Potential Hazards and Risks
Magnetic Field Risk The static magnetic field of the MRI system is exceptionally strong. A 1.5 T magnet generates a magnetic that is approximately 21,000 greater than the earth's natural field. In such an environment ferromagnetic metal objects can become airborne as projectiles. Small objects such as paper clips and hairpins have a terminal velocity of 40mph when pulled into a 1.5 T magnet and therefore pose a serious risk to the patient and anyone else in the scan room. The force with which projectiles are pulled toward a magnetic field is proportional to the mass of the object and distance from the magnet. Even surgical tools such as hemostats, scissors and clamps, although made of a material known as surgical stainless steel, are strongly attracted to the main magnetic field. Oxygen tanks, gurneys, floor buffing machines, and construction tools are highly magnetic and should never be brought into the scan room. However there are non-ferrous oxygen tanks and gurneys available, which are MRI compatible. Sand bags must also be inspected since some are filled, not with sand, but with steel shot which is highly magnetic.
Consumer products such as pagers, cell phones, cameras and analog watches may be damaged by the magnetic field. Pacemakers may be reprogrammed or turned off by the magnetic field. The magnet field erases credit cards with magnetic strips. Patients with ferrous intra-cranial vascular clips may be at risk due to the possible movement of the clip. See Contraindications for MRI below.
Radio-frequency (RF) Field Risk The radio-frequency field may induce currents in wires that are adjacent or on the patient, causing skin burns. It may induce currents in intra-cardiac leads, resulting in inadvertent cardiac pacing. Prolonged imaging may cause the patient's core body temperature to rise. In practice, significant patient heating is only encountered in infants.
Due to the risk of RF current induced thermal burns:
- To minimize the risk of synthetic fibers in clothes acting as a current inducer, all patients having a MR exam must be changed into hospital provided clothing (gowns) prior to imaging.
- All patients having a MR exam must be padded during imaging in accordance to manufacturer guidelines to minimize skin to skin, and skin to magnet bore contact.
- All patients must be provided a working alert device (squeeze ball), to able to communicate with the MRI technologist during imaging when in distress.
Cryogen Risk During a planned or accidental shutdown of the magnetic field (aka "quench"), the liquid Helium in the magnet turns into gas and may escape into the scan room displacing the oxygen in the room leading to asphyxia.
Biological Effects Due to Magnetic Field For the static magnetic fields currently used in MRI up to 2 Tesla, there are no known biological effects. The majority of studies show no effects on cell growth and morphology. Data accumulated by the National Institute for Occupational Safety, the World Health Organization, and the US State Department show no increased risk for leukemia or other cancer. Some reversible biological effects have been observed on human subjects exposed to 2.0 T and above. These effects include fatigue, headaches, hypotension and irritability.