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Anaesthesia for CT and MRI
Indications for Anaesthesia or Sedation
| Patient group |
Typical indication |
Preferred technique |
| Infants < 2 mth (after feeding) |
Often remain motionless without drugs |
Feed-and-wrap ± sucrose |
| Children 2 mth–6 yr, developmental delay, movement disorders |
Diagnostic CT / MRI, radiotherapy planning |
Dexmedetomidine–propofol sedation or GA with LMA/ETT |
| Critically ill / intubated |
Trauma, neuro-monitoring, chest/abdominal CT |
Maintain existing airway & ventilation; full ICU monitoring |
| Interventional (biopsy, drain, ablation) |
Painful/stimulating procedure |
GA or deep MAC ± regional block |
Generic NORA Requirements (ASA 2023)
- Same monitoring as theatre: ECG, NIBP, SpO₂ plus capnography for any sedation deeper than minimal.
- MRI-conditional equipment and infusion pumps; spare full O₂ cylinder attached.
- Immediate access to difficult-airway cart and resuscitation drugs.
- Defined extraction plan (cardiac arrest → pull patient beyond 0.5 mT line before defibrillation).
Anaesthesia for CT
Practical Set-up
| Consideration |
Best practice |
| Space & line-of-sight |
Position compact machine and MRI-safe monitor on gantry side; keep cables out of scanner aperture. |
| Airway access |
Head remains outside gantry → LMA acceptable for brief scans; intubate if aspiration risk or PPV required. |
| Radiation safety |
Anaesthetist usually controls ventilation from console; wear lead apron if room entry essential. |
| Topic |
Current recommendation (ACR Manual 2024) |
| Volume |
Adults 1.5-2 mL kg⁻¹ (max 150 mL 320 mg I mL⁻¹); children 1.5 mL kg⁻¹ (max adult dose). |
| Renal risk |
Hold metformin only if eGFR < 30 mL min⁻¹ or patient acutely unwell. Hydrate high-risk patients; no prophylactic N-acetylcysteine. |
| Allergy / prior reaction |
50 mg prednisone 13 h + 7 h + 1 h pre-scan, or IV hydrocortisone 200 mg + antihistamine 1 h pre-scan. Low-osmolar agents reduce risk seven-fold. |
| Central lines |
Do not use power injector through CVC or PICC > 325 psi; hand-inject only. |
Anaesthesia for MRI
MRI Safety Essentials
| Hazard |
Mitigation |
| Projectile risk |
2-stage screening (patient & staff) for ferromagnetics; all trolleys and cylinders MR-safe. |
| Implanted devices |
Check manufacturer database for MR-conditional parameters (field strength, SAR, positioning). |
| Burns from leads/lines |
Use MRI-compatible ECG leads; place IV loops straight, padding between skin and cables. |
| Acoustic injury |
Foam earplugs + earmuffs for all patients; neonatal moulded putty + mini-muffs. |
| Thermal stress / hypothermia |
Ambient 19 °C; use warmed blankets and monitor core temp in infants. |
Anaesthetic Workflow
- Induction in Zone III (safe area) with standard machine.
- Transfer on non-magnetic trolley; connect to MRI ventilator (or manually ventilate with 7 m length circuit).
- Monitoring–MRI-conditional ECG, fibre-optic plethysmography, long-hose ETCO₂, NIBP.
- Maintain GA with sevoflurane in O₂/air (use DEDICATED gas pipeline) or propofol TCI (PaedFuser/Eleveld).
- Emergence outside 0.5 mT line; recover in PACU with full monitoring
Sedation Protocols (children)–Red Cross Children’s Hospital 2024
| Drug |
Dose & route |
Notes |
| Dexmedetomidine |
1-2 µg kg⁻¹ IV over 5 min (top-up 0.5-1 µg kg⁻¹) or 3-4 µg kg⁻¹ intranasal |
Bradycardia common but usually benign |
| Propofol |
1-2 mg kg⁻¹ IV bolus or TCI Ce 1.8–3 µg mL⁻¹ |
Add if motion or paradoxical agitation after gadolinium |
| Clonidine (≥ 5 yr) |
5 µg kg⁻¹ PO 60 min pre-scan |
Good for radiotherapy sessions |
- Continuous capnography via 20 G cannula at nares; HFNO 30–40 L min⁻¹ for OSA or high BMI.
Godolinium Contrast
- Macrocyclic agents (gadobutrol, gadoterate) carry negligible NSF risk even when eGFR < 30 mL min⁻¹.
- Standard paediatric dose 0.1 mmol kg⁻¹ (0.2 mL kg⁻¹).
- Observe for rare anaphylactoid reactions; treat with IM epinephrine 10 µg kg⁻¹.
Disease-specific Considerations
| Condition |
Sedation / GA tip |
| Severe OSA / morbid obesity |
Avoid supine sedation with unsecured airway; intubate or use HFNO + jaw support; keep FiO₂ ≤ 0.3 inside bore to minimise fire risk (rare but reported). |
| Raised ICP / posterior fossa lesion |
GA with intubation, controlled PaCO₂ 4.0–4.5 kPa. Avoid ketamine and high PEEP. |
| Seizure disorder |
Continue anticonvulsants; propofol infusion preferred (dexmedetomidine may trigger bradycardia–asystole in Lennox-Gastaut). |
| Renal failure |
Use macrocyclic gadolinium; avoid Na-phosphate bowel prep for CT colonography. |
| Cardiac pacemaker |
Only MR-conditional models; programme to asynchronous mode; continuous ECG, pulse oximetry and external defibrillator outside 0.5 mT zone. |
Emergency Algorithms
| Event |
Immediate action |
| Cardiac arrest in MRI |
Stop scan, disconnect coils, pull trolley straight out to Zone II; begin ACL with standard defibrillator. |
| Airway obstruction (sedation) |
Jaw thrust & HFNO ↑; if no improvement → remove from bore, insert LMA. |
| Anaphylaxis (contrast) |
Adrenaline 10 µg kg⁻¹ IM (max 0.5 mg), high-flow O₂, IV fluids; extract to resus bay for IV therapy. |
Links
Past Exam Questions
Cardiac Arrest Management in the MRI Scanner
A 50-year-old patient who is post intracranial aneurysm surgery suffers a cardiac arrest whilst in the MRI scanner.
List 10 steps you would take to manage the patient. (10)
References:
- Allman K, Wilson I, O’Donnell A. Oxford Handbook of Anaesthesia. Vol. 4. Great Clarendon Street, Oxford, OX2 6DP, United Kingdom: Oxford University Press; 2016.
- Reddy, U., White, M., & Wilson, S. (2012). Anaesthesia for magnetic resonance imaging. Continuing Education in Anaesthesia Critical Care &Amp; Pain, 12(3), 140-144. https://doi.org/10.1093/bjaceaccp/mks002
- Wilson, S., Shinde, S., Appleby, I., Boscoe, M. J., Conway, D. H., Dryden, C. F., … & Wright, E. (2019). Guidelines for the safe provision of anaesthesia in magnetic resonance units 2019. Anaesthesia, 74(5), 638-650. https://doi.org/10.1111/anae.14578
- ASA Task Force on Sedation and Analgesia for Non-Anaesthesiologists. Practice guidelines 2023.
- ACR Committee on Safety. Manual on Contrast Media (Version 16.2), 2024.
- Kanal E, Barkovich AJ. ACR Guidance Document on MR Safe Practices: 2023 Update. J Magn Reason Imaging. 2023.
- Morton NS, Lardner DR. Remote-site paediatric anaesthesia. BJA Educ. 2022;22:419-26.
- Malviya S, et al. Dexmedetomidine in paediatric MRI: prospective cohort. Anesthesiology. 2021;134:100-12.
- Stonemetz J, et al. Anaesthesia for CT and MR–2025 review. Curr Opin Anaesthesiol. 2025;38:279-86.
- FRCA Mind Maps. (2024). Retrieved June 5, 2024, from https://www.frcamindmaps.org/
- Anesthesia Considerations. (2024). Retrieved June 5, 2024, from https://www.anesthesiaconsiderations.com/
Summaries:
MRI
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