|TERMINOLOGY USED FOR IMPLANTS AND DEVICES RELATIVE TO MRI TESTING|
The terminology applied to implants and devices relative to the MRI environment has evolved over the years. In 1997, the Food and Drug Administration, Center for Devices and Radiological Health, proposed definitions for the terms “MR Safe” and “MR Compatible” (1). 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 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 which 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 the device, when used in the MRI environment, has been demonstrated 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, MRI 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 (2).
Over the years, manufacturers generally used the terms “MR safe” and “MR compatible” to label medical devices. However, in time 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 importantly, 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, as follows.
MR Safe - an item that poses no known hazards in all MRI environments. Using the new terminology, “MR safe” items include non-conducting, non-metallic, non-magnetic 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), radio frequency (RF) fields, and specific absorption rate (SAR). 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 item 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, and 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. MR Unsafe items include magnetic items such as a pair of ferromagnetic scissors.
In addition to the new terms, the ASTM International document introduced corresponding icons, consistent with international standards for colors and shapes of safety signs. The icons are intended for use 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, however 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 item labeling must include results of testing sufficient to characterize the behavior of the item in the MRI environment.
Use of Terminology
The new terminology is intended help elucidate matters related to biomedical implants and devices in order to ensure the safe use of MRI technology. Importantly, it should be noted that this new terminology is not being applied retrospectively to implants and devices that previously received Food and Drug (FDA) approved labeling using the terms “MR safe” or “MR compatible” (i.e., in general, this applies to those objects tested prior to December, 2005). Accordingly, this should be understood to avoid undue confusion regarding the matter of labeling for “older” vs. “newer” implants.
(1) United States Food and Drug Administration, Center for Devices and Radiological Health, A Primer on Medical Device Interactions with Magnetic Resonance Imaging Systems. http://www.fda.gov/cdrh/ode/primerf6.html, 1997.
(2) Shellock FG. Reference Manual for Magnetic Resonance Safety Implants and Device: 2019 Edition. Biomedical Research Publishing Group, Los Angeles, CA, 2019.
(3) Shellock FG, Crues JV. Commentary: MR safety and the American College of Radiology White Paper. American Journal of Roentgenology 2002;178:1349-1352.
(4) American Society for Testing and Materials (ASTM) International, Designation: F2503. Standard Practice for Marking Medical Devices and Other Items for Safety in the Magnetic Resonance Environment. ASTM International, West Conshohocken, PA.
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