Anesthesia for MRI and CT

MRI/CT Environment & Risks - Remote Control Anesthesia

• MRI Suite Hazards:
  • 📌 MRI Zones (I-IV):
    • Zone I: Public.
    • Zone II: Unscreened pts.
    • Zone III: Screened personnel/patients; restricted access.
    • Zone IV: Scanner room; >5 Gauss line; highest risk.
  • Projectile Risk: Ferromagnetic objects become dangerous projectiles.
  • RF Heating: Tissue burn risk (implants, ECG leads).
  • Acoustic Noise: 90-130 dB; hearing protection essential.
  • Quench: Helium escape; asphyxia, frostbite risk.

  ⭐ The main static magnetic field in clinical MRI scanners typically ranges from 1.5T to 3T.

• CT Suite Hazards:
  • Ionizing Radiation: ALARA principle.
  • Contrast Media: Allergy/CIN risk.
  • Patient Access: Limited.
• Remote Anesthesia:
  • MRI-compatible equipment (anesthesia machine, monitors) essential.
  • Long breathing circuits; vigilant remote monitoring crucial.

Patient Screening & Prep - Shield & Screen Carefully

MRI Safety:

• Screening: Implants (clips, coils), pacemakers/ICDs (verify MRI-conditional status), ferromagnetic foreign bodies (ocular, shrapnel), pregnancy.
• 📌 Mnemonic: NO METAL - Metallic implants, Electronic devices, Tattoos (old, metallic ink), Aneurysm clips, Loose foreign bodies.

CT Specifics:

• Contrast:
  • Allergy Hx: Premedicate if prior mild reaction.
  • Renal: Check eGFR. Gadolinium: avoid if eGFR < 30 mL/min/1.73m². Iodinated: Contrast-Induced Nephropathy (CIN) risk.
• Metformin: Hold if eGFR < 30 mL/min/1.73m², AKI, or iodinated contrast given with eGFR < 45 mL/min/1.73m².

Universal Prep:

• ASA Fasting Guidelines adhered to.
• Informed Consent obtained.

⭐ Absolute contraindications for MRI include most cardiac pacemakers (unless MRI-conditional and programmed appropriately), cochlear implants, and intraocular metallic foreign bodies.

Anesthetic Techniques & Drugs - Tunnel Vision Techniques

• Anesthesia Spectrum:
  • General Anesthesia (GA): For complete immobility, controlled ventilation.
  • Sedation: Deep (unresponsive, purposeful response to pain) vs. Conscious (responds to verbal commands).
• Key Agents & Delivery:
  • Propofol: TIVA/TCI (Targets: Sedation 1.5-4.5 mcg/mL; GA 3-6 mcg/mL).
  • Dexmedetomidine: Sedation with minimal respiratory depression.
  • Remifentanil: Ultra-short-acting opioid, adjunct.
  • Volatiles: Sevoflurane/Desflurane (low flow, efficient scavenging essential).
• Airway Management:
  • LMA: MRI-compatible (non-ferromagnetic materials).
  • ETT: For definitive airway control, especially if aspiration risk.
• Equipment Considerations:
  • MRI Suite: MRI-compatible anesthesia machine, ventilator, and monitors.
  • CT Suite: Standard machine; long breathing circuits may be needed.

⭐ Total Intravenous Anesthesia (TIVA) with propofol and remifentanil is often preferred for MRI to avoid interference from volatile agents and ensure rapid recovery.

NORA Monitoring & Crisis - Eyes, Ears, Emergencies

Monitoring Essentials:

• MRI Suite:
  • ECG: Special filtered leads/algorithms.
  • SpO2: Fiber-optic probes.
  • NIBP: Extended tubing, automated.
  • EtCO2: Long sampling lines, sidestream.
  • Temperature: Fiber-optic/non-metallic probes.
• CT Suite:
  • Standard ASA monitors; often requires remote patient observation.

Key Complications & Management:

• MRI-Specific Hazards:
  • Thermal Burns: From looped wires or direct cable-skin contact. 📌 NO LOOPS (Avoid looping wires).
  • Acoustic Damage: Noise >90-120 dB (use ear protection).
  • Projectile Effect: Ferromagnetic objects become missiles.
  • Quench: Cryogen release → asphyxia, frostbite, barotrauma.
  • Claustrophobia.

  ⭐ To prevent RF-induced burns in MRI, ensure no direct skin contact with cables, avoid cable loops, and use appropriate padding.

• CT-Specific Hazards:
  • Contrast Media Reactions: Allergic to anaphylactic.
  • Radiation Exposure: Patient and staff.
• General NORA Risks:
  • Hypoxia, hypotension, airway obstruction, emergence delirium/agitation.

Flowchart: Managing Acute Contrast Reaction

High‑Yield Points - ⚡ Biggest Takeaways

• MRI environment: Potent magnetic fields, RF energy, and loud noise (>100 dB) are major hazards.
• Strictly no ferromagnetic items; ensure all equipment is MRI-compatible.
• Patient monitoring is challenging due to limited access and RF interference.
• Gadolinium contrast may cause Nephrogenic Systemic Fibrosis (NSF) in renal impairment.
• CT anesthesia aims for immobility, with attention to radiation safety for all.
• Total Intravenous Anesthesia (TIVA) is often favored for MRI; hearing protection is vital.
• Emergency preparedness for events like quenching is crucial.