MR Safety Basics

Three Fields, Three Kinds of Risk

An MRI scanner uses three electromagnetic fields. Each can interact with people, implants, and objects in a different way.

Sources: 1, 2, 3

The Four MR Safety Zones

The ACR divides an MRI facility into four zones, with access becoming stricter closer to the scanner.

Tip The area around external cryogen vents is a separate Cryogen Venting Zone that also needs signage and controlled access.

Sources: 1

Who Can Enter: MR Personnel Levels

The ACR sorts people into three groups by their MR safety training. The group sets how they may move through Zones III and IV.

Tip Each facility's MR Medical Director (MRMD) defines the training for these levels and appoints the MR Safety Officer (MRSO). The levels are facility roles, separate from professional certification.

Sources: 1

Screening Before Zones III and IV

• Every patient, visitor, and staff member who is not MR Personnel must pass MR safety screening before entering Zone III. It includes a written form and a verbal interview.
• Conscious patients who are not emergencies are screened twice, at least once by Level 2 MR Personnel.
• MR Personnel are screened when they start and reviewed every year. They must report changes, such as a new implant, before returning to the MR environment.
• The ACR recommends checking for ferromagnetic objects by direct inspection and with a ferromagnetic detection system (FMDS) before Zones III and IV. Conventional metal detectors, which cannot tell ferromagnetic metal from other metals, are not recommended.

An FMDS supports thorough screening and does not replace it. Implanted and on-body devices, whether MR Conditional or MR Unsafe, may contain ferromagnetic parts, including batteries, that set off the detector. The ACR offers a sample screening form on its MR Safety webpage.

Sources: 1

Emergencies: Key Principles

Every MR facility should have written emergency procedures and practice them in regular drills. The ACR describes these principles.

• Assume the magnet is on. In a fire or other emergency, the magnetic field is present and fully operational. Air tanks, axes, conventional fire extinguishers, and other ordinary equipment must stay out of Zones III and IV.
• Control access. Emergency calls from the MR facility should also reach a designated member of the MR staff, who keeps responders from entering Zones III and IV freely.
• Move the patient out for resuscitation. In a cardiac arrest or other medical emergency in Zone IV, the patient is moved at once to a predetermined, magnetically safe location for life support. MR Unsafe items such as defibrillators are not brought into Zone IV.
• Quench only for danger that cannot wait. The Emergency Magnet Off (quench) is for imminent risk of injury or death in Zone IV when the person cannot be removed safely, such as entrapment against the scanner by a ferromagnetic object. A medical emergency alone is not a reason to quench.
• Know what each switch does. Emergency Table Stop and Emergency Power Off do not turn off the magnetic field. A quench does, but not instantly, and it has its own hazards, such as cryogen gas in the room if venting fails.

These are principles, not a procedure. In an emergency, follow your facility's emergency procedures.

Sources: 1

Device Labels: MR Safe, MR Conditional, MR Unsafe

• MR Safe: the object or device is safe in all MR environments. It must be nonmetallic, nonconducting, and nonmagnetic.
• MR Conditional: the object or device may be safe in the MR environment if its conditions for safe use are met. Every condition in its labeling must be followed.
• MR Unsafe: the object or device presents safety risks in the MR environment.
• Safety in MRI Not Evaluated: FDA labeling for some passive devices that have not been tested in the MR environment. Their labeling states that safety in the MR environment is unknown.

The FDA advises that any device with an unknown MRI safety status should be assumed to be MR Unsafe until its status is established. For implanted systems, such as a pulse generator with leads, the ACR notes that "if any element of the system is not MR Conditional, the entire system is not MR Conditional."

Sources: 1, 2, 3, 4

Measuring RF Exposure: SAR and B1+rms

• SAR (specific absorption rate) estimates the rate of RF energy absorbed per unit mass of tissue, in watts per kilogram (W/kg). The scanner estimates it with vendor-specific models that can vary widely.
• Operating modes set SAR limits. Normal Operating Mode limits whole-body SAR to less than 2 W/kg. First Level Controlled Operating Mode allows less than 4 W/kg and requires medical supervision of the patient.
• B1+rms describes the transmitted RF field itself, in microtesla (µT). Because it is not a vendor-specific estimate, it is increasingly used as an MR condition for scanning implants.

The operating mode does not prevent burns from focal heating, for example from incorrect patient positioning or a conductor placed on the patient. SAR and B1+rms are different measures, and one cannot simply be converted to the other. Follow the measure that the device labeling specifies.

Sources: 1

The 9-Gauss Line

The static field extends beyond the scanner room, including to floors above and below. The ACR uses the 9-gauss line, consistent with the current IEC standard, to mark the area of static-field hazard for people with certain active implants. Older guidance used 5 gauss.

Sources: 1