MRI Principles
MRI: strong magnetic field (1.5T, 3T, up to 7T), radiofrequency pulses, no ionizing radiation. Images: hydrogen protons in tissues. T1: fat bright, fluid dark. T2: fluid bright. Contrast: gadolinium (different from iodinated CT contrast). Duration: 30-60 minutes. Claustrophobia: common, mild sedation, open MRI, communication (intercom, call ball). Noise: loud knocking, earplugs. Movement: blur/artifact — must lie still. Breath hold for abdominal.
Screening
Mandatory: EVERYONE who enters MRI suite — patients, staff, family. Form: detailed history. Absolute contraindications: pacemakers (some are MRI-conditional), cochlear implants, cerebral aneurysm clips (ferromagnetic), metallic foreign bodies (eye, shrapnel), some implanted pumps. Relative: joint replacements (usually OK), heart valves (OK), stents (OK after 6 weeks). Screening: MRI safety form, X-ray/CT if uncertain. Remove: jewelry, watches, credit cards, hearing aids, wigs, body piercings, drug patches (foil backing).
Safety Zones
Zone I: public access. Zone II: interaction with MRI staff, screening. Zone III: controlled access, MR personnel only, no ferromagnetic objects. Zone IV: scanner room, MR-trained personnel only, constant communication. Projectile effect: ferromagnetic objects become dangerous projectiles. Quench: helium release (emergency, can cause frostbite, asphyxiation). Contrast: gadolinium (nephrogenic systemic fibrosis risk in severe CKD). Emergency: code/crash outside Zone IV, bring patient out. Fire: no oxygen in Zone IV (fire risk with RF).
Important Notes
- Everyone entering the MRI suite must be screened for ferromagnetic implants — projectile effect can cause fatal injuries.
- Gadolinium contrast can cause nephrogenic systemic fibrosis (NSF) in patients with severe CKD (eGFR <30) — screen renal function.
Clinical Tips
- Cerebral aneurysm clips are an absolute contraindication for MRI — the magnetic field can dislodge the clip, causing fatal hemorrhage.
- In a code/crash, remove the patient from Zone IV (scanner room) before starting CPR — bring crash cart to Zone III, not Zone IV.
Summary
MRI safety requires mandatory screening for everyone entering the suite, identifying absolute contraindications (pacemakers, aneurysm clips, metallic foreign bodies), and respecting safety zones.
