1. General MRI hazards and anesthetic considerations
2. Pediatric sedation and anesthesia for MRI specifically
3. Equipment requirements
4. Monitoring standards

Anesthetic Considerations for Anesthesia in MRI - Management of a 6-Month-Old Child

(MD Anesthesiology - 10 Marks)

PART A: GENERAL ANESTHETIC CONSIDERATIONS FOR MRI

1. The MRI Environment - Unique Challenges

• MRI uses a powerful static magnetic field (typically 1.5-3 Tesla; 1 T = 10,000 gauss)
• Ferromagnetic objects become dangerous high-velocity projectiles ("missile effect") - gas cylinders, laryngoscopes, IV poles, scissors, pens, stethoscopes
• Implanted ferromagnetic devices (cardiac pacemakers, aneurysm clips, cochlear implants, older prosthetic joints) may shift, heat, or malfunction
• The field is always on - it never switches off

• RF pulses used for imaging cause peripheral nerve stimulation (PNS) - sensations ranging from tingling to pain
• Ordinary metal ECG leads and pulse oximeter cables act as antennas - attract RF energy, distort imaging, and can cause thermal burns
• Loops of wire or cable on the patient's skin cause induction heating and burns
• Tattoos with ferromagnetic inks can heat and cause skin burns

• ECG: severe artifact from rapidly changing magnetic gradients; nearly impossible to eliminate all artefacts completely
• Standard pulse oximeters, infusion pumps, ventilators malfunction near the magnet
• Electrodes must be placed close together, toward the center of the field, with leads insulated from skin

• Once inside the bore of the magnet, access to the airway and IV lines is severely limited
• Remote from main OR: emergency help, difficult airway carts, defibrillators, and drugs may not be immediately at hand
• Resuscitation must occur outside the scanner - a defibrillator and standard laryngoscope cannot be taken near the magnet

• Rapidly alternating gradient currents generate noise exceeding 99-140 dB - requires ear protection for patient and staff
• Inside the magnet coil it can reach 80°F (27°C) - this is a special risk for infants who are prone to hyperthermia
• Poor lighting inside the bore

2. MRI-Compatible Equipment (Mandatory Requirements)

3. Zones of MRI Safety (ACR 4-Zone Framework)

• Zone I: Freely accessible to public
• Zone II: Interface/transition area (screening occurs here)
• Zone III: Strong magnetic field present - restricted, requires escort
• Zone IV: Inside the magnet bore - highest risk, most restricted

4. Pre-Anesthetic Screening (All Patients)

• Cardiac pacemakers or implantable defibrillators (ICD)
• Intracranial aneurysm clips (older ferromagnetic clips are an absolute contraindication)
• Cochlear implants
• Metallic foreign bodies (orbital, intraocular)
• Spinal cord stimulators, neurostimulators
• Older prosthetic joints, surgical staples, stents
• Insulin pumps, medication patches

5. Anesthetic Technique Options in MRI

• Suitable for cooperative adults, short procedures
• Agents: oral benzodiazepines, IV midazolam, IV propofol infusion, dexmedetomidine
• Risk: loss of airway in a location with poor access - deep sedation can be catastrophic
• End-tidal CO2 monitoring via nasal cannula sampling line is essential
• Mandatory: equipment for emergency conversion to general anesthesia must be immediately available

• Preferred for pediatric patients and uncooperative adults
• Provides airway protection with less invasiveness than ETT
• Long breathing circuit needed to keep machine outside the strong field

• For full airway protection: elevated ICP, full stomach risk, OSA, obese patients
• MRI-compatible ETT (no wire reinforcement with ferromagnetic material)

6. Monitoring Standards in MRI

• Continuous pulse oximetry (MRI-compatible fiberoptic probe)
• ECG (MRI-compatible electrodes, carbon leads)
• Non-invasive blood pressure (extra-long tubing)
• End-tidal CO2 (if intubated or via nasal sampling)
• Temperature (especially in children)
• Continuous precordial stethoscope (plastic, non-metallic)
• Qualified anesthesia personnel must be continuously present

7. Contrast Agents (Gadolinium)

• Gadolinium-based contrast agents (GBCAs) for MRI have a lower reaction rate than iodinated CT contrast
• Hypersensitivity: ~5.9 per 10,000 injections; severe reactions 1:10,000-1:40,000
• Contraindicated or used with extreme caution in renal insufficiency (risk of Nephrogenic Systemic Fibrosis - NSF)
• Premedication protocol for known sensitivity: corticosteroids + antihistamines
• Emergency management of severe reactions: stop infusion, O2, airway support, IV fluids, vasopressors, bronchodilators

PART B: MANAGEMENT OF A 6-MONTH-OLD INFANT FOR MRI

Special Considerations in Infants

1. Pre-Anesthetic Assessment

• Gestational age (preterm infants have immature respiratory drive - risk of post-anesthetic apnea up to 60 weeks post-conceptual age)
• Birth history, congenital anomalies, cardiac defects
• Reason for MRI (neurological, structural anomaly)
• Previous anesthetic history
• Current medications
• Vaccination status (not directly relevant but reflects overall health)
• Family history of malignant hyperthermia or pseudocholinesterase deficiency

• Weight (for drug dosing - at 6 months, expected ~7-8 kg using 2n+4 formula or growth chart)
• Airway assessment: small mouth, large tongue, short neck, high anterior larynx (C3-C4), large occiput causing neck flexion in supine position
• Heart: rule out congenital heart disease (murmur, cyanosis)
• Respiratory status: any upper or lower respiratory tract infection (URTI/LRTI) - defer if active
• IV access assessment

• Hemoglobin (physiological nadir at 6 months)
• Blood glucose (infants prone to hypoglycemia with fasting)
• Echocardiogram if cardiac anomaly suspected

2. Fasting (NPO) Guidelines for a 6-Month-Old

3. Premedication

• EMLA cream over the dorsum of the hand/foot 45-60 minutes before for IV cannulation
• Oral midazolam 0.5 mg/kg (max 15 mg) 30-45 minutes before for anxiolysis if the child is old enough to be apprehensive (more relevant from 8-10 months onward)
• At 6 months: separation anxiety is minimal - premedication may be omitted; inhalational induction without prior separation anxiety
• Atropine 0.02 mg/kg IV (minimum 0.1 mg) to prevent bradycardia (infants are vagotonic and bradycardia is the primary cardiac response to hypoxia and laryngoscopy)

4. Anesthetic Plan

• Difficult to titrate accurately
• Prone to airway obstruction
• No reliable communication
• Risk of hypoxia is high in an inaccessible environment
• Temperature regulation is poor

• Sevoflurane 8% in 100% O2 via a well-fitting face mask
• Smooth, rapid induction
• Reduce to maintenance concentration once unconscious

• Propofol 2-3 mg/kg IV, or
• Ketamine 1-2 mg/kg IV (useful if hemodynamically unstable or cardiac disease)

• LMA (Laryngeal Mask Airway) is the preferred airway device for MRI:
  • Size 1 or 1.5 for a 6-month-old (~5-6 kg = size 1; 6-10 kg = size 1.5)
  • Allows spontaneous ventilation
  • Less stimulating than ETT
  • Appropriate for non-aspiration-risk infants
• ETT if:
  • Full stomach (emergency), GERD, high aspiration risk
  • Procedures requiring controlled ventilation (elevated ICP)
  • Anticipated airway difficulty

• Sevoflurane 2-3% in 50:50 O2:Air via MRI-compatible circle system or Mapleson D (Jackson-Rees) circuit with long breathing tubes
• Spontaneous breathing or assisted ventilation
• Avoid N2O (distends bowel, increases pulmonary vascular resistance, risk of PONV)

• Atropine: 0.1 mg IV (minimum dose)
• Propofol induction: 14-21 mg IV
• Sevoflurane: 2.5-3% maintenance
• Fentanyl: 1-2 mcg/kg if procedure is painful or for smooth emergence
• Ondansetron 0.1 mg/kg IV for PONV prophylaxis

5. Intraoperative Monitoring

• Pulse oximeter: MRI-compatible fiberoptic probe (wrap in foam to avoid heating)
• ECG: MRI-compatible carbon-lead electrodes, placed centrally
• NIBP: extra-long tubing, 5 min intervals
• Temperature: rectal or esophageal MRI-compatible probe - critical in infants (risk of both hypothermia and hyperthermia inside the bore at 80°F)
• Precordial stethoscope: plastic, non-metallic
• End-tidal CO2: via breathing circuit sidestream sampling
• Inspired gas analysis: O2, volatile agent concentration
• Continuous visual observation via MRI-suite window and camera

6. Specific Hazards Unique to the 6-Month-Old in MRI

7. Post-Anesthetic Care

• Transfer to recovery room with skilled anesthesia personnel accompanying
• Standard PACU monitoring: SpO2, HR, BP, temperature, level of consciousness
• Airway support: LMA or ETT removed when airway reflexes fully returned (child cries, opens eyes, moves purposefully)
• Oral fluids when alert and gag reflex present
• Discharge criteria: standard PACU Aldrete/modified Aldrete score ≥9
• Watch for:
  • Post-obstructive pulmonary edema
  • Laryngospasm on emergence
  • Hypothermia
  • Hypoglycemia
  • Delayed emergence (especially with propofol in infants)

8. Emergency Management in MRI

1. Immediately stop the scan and notify MRI technician
2. Remove patient from bore rapidly (slide table out)
3. Move to Zone II or outside the scanner room
4. Begin resuscitation outside the scanner - defibrillator, standard laryngoscope, and metal oxygen cylinders cannot enter Zone IV
5. Call for emergency help via dedicated two-way communication system (mandatory in all MRI suites)

Summary Table: Key Points for 6-Month-Old in MRI

• Barash, Cullen & Stoelting's Clinical Anesthesia, 9e - Chapter 33: Nonoperating Room Anesthesia (pp. 2652-2678)
• Morgan & Mikhail's Clinical Anesthesiology, 7e - Case Discussion: Monitoring During MRI (pp. 252-254)
• Miller's Anesthesia, 10e - Chapter: Non-Operating Room Anesthesia