What This Article Covers
In this article, we will break down the key components involved in interpreting implant conditions for MRI, including both foundational concepts and real-world clinical application:
- Understanding ASTM implant labeling and what each classification means in practice
- How to interpret MRI implant conditions and translate IFU requirements into clinical decisions
- Common challenges in implant condition interpretation, including inconsistent or ambiguous labeling
- Considerations for off-label scanning and how it fits into risk/benefit decision-making
- A practical framework for evaluating implants and supporting clinical escalation when needed
MRI safety is no longer limited to determining whether a device is labeled MR Safe, MR Conditional, or MR Unsafe. When scanning patients with implants, the challenge extends far beyond identifying the ASTM classification alone. MR teams must also interpret increasingly complex and inconsistently formatted device conditions, determine whether those conditions can realistically be met within the clinical environment, and assess when a case should be escalated for a formal risk/benefit evaluation.
Understanding ASTM Labeling
Before we can perform any of these tasks, we first have to understand what ASTM labeling means and which terms are not applicable to implants or devices. In a document published by the FDA on October 10, 2023, titled Testing and Labeling Medical Devices for Safety in the Magnetic Resonance (MR) Environment, the following ASTM labels are defined:
- MR Conditional: “a medical device with demonstrated safety in the MR environment within defined conditions including conditions for the static magnetic field, the time-varying gradient magnetic fields, and the radiofrequency fields”
- MR Safe: “a medical device that poses no known hazards resulting from exposure to any MR environment. MR Safe medical devices are composed of materials that are electrically nonconductive, nonmetallic, and nonmagnetic.”
- MR Unsafe: “a medical device which poses unacceptable risks to the patient, medical staff, or other persons within the MR environment.”
These are the terms we should use when discussing implanted devices or objects entering the MR environment. The term “MR Compatible” is outdated, and we should refrain from using it. Using the correct terminology removes ambiguity and helps ensure the device or implant is handled appropriately within the MR environment. If we are labeling a device as MR Safe, we need to be certain it poses no known hazards from any of the magnetic, gradient, or RF fields present in MRI.
Building a Framework
Now that we understand ASTM labeling, we can begin to build a framework for assessing implants or devices that need to be introduced into the MR environment. For implants, operative notes, implant cards, and medical records can help identify the device. In most cases, you will need at least one, or a combination, of the following: manufacturer, model number, serial/lot number, implant location, and date implanted. For devices, you’ll either need the IFU containing the ASTM labeling, or you can test the device for ferromagnetic properties using a 1000 gauss handheld magnet. This will help determine if the device will become a projectile when introduced into the MR environment. From there, if the device is electrically conductive, additional considerations should be made prior to allowing entry into the bore.
Once you’ve identified the exact implanted device, you’ll want to locate the instructions for use (IFU) for an MRI. Assessing these conditions is where things can get a bit tricky as there is no standardized framework for manufacturers creating implant conditions. Each one may present differently, but most will include the following: static magnetic field strength, SAR/B1+rms limits, gradient restrictions, coil restrictions, scan duration, landmarking/isocenter restrictions, wait periods post-implantation, device programming requirements, and monitoring requirements.
Common Challenges
When assessing the conditions for introducing an implanted device into the MR environment, I see some questions arise more frequently than others. “I don’t see a wait time post-implantation. How long do I wait?” I saw David Bishop once compare this to stopping on the road without a stop sign. If there isn’t one, you don’t stop. In that same conversation, Kris Seitz said, “Don’t chase conditions that don’t exist. Don’t make things more difficult than they already are.” Sage advice from both David and Kris that I believe is worth repeating (over and over, if needed). So, if you don’t see a wait time post-implantation, then there likely isn’t one.
The next question I see frequently is interpretation of scan duration. “Does x time limit mean per pulse sequence? Or total scan time?” This condition can be especially confusing, because manufacturers describe it in many different ways. You may see terminology such as continuous scan time, active scan time, total scan time, RF exposure time, or cumulative scan duration. Continuous scan duration typically refers to the amount of time allowed per pulse sequence before a cool down period is required. Active scan time generally refers only to the period during which RF energy is actively being transmitted. Total scan time usually refers to the cumulative duration of all performed pulse sequences combined.
Some manufacturers provide very clear guidance in this area by listing allowable active RF exposure time per sequence followed by a required cool down period. Others leave significantly more room for interpretation.
Additional challenges arise with implants, particularly active implanted devices, where conditions change depending on implant location and the anatomy being imaged. Coil restrictions can also become unnecessarily complicated depending on how they are presented within the IFU. Even RF limitations may vary considerably between manufacturers. One IFU may specify a SAR limit of 2 W/kg while another simply states Normal Operating Mode.
Thinking Beyond the Implant Conditions
An important thought to keep in mind when assessing these implants is that they are not tested to failure; they are tested for approval to go to market. Any scanning performed outside the IFU is considered “off-label,” but off-label does not automatically mean unsafe.
This is where understanding the three energies in MRI, coil selection, MRI physics, and potential device interactions becomes critically important. When you can assess each of those variables while also interpreting the implant conditions, you are often able to present a thoughtful and well-supported case to the radiologist or MRMD for off-label scanning consideration. And because implants are not tested under every conceivable clinical scenario, this becomes an imperative skillset to possess.
As MR professionals, it is crucial that we approach these situations through the lens of risk mitigation rather than rigid limitation alone.
When we understand the physics, understand the conditions, and understand the potential interactions, we place ourselves in a position to advocate for our patients while still maintaining safety as the highest priority.