Imaging Solutions practices the system of MRI terminology developed in 2005 by the American Society for Testing and Materials (ASTM). This adoption came about in an effort to clarify the terminology and, more important, because misuse of old terminology could result in serious accidents for patients and other individuals. This new system references the MRI environment where testing took place, which includes the location inside the static magnetic field and its magnitude, spatial gradients, time-varying magnetic fields, radio frequency (RF) magnetic field pulses and specific absorption rates (SAR).
Taking these conditions into consideration, three new categories were developed: MR Safe, MR Conditional, and MR Unsafe. In this system, “safe” and “unsafe” are the two extremes. “Conditional” is in the middle and characterizes the behavior of a medical device under specific conditions in the MRI environment as determined by testing. 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 new terminology and icon structure is intended to help clarify matters related to biomedical implants and devices to ensure the safe use of MRI technology.
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. Importantly, for “MR Conditional” items, the item’s labeling must include the parameters and results used for testing that are sufficient to characterize the behavior of the item in the MRI environment (13).
Further details and a comprehensive discussion of the labeling applied to passive implants are presented in the recent FDA document, Guidance for Industry and FDA Staff – Establishing Safety and Compatibility of Passive Implants in the Magnetic Resonance (MR) Environment (11).
MR Safe – an item that poses no known hazards in all MRI environments. Using the terminology, “MR Safe” items are non-conducting, non-metallic, and 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 MR environment with specified conditions of use. “Field” conditions that define the MR environment include static magnetic field strength, spatial gradient magnetic field, dB/dt (time rate of change of the magnetic field), 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 MR environment. In particular, testing for items that may be placed in the MR 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.