RF-Induced Heating of Implanted Medical Devices in MRI Systems

This sponsored article is brought to you by COMSOL.

Over 80 million magnetic resonance imaging (MRI) scans are conducted worldwide every year. MRI systems come in many different shapes and sizes, and are identified by their magnetic field strength. These scanners can range from below 0.55 tesla (T) to 3 T and beyond, where tesla is the unit for the static magnetic field strength. For patients with implanted metallic medical devices, the strong magnetic fields generated by MRI systems can pose several safety concerns.

For instance, high-powered magnets generate forces and torques that can cause the implant to migrate and potentially harm the patient. In addition, the gradient coils in MRI systems, used for spatial localization, can cause gradient-induced heating, vibrations, stimulation of the tissue, and device malfunction. Lastly, the large radiofrequency (RF) coil in MRI systems can cause the electrically conductive implant to electromagnetically resonate (called the antenna effect"), resulting in RF-induced heating that can potentially burn the patient (Ref. 1).

MED Institute, a full-service contract research organization (CRO) for the medical device industry, is using multiphysics simulation to better understand the effects of RF-induced heating of medically implanted devices for patients that need MRI scans (Ref. 2).

Standardized Test Methods for Medical Devices

MED Institute provides support throughout the entire product development cycle. Its MRI Safety team helps manufacturers evaluate and perform physical testing of their medical devices for safety and compliance in the MRI environment (Figure 1). The team works closely with the Food and Drug Administration (FDA), which oversees the development of medical products to ensure safe and effective use. Furthermore, the team complies with the standards of the American Society for Testing and Materials (ASTM) and International Organization for Standardization (ISO). Specifically, it follows the ASTM F2182 standard to measure RF-induced heating of a medical implant within a gel phantom (Figure 2) and follows ISO/TS 10974 to evaluate electrically active implantable medical devices (AIMD) during MRI.

The gel phantom used for testing is a rectangular acrylic container filled with a conductive gel that approximates the thermal and electrical properties of average human tissue (Ref. 3). The phantom is placed on the patient table inside the RF coil of an MRI scanner and fiber optic temperature probes (1 mm in diameter) are attached to the device before submerging it into the gel. The probes measure the temperature changes experienced by the device during the MRI scan. This type of physical experiment is used often, but it poses some potential problems. For instance, movement within the phantom can introduce uncertainty into the experiment, and inaccurate probe placement can lead to invalid results. In addition, depending on the materials of construction and their magnetic susceptibility, magnetic force could also be an issue (Ref. 4).

To help address these issues, the team at MED Institute uses computational modeling and simulation as an alternative to physical testing. David Gross, PhD, PE, Director of MRI Safety Evaluations and Engineering Simulations, leads a team of analysts that use simulation to gain a better understanding of physics-based problems. He says, The simulation provides us with 3D temperature contours anywhere within a volume of interest; we are not limited to discrete point-probe measurements, and we do not have to worry about the inaccuracies of the equipment or uncertainty of probe placement from the experiment."

The team has experience conducting these simulations for closed-bore MRI systems, in which a patient is contained in a compact tube. The team is now using simulation to perform these same analyses for open-bore systems (Figure 3), which have wider physical access, making them beneficial for imaging pediatric, bariatric, geriatric and claustrophobic patients", as is explained on the MED Institute website (Ref. 5).

Multiphysics Simulation for RF-Induced Heating

With COMSOL Multiphysics, MED Institute is able to evaluate the RF-induced temperature rise of implants and compare the results of various sizes and constructs of a device within a product family to determine a worst-case configuration. The analysts at MED can import a CAD file of a client's device using the CAD Import Module, an add-on to COMSOL Multiphysics. In terms of RF-induced heating, the team uses the RF Module and Heat Transfer Module add-on products to combine the physics of electromagnetics with transient heat transfer. For analyzing electromagnetics, the RF Module enables the use of Maxwell's equations to solve for the wave equation at every point within the model that is impacted by electromagnetic fields. This is done in a steady-state frequency domain, which is then sequentially coupled to the transient heat transfer. With the Heat Transfer Module, the team is also able to solve heat conduction equations.

In the example below, MED Institute imported a CAD file of a knee implant into the COMSOL Multiphysics software. The geometry of the implant included a stem extension, tibial tray, femoral tray, and other components. All of these components can have various sizes and can be assembled in various ways, and patients with the implant can be scanned in various MRI systems that create different electromagnetic fields. With the overwhelming amount of permutations that these variables can produce, it is often not clear which configuration would result in the worst-case RF-induced heating.

With our Medical Device Development Tool (MDDT), we can not only augment physical testing but even replace it with simulation in some cases. The immediate, positive results are that our clients are able to have their products evaluated quicker and at less cost because we are able to rely on the simulation."
-David Gross, MED Institute Director of MRI Safety Evaluations and Engineering Simulations

This is where the use of simulation comes in; you focus your efforts on the primary factors that can change the resonance of a particular implant," Gross says. By using the COMSOL software, the organization is able to better understand the relative bounds of where it would expect to see resonance and how the device behaves under different electromagnetic fields. This helps with performing sensitivity analyses, where the team can test what causes the change in resonance, such as modifying the diameter of the stem or other components of the implant. For this particular case, the team ran hundreds of simulations to determine the worst-case device size and worst-case RF frequency.

Using worst-case analysis is crucial in the verification process because it allows manufacturers to test different factors for a wide range of devices - such as determining which size brings the most complications - rather than conducting physical testing for every variant of one product (Ref. 6). Performing multiple physical experiments becomes very expensive and time-consuming, especially when you account for the hourly cost of using a physical MRI scanner," says Gross.

As shown in Figure 4, the electric field in the gel phantom of a 1.2 T open-bore system (upper left) is very different from a 1.5 T closed-bore system (upper right). The knee implant was simulated in both systems, where the results show a different resonance and maximum temperature rise at the end of the stem (lower images).

Using COMSOL allowed the team to better understand how a device behaves under electromagnetic fields. With these results, the team was then able to determine where they should place temperature probes while physically testing the device in an actual MRI system to obtain temperature rise results.

FDA Qualification of MED Institute's Virtual MRI Safety Evaluations

MED Institute's experience with using simulation to test RF-induced heating of medical devices has inspired development of a promising new simulation tool that accelerates the product development cycle. The MED Institute team submitted this simulation tool to the FDA's Medical Device Development Tool (MDDT) program, which allows the FDA to evaluate new tools with the purpose of furthering medical products and studies. As stated on the FDA website, The MDDT program is a way for the FDA to qualify tools that medical device sponsors can choose to use in the development and evaluation of medical devices." (Ref. 7) Once qualified, the FDA recognizes the tool as an official MDDT.

In November 2021, MED Institute was granted FDA qualification of its MDDT, Virtual MRI Safety Evaluations of Medical Devices". This is an evaluation process that involves using multiphysics modeling and simulation to test the interactions of medical devices in an MRI environment. The tool is used for modeling an RF coil of an MRI system, ASTM gel phantom, and a medical device placed within the gel. Simulation is then used to analyze the electromagnetics and the heat that generates around the device (Ref. 8).

After testing is complete, the labeling of the device is described by ASTM 2503 or, if it is an electrically active implant, by the ISO 10974 test. The labeling is placed on the device packaging and inside the instructions for use (IFU) so that an MRI technologist or radiologist can see the relevant information for a patient with an implanted device.

With our MDDT, we can not only augment physical testing but even replace it with simulation in some cases," says Gross.

Modeling and Simulation Support from the FDA

Over the years, MED Institute has evaluated many medical devices for MRI safety with COMSOL Multiphysics simulations. It has found that COMSOL is a powerful and efficient platform for solving complex multiphysics problems. The immediate, positive results are that our clients are able to have their products evaluated quicker and at less cost because we are able to rely on the simulation. It does not require them to send us the actual product to test for RF-induced heating," says Gross.

The FDA has been supportive of computational modeling and is willing to evaluate and accept data from simulation in lieu of physical testing. It is important for medical device sponsors to know that they have the encouragement and support of the Agency," Gross says. MED Institute has had the privilege of working alongside the FDA for many years for the benefit of patients. It goes to show that they are invested and believe in the power of modeling and simulation," Gross adds.

References

1. Thermal Injuries," Questions and Answers in MRI; https://mri-q.com/rf-burns.html#:~:text=Antenna%20effect.&text=Antennas-produce-standing-wave-patterns,likely-to-create-heating-problems
2. D. Gross, Top 10 Challenges for MRI Safety Evaluation," MED Institute Inc., June 2020; https://medinstitute.com/blog/top-10-challenges-for-mri-safety-evaluation/
3. Medical Device MRI Safety Testing," MED Institute Inc., April 2016; https://medinstitute.com/blog/medical-device-mri-safety-testing-where-should-a-hip-implant-be-placed-in-an-astm-f2182-test-to-measure-the-maximum-rf-induced-heating/
4. Keynote: RF-Induced Heating of Medical Devices in Open-Bore MRI," COMSOL; https://www.comsol.com/video/keynote-rf-induced-heating-of-medical-devices-in-open-bore-mri
5. Radiofrequency-Induced Heating in Open Bore MRI," MED Institute Inc., Aug. 2020; https://medinstitute.com/download/radiofrequency-induced-heating-in-open-bore-mri/
6. The Worst-case Scenario," Packaging Compliance Labs; https://pkgcompliance.com/the-worst-case-scenario/
7. Medical Device Development Tools (MDDT)," U.S. Food and Drug Administration; https://www.fda.gov/medical-devices/science-and-research-medical-devices/medical-device-development-tools-mddt
8. MDDT Summary of Evidence and Basis of Qualification Decision for Virtual MRI Safety Evaluations of Medical Devices," Apr. 2021; https://www.fda.gov/media/154181/download