An incident involving a magnetic resonance imaging (MRI) device illustrated the potential for injury to firefighters and the need for emergency responders to be aware of the “invisible forces” that can affect the safety of their work environment.

On August 7, 1994, at 0222 hours, the Stockton (CA) Fire Department responded to a report of a structure fire in a large medical office complex. On arrival, the first engine reported “heavy smoke showing” and potential “flashover conditions.” The company officer requested that a second alarm assignment be struck.

When the engine crew entered the structure, the condition they encountered suggested that the fire had been smoldering for some time. The seat of the fire was difficult to find due to the heavy smoke and extreme heat. On arrival of the first-due truck company, gasoline-powered blowers were set up and positive-pressure ventilation initiated. When the fire was finally found, it was rapidly extinguished.

During the overhaul, damaged acoustic ceiling tiles were removed from the building`s drop ceiling. One firefighter noted that the steel pike pole he was using was “irresistibly” drawn toward a large machine in the room he was working. He was aware that the MRI scanner was in the building, but smoke conditions were so bad he could not see the warning signs around the scanner. No one was aware of the power of the magnetic force associated with this medical diagnostic tool.

The firefighter said that he had to use both arms to free his pike pole from where it impacted the surface of the device. Concerned about other firefighters` safety, he informed the incident commander regarding the magnetic hazard he had just experienced. Before the IC could issue an “alert” broadcast to warn of the MRI device, another firefighter wearing a pickhead axe on his belt entered the magnetic field. He was forcibly pulled toward the center of the machine. The unfortunate firefighter stated that he was unable to resist the pull of the magnetic force 10 feet away from the scanner. Calling for help, he thought his clothes had been caught by some overlooked mechanical force. Two other firefighters were required to dislodge him from the machine`s iron grip.

Fortunately, emergency operations were completed without further incident or injury, but his experience generated questions regarding emergency workers` safety in an environment containing strong magnetic fields.


MRI scanners are medical diagnostic devices that use a magnetic field, radio frequency waves, and computers to produce remarkably clear images of internal anatomical structures that are far superior to conventional X-rays. Applying strong magnetic forces to the body alters the normal orientation of hydrogen atoms present in all organic tissue. This shifting of atomic orientation is scanned by radio frequency waves and translated into a visual image by a complex and powerful computer system. The MRI image quality is startling, clearly distinguishing various soft tissues and skeletal structures in the body. Since its development, MRI has become an invaluable asset in diagnosing many diseases and injuries.

The magnetic field of an MRI device emits no sound and cannot be seen, yet this force is very strong. Commonly, an MRI magnet exerts a force of 0.5 to 1.5 Tesla, which is 25,000 to 30,000 times the pull of earth`s gravitation. The MRI magnet can exert more than four tons of force at the center of the scanner core, which is more intense than that of crane electromagnets used in scrap metal recycling facilities.

Additionally, the pull or intensity of the magnetic force is exponential: The closer one is to the machine, the stronger the pull exerted on ferrous metal tools or objects that person is carrying. The point at which a person first senses the magnetic force depends on the nominal strength of the scanner. Recently, some facilities have begun using scanners rated at more than three Tesla. Close to the magnet`s bore (central opening), a human is literally unable to resist the force of the magnet.

This magnetic force is developed by flowing electical current through coiled wire conductors surrounding the scanner bore. The current necessary to operate an MRI scanner properly is roughly 1,300 amperes. Sustaining the magnetic field is accomplished by supercooling the conducting wires in a liquid helium (24507F) or liquid nitrogen (22327F) jacket.The supercooling drops the resistance in the wires to zero. Once the current is flowing and the cryogenic is introduced to the coils, no further electricity is required to maintain the magnetic force. Disconnecting the electrical service to an MRI lab does not affect the magnet!1

MRI scanners are usually shielded in a room constructed of heavy steel frame that surrounds the scanner and extends through the roof of the building. This steel frame prohibits outside radio frequencies from affecting the accuracy of the scanner. Typical fire department portable radios will not function in this shielded scanning room. Scan rooms must meet the MRI industry safety specifications. All MRI facilities are required to display a warning showing where magnetic hazards may be encountered. Usually, these consist of an exclusion line or signs on the walls or floor indicating where the effects of the magnetic force may be felt.


Fire department personnel responding to MRI facilities should consider several factors:

Familiarity with the MRI system, its users, and technicians and their emergency procedures will make the response safer for firefighters and MRI facility occupants. Fire departments should be aware of MRI facilities in their response districts and preplan their response. It is important to meet ahead of time with the local MRI scanner representatives to discuss operation contingencies. Twenty-four-hour emergency numbers for the MRI technologist, MRI manufacturer engineer, and MRI system physician should be available.

On entering an MRI structure, firefighters should consider the possibility of rapid flashover or backdraft conditions. MRI facilities are constructed to shield the environment from strong magnetic forces. The powerful computer equipment necessary for scanning often requires special air-conditioned rooms to operate adequately. These factors contribute to the possibility that a fire starting in an MRI building will create heavy heat and smoke conditions.

Air cylinders; axes; pike poles; and most other steel, iron, or ferrous metal objects can become deadly projectiles and must be excluded from the scanner area. The firefighters involved in this incident were very lucky to have avoided serious injury. Dr. Ray Ballinger, chief of MRI at the University of Florida/VA Medical Center, states: “A wrench released from the end of the scanner table will accelerate to about 45 to 50 mph by the time it reaches the bore of the magnet.”

If possible, extinguish the fire without bringing equipment or personnel into the scanning room. If the fire involves the scanner itself, the magnet may have to be “quenched.” The quenching process involves the release and vaporization of the cryogenic cooling medium, which will effectively stop the flow of electrical current through the coils. It will most likely seriously damage the MRI scanner, but so would a fire. If possible, an MRI engineer should quench the magnet. Fires involving the area outside the scanning room can be extinguished using conventional firefighting methods. It would be prudent to discuss your proposed firefighting strategy with the MRI professionals beforehand. Releasing cryogen may displace the air from the scanning room, potentially causing dangerous anoxic conditions for anyone in the room.

In a medical emergency, EMS responders should have an MRI technologist available to remove a patient from the scannning room. All scanning tables can be unlocked and rolled out of the scanner room. Responders should not attempt to render emergency care near the magnet, which may interfere with medical and other technical devices. Walking into the scanner room with oxygen cylinders, monitor-defibrillators, or gurneys could be disastrous for provider and victim alike.

Emergency responders with pacemakers, cerebral aneurysm clips, coronary artery stints, orthopaedic prostheses, and certain types of artificial heart valves should not work near MRI equipment. Also, those who have worked in welding, auto mechanics, or other similar work may have small metal fragments behind or embedded in the tissues of their eyes. While rare, there have been cases in which strong magnetic fields have twisted and heated these fragments, possibly damaging eye tissue. Firefighters who meet any of these criteria should avoid operations close to the magnet.

The IC should be aware that any personnel working inside the MRI scanning room will not be able to transmit or receive any radio communication. The IC should maintain communication by using runners or posting someone outside the scanning room to monitor the radio and communicate with personnel outside the building.

The strong magnetic field will interfere with electronic devices and cause digital displays to “freeze up.” It will also permanently erase any magnetically encoded information, such as that on credit cards, and render the cards useless.

MRI has become an important and useful addition to modern medical diagnosis. Scanners are rapidly being added to hospital facilities in most municipalities. MRI scanners are becoming larger and more powerful as the technology advances. It is understood that emergency responders need to be prepared for the worst-case scenario. Emergency service providers should become acquainted with the MRI facilities and staff in their community. Fire department training officers should plan drills at the MRI facility and educate the crews who will respond to these facilities about the possible scenarios they might experience. Making your personnel familiar with the hazards they can encounter will give them the knowledge and skills necessary to work safely. Do not let the “invisible forces” of MRI catch your department by surprise! n


1. GE Medical Systems Institute. Warning: Invisible forces: Magnet safety in MRI. Videotape. 1988, Sacramento, California: GE Medical Systems Service Training.

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