MRI Safety Info
• Implants and Electronics
• Missile Effect
Magnetic Resonance Imaging (MRI) uses powerful magnets and radio waves to produce detailed images of the body. While the magnets and radio waves are inherently safe, the powerful magnetic field can attract magnetic items and adversely affect electronic equipment.
The most dangerous hazard presented by the magnetic field is the Missile Effect. The Missile Effect occurs when magnetic items are quickly pulled into the bore (center) of the MR scanner. Standard, 1.5 Tesla MR scanners can attract items such as keys, paper clips or scissors at forty miles per hour. The more mass an item has, the stronger the attractive force on it will be. Heavier items, such as wheelchairs, can be pulled in just as quickly as smaller objects. The Missile Effect will not only damage equipment, but can fatally injure someone that is located between the item and the MR scanner.
The magnetic field poses other serious hazards if proper precautions are not taken. The magnet can twist and dislocate metallic components in electronics, power tools and surgically implanted medical devices. Aneurysm clips can be pulled or twisted from their original position, causing dangerous internal bleeding. Not all metallic objects are magnetic, but it is impossible to tell if an object is magnetic by sight alone. Testing is done on every item brought into the MR Suite to confirm its safety for that particular MR environment.
The magnetic field negatively affects electromagnetic devices brought into the MR suite. This poses a potentially fatal danger to those with surgically implanted medical devices. Pacemakers, neurostimulators, insulin pumps and intrathecal pumps can malfunction inside the MR scanner room.
While not as hazardous as the Missile Effect or the disruption of medical implants, the powerful magnet can also erase information from storage devices. Some common items that contain stored information are ID badges, credit cards, cell phones and flash drives.
Magnetic Resonance Imaging (MRI) uses radio waves and powerful magnets to create detailed images of the body. Because the magnetic field presents a hazardous atmosphere around certain items, MR facilities use an item classification system so that potentially dangerous items are not brought near the MR scanner.
Potentially magnetic items must be tested before entering the controlled access areas of the MR facility. Only items that are proven to be safe near that particular MR environment are allowed inside the MR Suite. When an item is tested, it is classified as MR Safe, MR Conditional or MR Unsafe. Specific icons, usually in the form of stickers, identify the category of each item. These icons may be reproduced in black and white; however, the use of color is encouraged because of added visibility.
MR Safe items pose no known hazards in all MR environments and are indicated by a green and white “MR” icon. Conversely, MR Unsafe items, such as any magnetic item, are unsafe in all MR environments. The MR Unsafe icon features “MR” inside of a red circle with a bar through it. Any items not clearly marked should be considered MR unsafe. Finally, MR Conditional items do not pose any known hazards in a specific MR environment with specific conditions of use. The MR conditional icon consists of “MR” inside of a yellow triangle.
Never bring MR Conditional items into the MR suite without prior approval from the MR staff. Most transports and medical devices will be designated as MR Conditional, so safety within the MR suite depends on knowing what the specific conditions are for each object.
Labeling items and following the MR item classification system helps prevent dangerous situations for both staff and patients within the MR Suite.
Magnetic Resonance Imaging (MR) facilities and hospitals have specific rules and precautions for the areas surrounding the MR Suite. However, some general rules apply to all MR facilities and hospitals.
The most basic and important rule is to only bring MR-compatible equipment into the controlled access areas. All items should be tested and labeled before entering controlled access areas of the facility. The greatest type of hazard is bringing magnetic items into the MR Suite, which can be rapidly drawn into the magnet of the scanner – known as the Missile Effect.
Any hand tools or housekeeping supplies used in the magnet room must be made entirely of non-magnetic material such as brass, plastic, rubber or carbon fiber. Never bring power tools or mechanical cleaners, such as floor buffers, into the MR suite. All furniture in the controlled access areas of the MR suite must be completely free of metal, including screws or fasteners.
Surgically implanted medical devices can be negatively effected by the magnet. Pacemakers can malfunction near the MR scanner and aneurysm clips can be pulled out of place. The magnet can negatively affect any metallic fragments, bullets or shrapnel inside the body.
Minor emergencies in the MR suite can turn into more dangerous situations (situations that are more dangerous) when proper procedures are not followed. During an emergency inside the MR suite, three simple steps should be followed: 1) Evacuate the MR scanner room; 2) secure the room to prevent entry, and 3) alert a senior technologist or manager.
If the emergency involves the health of a person, such as cardiac arrest or a seizure, remove the person from the magnet room using MR-compatible equipment before administering medical treatment. Do not attempt to resuscitate or defibrillate anyone inside the MR scanner room.
If a fire occurs in the MR suite, evacuate the scanner room, secure the door, sound the alarm and call nine-one-one or your local fire department. Once the MR suite is evacuated, only properly trained personnel should attempt to contain the fire, using only MR tested and approved extinguishers. Untrained or unsupervised personnel should never enter the MR Suite. Not all firefighters or other first responders are aware of the hazardous magnetic field.
If the Missile Effect pulls an item into the scanner, a potential emergency situation may occur. Only after an object has completely settled on the magnet should a careful attempt be made to remove it. If the object can not be removed by two people without the aid of tools, an MRI service company should be called. The service company may have to ramp-down the MRI system, which slowly reduces the strength of the magnetic field.
A system quench rapidly shuts down the magnet, but is the last resort because it is both dangerous and can permanently damage the scanner. Unless it is a life-threatening situation, a quench must be authorized by the appropriate personnel. Evacuate the magnet room before a quench, or as soon as possible after the quench begins. Only trained and qualified personnel may enter the MR suite after a quench is initiated.
Pressure from released cryogen gas can force inward-swinging doors shut. If this occurs while inside the MR suite, break the glass or a window to release the pressure. Remember, the priorities in an emergency are to evacuate the MR suite, sound the alarm, secure the scanner room,and alert the proper personnel.
Cryogens are chemicals with very low boiling points used to cool the magnets on the MR scanner. Cryogens allow the magnet to remain in a superconducting state, drastically reducing the amount of power needed to control the MR scanner. The most common cryogen used in Magnetic Resonance Imaging is liquid helium. Cryogens are necessary to keep the scanner cool and working properly, but also pose physical hazards.
With temperatures hundreds of degrees below freezing, cryogens are cold enough to freeze human tissue within seconds, and pose serious cold burn and frostbite hazards. As the cryogens are released to keep the magnet cool, they evaporate into odorless, colorless and tasteless gases. Most of the gas is recaptured, but some escapes during the process. These gases are still extremely cold and are normally vented safely out of the building.
The greatest risk of exposure to cryogens is during an emergency shut down of the magnet, called a system quench. During a quench, all of the cryogen evaporates quickly, causing a loss of superconductivity in the magnet. Emergency venting systems direct the escaping cryogens through a quench pipe out of the building.
If there is a problem with the emergency venting system, or the cryogen tank malfunctions, the liquid helium could be released into the MR scanner room. The rapidly expanding helium will cause displacement of oxygen and present an asphyxiation hazard. The force of quenching can be strong enough to destroy MR equipment or the walls of the scanner room.
Unintentional quenches are extremely rare and seldom cause incidents. The most common causes of unintentional quenches are equipment malfunctions, improperly filling the cryogen tank, contaminants inside the cryostat and extreme magnetic or vibrational disturbances.
Due to the powerful magnetic field, many Magnetic Resonance Imaging (MRI) facilities and hospitals restrict access to the MR Suite by establishing four conceptual zones around the MR scanner. Each boundary zone in this four-zone safety system is defined by its purpose and distance from the MR scanner. Since the magnetic field extends in three dimensions, some zones may extend into other areas or floors of the facility.
Zone One consists of all areas freely accessible to the general public. This zone includes the entrance to the MR facility and the magnet poses no hazards in these areas.
Zone Two acts as a buffer between Zone One and the more restrictive Zone Three. Here, patients are under the general supervision of MR personnel. Normally, these areas are also safe from all magnetic field hazards. Zone Two may include the reception area, dressing room and interview room.
Access to Zone Three should be restricted by a physical barrier. Only approved MR personnel and patients that have undergone a medical questionnaire and interview are allowed inside Zone Three. The MR control room and/or computer room are located within Zone Three.
Zone Four is strictly the area within the walls of the MR scanner room, sometimes called the magnet room. Access into the MR scanner room should only be available by passing through Zone Three. Zone Four is sometimes considered to be inside of
Zone Three because it does not have a direct entrance to unrestricted areas.
Zone Three and Zone Four are sometimes collectively referred to as the MR Suite. Inside the MR Suite is an invisible boundary defined by the magnetic field’s five Gauss line. The five Gauss line is the point at which the magnetic field begins to affect electromagnetic devices, such as pacemakers.
Because the magnetic field extends in all directions, the five Gauss line can also extend to areas outside of the MR Suite, including other floors, if the magnetic field is large enough. Magnetic fields cannot be seen or felt, so the five Gauss line is sometimes marked on floors or walls for safety. Marking the five Gauss line is particularly important when it extends beyond the walls of the MR scanner room.