.
Thermal injury + RF burns
- conductive loops (ECG cable looped on skin, fingers touching fingers, wet skin contact with bore wall)
- broken/frayed wires
- tattoos (older inks contain iron oxide)
- transdermal patches with metallic backing (nicotine, clonidine, fentanyl — REMOVE before scan)
- pulse oximeters with non-MR-compatible cables
Place fiberoptic SpO2 leads on a finger, not looped.
Pad the bore walls.
Insulate ECG electrodes.
Remove all metallic-backed patches.
Anesthesia in the MRI environment
Long ventilator and IV extension circuits (3-5 m) allow the patient in the bore while the machine stays in Zone 3 — confirm circuit integrity before induction.
Remote monitor in the control room.
Pediatric MRI sedation is the most common anesthesia application: propofol infusion 100-150 mcg/kg/min for diagnostic scans, OR dexmedetomidine 1-2 mcg/kg load then 1-2 mcg/kg/hr infusion (preserves airway better in spontaneous-ventilation cases).
Always have a plan for emergency: code carts must stay OUT of Zone 4; if arrest occurs, drag the patient out of Zone 4 to the resuscitation area before starting compressions with non-MR equipment.
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