NMR and MRI Medical Scanners: Surviving the "Invisible Force"

Magnetic resonance imaging (MRI) is a test that uses a magnetic field and pulses of radio wave energy to make pictures of organs and structures inside the body. MRI and nuclear magnetic resonance (NMR) scanners are found in hospitals, nuclear medicine...


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In 2006, two people were injured when an MRI machine they were moving exploded. It is reported that nitrogen and helium still in the machine mixed, causing the explosion. One of the injured had shrapnel in his head and the other an injured arm. Firefighters were called to the facility following the explosion to investigate and treat the injured.

Beware of the force

The power of these magnets should not be underestimated and great care should be exercised by emergency response personnel working near them. All ferromagnetic metals should be removed without exception before entering the facility magnet area. If metal cannot be removed, personnel should not enter.

Magnets at MRI and NMR facilities are always on and there is always a magnetic field present, even if power to the facility or the magnet is disconnected. The only way magnets can be shut down is by quenching. This can occur by instigation through a quench switch located in facility or spontaneous quenching caused by a malfunction in the system. Instigation of a quench using the switch can discharge the magnet in about 20 seconds. Even a partially quenched magnet can pose a hazard and attract ferrous objects such as gas cylinders.

Instigated quenching is the removal of the nitrogen and helium from the magnet and exhausting to the outside of the building. Spontaneous quenching results from a breakdown in the system, which may result in vapors of nitrogen and helium displacing the oxygen in the room and causing simple asphyxiation to anyone not wearing SCBA. When entering an area where oxygen deficiency is suspected or possible, portable oxygen monitoring equipment should be used. During instigated quenching it has also been reported that exhausting equipment has failed resulting in vapors in the room as well.

Quenching cannot only be a dangerous operation, but is very expensive. It can cost over $50,000 to reenergize the magnet and result in a one- to two-month down time. If the magnet needs repair as a result of the quenching, it can cost over $100,000 with several months of down time. If the machine cannot be repaired, replacement can cost more than $2 million. Quenching should only be done under the supervision of facility personnel when there is a life-threatening situation, like a person pinned to the magnet by a large object that cannot be removed by hand or as the result of a large fire so that firefighters can enter the room safely. If an emergency quench is necessary, pressure generated by the quench may prevent doors from opening, so prop open the magnet room door. Allow no others to enter the room through the open door. All personnel must know to leave the room and not return until the cryogenic gases have dissipated and the room is safe to reoccupy.

Ultra-cold materials

In addition to the asphyxiation hazards they present, cryogenic liquid nitrogen and helium are very cold materials and can cause thermal burns and solidification of body parts. Contact with the liquids can solidify personal protective equipment (PPE) normally worn by emergency responders. Avoid contact with the materials. Special gloves are used within the facilities for handling cold or frosted surfaces. These gloves will not provide any protection if submerged in the liquids themselves. Solidification of hands and fingers can occur quickly.

Cryogenic liquids also have large liquid-to-vapor expansion ratios and a small amount of liquid can cause a significant amount of vapor. Helium has a boiling point of 452 degrees below zero. Helium is the only substance on Earth that does not exist as a solid. There is nothing colder to solidify helium. Helium’s expansion ration is 754-1, meaning one gallon of liquid helium would expand to 754 gallons of helium vapor. Nitrogen has a boiling point of 321 degrees below zero and an expansion ratio of 697-1. Extreme caution should be exercised when working in areas where cryogenic liquid or vapor has been released. Both helium and nitrogen gases are odorless and colorless. However, cryogenic gases are so cold they can freeze water vapor in the air, causing a vapor cloud appearance.

Response precautions