FRCA Notes

Magnetic Resonance Imaging

An SAQ on anaesthesia for MRI came up in the 2018 Final written exam (38% pass rate).

Examiners criticised of a lack of understanding of the risks of MRI, and that answers were generic to remote anaesthesia rather than MRI-specific.

One may wish to review Primary FRCA material relating to magnetism, induction of magnetic fields and the physics of MRI.

Resources

Equipment (in general)

  • Medical equipment with ferromagnetic components may be displaced and/or torqued by the powerful magnetic fields of the MRI scanner
    • As such, no ferromagnetic material should ever be taken beyond the 50G line
  • Equipment may also resonate owing to the radiofrequency energy emitted by the MRI scanner
  • To ensure appropriate equipment is used, it is classified as either:
    1. MRI Safe
      • Completely free of metallic components
      • Non-metallic, non-conductive, non-radiofrequency reactive
      • No hazard in any MR environment

    2. MRI Conditional
      • Safe under certain tested magnetic conditions that are written on the product (i.e. the magnetic field strength within which that product may be safely used)

    3. MRI Unsafe
      • Poses a hazard in any MR environment

Implants

  • Ferrous implants are also subject to displacement and torque forces, while non-ferrous implants may become heated by the radiofrequency energy emitted by the MRI scanner
  • Both ferrous and non-ferrous implants will cause image distortion
  • The suitability of any individual implant will depend on its nature, although a non-exhaustive list is found below
    • Green lines indicate typically MRI safe implants
    • Yellow lines indicate typically MRI conditional implants
    • Red lines indicate typically MRI unsafe implants
Device Notes
Programmable hydrocephalus shunts The magnetic field may affect pressure sensing
Programmer should be available to re-program post-scan
Joint replacement Large metallic implants may generate heat and degrade image quality
Careful monitoring is required
Heart valves Low risk of displacement
Intra-uterine devices Usually copper and plastic i.e. safe
Clips and staples Nature of the clip/staple should be known and managed accordingly
Aneurysm clips Need positive documented evidence that the clip is non-ferromagnetic for scanning to proceed
Intravascular devices Non-ferromagnetic devices are safe
Magnetic devices typically safe 6 - 8 weeks post-op. as securely attached to a vessel wall
Tissue expanders/implants Injection ports are often metallic; caution should be taken
Pacemakers & ICD Should not enter within the 5 Gauss line due to concerns about disruption of function
Some MRI conditional pacemakers are available
Neurostimulators Batteries and implanted electrodes contraindicate MRI
Implanted drug infusion pumps Ferromagnetic components and switches
Cochlear implants Ferromagnetic components
Ocular foreign bodies
Ocular implants		 Foreign bodies a contraindication as may cause vitreal haemorrhage
Implants have high risk of movement/dislodgement
Penile implants Risk of dislodgement(!)


Time-varying magnetic gradient fields

  • Small, dynamic magnetic fields can induce a current
  • These currents may be sufficient to stimulate peripheral nerve and muscle cells, causing discomfort

Acoustic noise

  • Caused by force generated on coils by alternating currents
  • Both patients and staff need ear protection as switching of the gradient fields creates noise >85dB (often >95dB)

Radiofrequency heating

  • Power is dissipated within the patient, causing an increase in body temperature
  • Risk of severe, rapid burns from any conductive material on the patient's skin including ECG leads and metal in clothing

Helium escape

  • The scanner is cooled by liquid helium and the environment tends to be cold, so preventative measures must be used to avoid hypothermia
  • Spontaneous emergency shutdowns ('quenches') cause liquid helium and nitrogen to expand to gas
  • They must be vented rapidly, though this may generate a hypoxic environment within the MRI scanner

IV gadolinium contrast

  • Less likely to cause allergic reactions (incidence 0.01%) than iodinated contrast
  • Mild side-effects (1-5%) including nausea, vomiting, headaches and pain on injection
  • Can cause nephrogenic sclerosing fibrosis, which is more likely in those with advanced renal or liver disease
  • Is contraindicated in pregnancy

Patient cohort

  • Paediatric patients, including those with neurological disease, vascular malformations, or oncological tumour growth
  • Patients with learning difficulties, anxiety, claustrophobia or movement disorders requiring GA to facilitate MRI
  • (Unstable) ICU patients

Monitoring issues

  • Use of a powerful magnet precludes use of equipment with metallic components
    • Metallic objects will either interfere with MRI signal or be accelerated dangerously fast
    • Therefore MR safe ECG electrodes, BP cuffs and pulse oximeters are required

  • A radio-transmitter is used to communicate with the control room, which may interfere with patient monitoring systems
  • A separate radio-transmitter can be used to transfer ECG recordings
  • However as the whole MRI room is encased in a (copper/aluminium) Faraday cage, there is still interference
Monitor Issue
Sats probe Fibre-optic probe with long cables or Bluetooth signal
EtCO2 Long response time due to length of cable needed
ECG Currents can be induced in cables so braided cables required
Ag/AgCl electrodes can cause burns; carbon-fibre electrodes used
Aorta acts as a conductor inducing T-wave or ST abnormalities so high risk patients require pre-/post-ECG
NIBP Plastic rather than ferromagnetic connections required
Arterial line Transducer needs to be far from the magnet, but long tubing increases damping
Pilot balloon Contains a small ferromagnetic spring so must be taped well out of the way

Other issues

  • Anaesthetic machines
    • If a standard machine is used, it is housed outside the Faraday cage and an extra-long Bain circuit (5.4m) is required
    • An MR conditional machine can be used within the scanner, but outside the 5G line

  • MR conditional infusion pumps should be used
    • Standard pumps outside the 100G line can also be used
    • TIVA pumps are typically strongly ferromagnetic so require long infusion lines

  • Environmental factors
    • Induction occurs in a separate room, so a transfer is required
    • There is often limited space for anaesthetic equipment
    • There is limited visibility of and access to the patient, especially once scanning begins, and scanning often takes a long time 20 - 30mins
    • Patients are removed to a remote, isolated and unfamiliar area