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Developing the right MRI safety assessment strategy

Medical devices manufacturers are regularly asked about the safety of their products when introduced into an MRI scanner (Magnetic Resonance Imaging).

These questions usually come from competent authorities, notified bodies or medical teams. This is the subject of a regulatory strengthening and forms part of a growing number of standards (ISO 14630, ISO 14708, EN 45502, etc.).

The placing on the market of an implantable device is now impossible in some countries without prior information on the product safety during an MRI exam.

Therefore it becomes a major issue for MD manufacturers, especially with regard to implantable Medical Devices.

  

 

Clinical applications of MRI are expanding. This exam tends to replace other imaging techniques, in particular for the detection of cancer. In 2013, on average 55 inhabitants out of 1000 underwent an MRI, against 28 inhabitants 10 years ago.

France was behind concerning the number of annual examinations but is now part of the leading pack with 90.9 examinations per 1,000 inhabitants in 2013 against 32 in 2006 (source: OECD).

MRI and Medical Devices

To acquire an MRI image, some electromagnetic fields are needed: a static magnetic field, some magnetic field variations (called "gradients") and some radiofrequency waves. The intensity of each of these waves can be extremely high. During the exam the protons in the water molecules composing the human tissues are excited by the radiofrequency waves. When they return to their equilibrium, these will reissue waves whose capture enables the image building.

Any conductive material introduced into the MRI environment will interact with these electromagnetic waves. The intensity and nature of these interactions depend on multiple parameters (material, design, positioning in the MRI, settings of MRI, etc.) and multiple accidents due to these interactions are recorded.

The risk associated with the attraction of ferromagnetic materials by the magnetic field is the easier to understand.

Other risks comprise torque phenomena (tendency of the device to rotate in a magnetic field), a temperature rise, vibrations and some induced currents. The latter can induce malfunctioning or destruction of the electronic parts, and even induce unintended stimulation of the organs.

Finally the presence of the device may trouble the MRI images by generating some "artifacts" that will prevent the radiologist from completing their diagnosis.

The responsibility falls on the manufacturer of the medical device to analyze and evaluate the risks associated with the introduction of the device in an MRI environment. The current changes in notified bodies constitute an opportunity for part of the manufacturers to incorporate this concept in their files.

Strategy to be adopted for assessing the MRI-related risks

A campaign on assessment of MRI-related risks aims to mark the device and specify the conditions ensuring patient’s safety during an MRI exam in its users’ instructions. This marking is usually based on ASTM F2503, which defines three categories:

  • MR safe if the device can be inserted into any type of MRI without any risk. This category is usually reserved for devices consisting solely of non-conductive materials.
  • MR Unsafe if the device generates risk when introduced in MRI.
  • MR Conditional: a category that defines the conditions under which the device can be introduced without risk.

To achieve this marking, the manufacturer must rely on technical data usually derived from MRI safety and compatibility testing. The Food and Drug Administration (FDA), the leading authority in this field, published in December 2014 a guide to clarify its position: "Establishing Safety and Compatibility of Passive Implants in the Magnetic Resonance (MR) Environment". This guide is dedicated to passive implants and asks manufacturers to focus on the phenomena of attraction, torque, temperature rise due to radiofrequencies and artefacts. Performing tests according to ASTM standard is recommended in most cases.

Among its activities, Healtis assists clients in developing its risk assessment strategy to MRI. We adopt an approach based on the identification of potential interactions between MRI and the device before determining whether these interactions can cause a risk and to propose a method of assessing risk (testing or writing a scientific argument).

It's a bespoke approach, based on the most detailed texts (ISO / TS10974) which allows taking into account the specificities of the product, sometimes reducing the costs of the study and above all avoiding certain hazards to be omitted.

Training employees

Manufacturers are faced with various difficulties during the process of assessing the MRI safety of their products: analysing the risks, elaborating an evaluation strategy of MRI risks, choosing a test laboratory, monitoring the tests, understanding and interpreting their results, drafting the users' instructions and then responding to the authorities.

They can be assisted by technical experts but should then be able to understand their remarks or even assess their relevance. I recommend the companies to provide technical and regulatory training to their staff in order to acquire the necessary knowledge to discuss with their subcontractors and support the marketing authorization file.

For an active implants manufacturers, ensuring and assessing the MRI safety is a complex and important issue, and justifies the creation of a dedicated team.

Contribution of computer modeling to MRI compatibility studies

Each MRI safety test is performed on a sample of the product. When the device is available in several variants the test must be performed on the worst case. The rationale for this worst-case is a key point of the testing campaign.

It is usually easy to define a worst-case with regard to the interactions with the magnetic field and the magnetic field gradients (attraction, torque, vibration, etc.). Phenomena related to absorption of radiofrequencies, especially the temperature rise, are almost impossible to predict.

prothesis modeling medical deviceThe computer modeling approach is generally used when the device features a complex shapes, is made of different materials, has several different sizes or is composed of different parts. Computer modeling can predict the radiofrequency energy absorption by the device, which is linked to the temperature rise.

This technique can be used to determine or confirm test conditions. For example, it is often used to locate the hottest spots, the temperature of which will be monitored during the test, or search the position and orientation in the MRI that will maximize the heating effect.

Computer modeling is also used to find the worst case for multi-configuration passive medical devices. For example, a range of knee prosthesis consists of a femoral stem, a tibial tray, a femoral condyle, an insert and sometimes other elements to fit all clinical situations.

Each of these elements is available in various sizes and sometimes in different materials while the various ranges are sometimes compatible with one another. There may be several billion possible configurations and it is impossible to determine through an empirical approach the case that will be subjected to the most significant temperature rise.

In such situations, computer modeling enables to find the worst-case by using a parametric approach. This study consists in determining the changing parameters between two configurations and to study the influence of each parameter on the energy absorbed by the prosthesis.

This approach is now described in the draft of the FDA guide « Assessment of Radiofrequency-Induced Heating in the Magnetic Resonance (MR) Environment for Multi-Configuration Passive Medical Devices. »

Getting started with MRI compatibility

The subject is vast not so simple. For the passive implants manufacturers, the FDA guidance can be a good start: "Establishing Safety and Compatibility of Passive Implants in the Magnetic Resonance (MR) Environment". You can also save valuable time by contacting directly a test laboratory, such as Healtis.

Author

Yannick Ponvianne
CEO of Healtis
info@healtis.com

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