Safety of Magnetic Resonance Imaging

Contraindication

• Cardiac pacemaker

• Aneurysm clips

• Claustrophobia

• Metallic fragments in the eye

• Cochlear implants

• Internal drug infusion pumps

• Neurostimulators

• Bone growth stimulators

MRI Magnetic Fields exhibits

threshold dose-response relationship.

Three physical fields that might suspect of producing biological response

• Strong static magnetic field

• Time-varying gradient magnetic field

• RF emission

Strong static magnetic field (B₀) unit

Tesla

Strong static magnetic field (B₀) affects

biological processes:

•  Membrane permeability

• Enzymes kinetics

• Nerve conduction

Time-varying gradient magnetic field (B₁) Unit

Tesla/second (T/s)

Time-varying gradient magnetic field (B₁) Affects

sensory responses:

• Visual phosphenes

• Bone healing

• Cardiac fibrillation

RF emission Unit

Watts/kilogram (W/kg)

RF emission

Causes tissue heating

Heat

the principal result of the interaction between RF field and tissue.

Heating

expressed as specific absorption rate (SAR)

Magnetic field precautions

avoid metallic objects in MRI room.

Patient preparation

ensure no internal or external metal.

Warning signs

required in MRI areas.

Quenching

The events that occur when the liquid cryogens that cool the magnet coils boil off rapidly.

Quench causes

magnet fault or emergency stop activation.

Quench effects

• helium release

• loud noise

• risk of asphyxiation.

Quench response

evacuate immediately.

Magnet recovery

takes about 72 hours to restore full field strength.