MRI: New Terminology for Implants and Devices

April 20, 2006



Prior to allowing patients and other individuals into the MRI environment, careful screening determines the presence of implants or devices that may pose hazards or risks.(1) Once an implant or device is identified, screeners must examine the label for a given object and decide whether it is acceptable for the patient to undergo an MRI procedure or for an individual to enter the MRI environment. A classification system set up in 1997 created two categories: MR Safe and MR Compatible. But this resulted in confusion. A new system developed by the American Society for Testing and Materials offers three categories: MR Safe, MR Conditional, and MR Unsafe. This may provide better guidance to users.

'OLD' TERMINOLOGY

The terminology applied to implants and devices relative to the MRI environment has evolved over the years. In 1997, the FDA, Center for Devices and Radiological Health, proposed definitions for the terms

MR Safe

and

MR Compatible

.(2) These terms are defined as follows:

MR Safe:

The device, when used in the MRI environment, has been demonstrated to present no additional risk to the patient or other individual, but it may affect the quality of the diagnostic information. The MRI conditions in which the device was tested should be specified in conjunction with the term

MR Safe,

since a device that is safe under one set of conditions may not be found to be so under more extreme MRI conditions.

MR Compatible:

A device shall be considered

MR Compatible

if it is MR safe and has been demonstrated, when used in the MRI environment, to neither significantly affect the quality of the diagnostic information nor have its operations affected by the MR system. The MRI conditions in which the device was tested should be specified in conjunction with the term

MR Safe,

since a device which is safe under one set of conditions may not be found to be so under more extreme MR conditions. Using this terminology, MR safety testing of an implant or object involved assessments of magnetic field interactions, heating, and, in some cases, induced electrical currents, while MR compatibility testing required all of these as well as characterization of artifacts. In addition, it may have been necessary to evaluate the impact of various MRI conditions on the functional or operational aspects of an implant or device.

'NEW' TERMINOLOGY

Over the years, manufacturers generally used the terms

MR Safe

and

MR Compatible

to label medical devices. In time, however, it became apparent that these terms were confusing and were often used interchangeably or incorrectly.(3) Therefore, in an effort to clarify the terminology and, more important, because the misuse of these terms could result in serious accidents for patients and other individuals, the MR task group of the American Society for Testing and Materials (ASTM) International developed a new set of terms with associated icons (4). The new terms,

MR Safe, MR Conditional,

and

MR Unsafe

are defined by the ASTM document(4) as follows.

MR Safe:

An item that poses no known hazards in all MRI environments. Using the new terminology,

MR Safe

items include nonconducting, nonmetallic, nonmagnetic items such as a plastic Petri dish. An item may be determined to be

MR Safe

by providing a scientifically based rationale rather than test data.

MR Conditional:

An item that has been demonstrated to pose no known hazards in a specified MRI environment with specified conditions of use. Field conditions that define the MRI environment include static magnetic field strength, spatial gradient, dB/dt (time varying magnetic fields), radiofrequency fields, and specific absorption rate. Additional conditions, including specific configurations of the item (e.g., the routing of leads used for a neurostimulation system), may be required.For

MR Conditional

items, the labeling includes results of testing sufficient to characterize the behavior of the item in the MRI environment. In particular, testing for items that may be placed in the MRI environment should address magnetically induced displacement force and torque and RF heating. Other possible safety issues include, but are not limited to, thermal injury, induced currents/voltages, electromagnetic compatibility, neurostimulation, acoustic noise, interaction among devices, the safe functioning of the item, and the safe operation of the MR system. Any parameter that affects the safety of the item should be listed, and any condition that is known to produce an unsafe condition must be described.

MR Unsafe:

An item that is known to pose hazards in all MRI environments. This includes magnetic items such as a pair of ferromagnetic scissors.In addition to the new terms, the ASTM document introduced corresponding icons, consistent with international standards for colors and shapes of safety signs. The icons are intended to be used on items that may be brought into or near the MRI environment as well as in product labeling. The icons may be reproduced in color or in black and white, although the use of color is encouraged because of the added visibility.The

MR Safe

icon consists of the letters 'MR' in green in a white square with a green border, or the letters 'MR' in white within a green square. The

MR Conditional

icon consists of the letters 'MR' in black inside a yellow triangle with a black border. The

MR Unsafe

icon consists of the letters 'MR' in black on a white field inside a red circle with a diagonal red band. For

MR Conditional

items, the labeling must include results of testing sufficient to characterize the behavior of the item in the MRI environment. Notably, the new terminology is not being applied retrospectively to implants and devices that have already received FDA approved labeling using the terms

MR Safe

or

MR Compatible.

The use of this new terminology should serve to elucidate matters related to biomedical implants and devices and help to ensure the safe use of MRI technology.Dr. Shellock is an adjunct clinical professor of radiology and medicine at the Keck School of Medicine and director of MRI studies of Biomimetic MicroElectronic Systems (BMES) Implants, National Science Foundation BMES Engineering Research Center, University of Southern California, and Institute for Magnetic Resonance Safety, Education, and Research.
www.MRIsafety.com
www.IMRSER.org

REFERENCES

1. Shellock FG.

Reference manual for magnetic resonance safety, implants, and devices: update 2006.

Los Angeles: Biomedical Research Publishing Group, 2006.
2. U.S. Food and Drug Administration, Center for Devices and Radiological Health.

A primer on medical device interactions with magnetic resonance imaging systems.

, 1997.
3. Shellock FG, Crues JV. Commentary: MR safety and the American College of Radiology White Paper. AJR 2002;178:1349-1352.
4. American Society for Testing and Materials (ASTM) International, Designation: F2503-05.

Standard practice for marking medical devices and other items for safety in the magnetic resonance environment.

ASTM International, West Conshohocken, PA, 2005.