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by the NFCC

Hazard

MRI scanners

Hazard Knowledge

MRI (magnetic resonance imaging) scanners will most commonly be found in medical facilities, such as hospitals. However, they may also be found in animal facilities, such as veterinary clinics and hospitals. Refer to the supplementary information for further detail about MRI scanners.

The main hazards relating to MRI scanners are:

  • Electricity
  • Strong magnetic fields
  • Super-conducting magnets
  • Cryogenic materials
  • Asphyxiation
  • Impact on communications

Electricity

Fixed and mobile scanners require a three-phase 400V electricity supply. Mobile scanners may have power supplied by a generator or by cable from an adjacent building.

It is important to understand that isolating the electrical supply will not stop the magnetic field being generated for several hours.

Strong magnetic fields

MRI scanners produce strong magnetic fields; these are emitted in all directions around the equipment but the strongest fields are towards the centre of the scanner where the patient enters.

Any metallic material may be strongly attracted to the core of the scanner. Metallic objects can become projectile hazards. Even non-magnetic materials, such as aluminium, can be affected and may twist violently in the magnetic field.

The magnetic field may affect:

  • Fire and rescue service equipment
  • Communication equipment, such as mobile phones and radios
  • Any metal implants or fragments, such as:
    • Surgical pins or plates
    • Cardiac pacemakers
    • Dental bridges or implants
    • Welding fragments
    • Metal splinters
  • Jewellery including piercings and watches
  • Metal fastenings on clothing

MRI magnetic fields can erase bank card encoding, and cause a malfunction in any battery-operated equipment such as watches.

Although the magnetic field cannot be shut down quickly, there are other emergency procedures that will shut down the magnetic field within several minutes. During that time a high magnetic field will still exist.

Super-conducting magnets

The magnets within the scanner consist of a large number of tightly wound wires, through which a current passes to produce a magnetic field. MRI scanners have a helium liquid in the magnet, which usually vents through ducting. If it fails, helium gas will be released into the room, depleting oxygen levels.

Cryogenic liquids

The super-conducting magnets are kept at extremely low temperatures by being immersed in liquid helium (-269oC), which is sometimes surrounded by an insulating layer of liquid nitrogen (-196oC). For further information on dealing with cryogenic hazards see Cryogenic materials.

Asphyxiation

During an emergency shutdown (or quenching) of an MRI scanner the cryogenic liquids are allowed to 'gas off' and are rapidly vented outside the facility. However, it is possible that some of the gases could be released into the room containing the scanner. This may present an asphyxiation hazard as oxygen is displaced from the atmosphere.

Impact on communication

The scanner room is constructed as a 'Faraday cage' to ensure that the magnetic field remains confined within the room. It continues to work even after electrical power is isolated, and will also block all radio signals, incident ground radios, breathing apparatus (BA) telemetry systems and mobile phones.