The Food and Drug Administration (FDA or the Agency), the US regulating authority in the sphere of medical devices and other healthcare products, has published a guidance document dedicated to the assessment of radiofrequency(RF)-induced heating in the magnetic resonance (MR) environment for multi-configuration passive medical devices.

The latest version of the guidance was issued in March 2016. Due to its legal nature, the document itself does not introduce new rules the parties involved shall follow but provides additional clarifications regarding the applicable legislation and recommendations to be considered by interested parties in order to ensure compliance with the respective regulatory requirements.

Moreover, an alternative approach can be applied, provided such an approach meets current requirements and has been approved by the regulating authority in advance. The document could be subject to changes reasonably necessary to reflect amendments to the underlying legislation.

Regulatory Background 

The present document describes the suggested approach to the assessment of RF-induced heating in the MR environment for both multi-component and single-component device types. The Agency pays special attention to multi-component medical devices since it becomes quite difficult to assess the RF-induced heating due to the complex design of such devices and the combination of several components with different characteristics. In such cases, numerous possible configurations should be subject to the assessment. The present document describes how such an assessment could be carried out in the most efficient manner. 

The guidance also contains references to the FDA-recognized voluntary consensus standards the medical device manufacturers may use in order to demonstrate compliance with the applicable regulatory requirements. 

The present FDA guidance describes how the manufacturer can identify the worst-case device configurations that could be subject to testing and also provides detailed clarifications regarding the methods to be used for the assessment of RF-induced device heating. The recommendations provided in the guidance could be applied to multi-configuration passive devices consisting of multiple components, as well as single-component devices, which can be used in multiple configurations. 

For the purpose of the document, a passive medical device refers to one that does not require the supply of electrical power to be used for its intended purpose. 

In general, the medical devices covered by the scope of the guidance fall into three categories: 

  • Completely implanted,
  • Partially implanted, or 
  • Used entirely externally.  

The scope of the document also covers electrically conductive multi-configuration passive medical devices. Any devices requiring an external power supply for their operations fall outside the scope of the guidance. The authority additionally mentions that the document describes the general approach for the assessment of RF-induced heating, but does not provide acceptance criteria for the particular medical devices. 

RF-induced Heating Assessment 

As mentioned, the guidance provides recommendations regarding the method to reduce the number of possible configurations subject to the assessment and how such an assessment should be carried out.

 According to the document, the medical device manufacturer as a party responsible for the product subject to review shall:

 

  1. Define and describe the MR Conditional scan conditions for the proposed labeling. This includes such parameters as the strength of a magnetic field, the Specific Absorption Rate (SAR) levels, and coil types. 
  2. Provide a scientific rationale based on benefit-risk considerations, using information from previous human trials, animal data, and/or published literature to establish the heating acceptance criterion. According to the document, the manufacturer shall refer to a 15-minute period when evaluating the temperature peak rise. 
  3. Define and describe all possible device configurations and combinations using tables, lists, and/or drawings. In particular, the manufacturer shall provide a description of all important characteristics of the device, such as size, properties of the surface, as well as the details on any and all materials used. If a wide range of configurations is possible, it is allowed to indicate the limits instead of describing each configuration possible. 
  4. Use a scientific rationale or a scientific method to reduce configurations and configurations available (CCall) to a subset of potential worst-case device configurations and combinations (CCtest) for heating assessments. According to the document, the manufacturer shall also provide a detailed description of an algorithm used to perform such a reduction. In this regard, the manufacturer shall also consider the risks associated with the RF-induced heating and harm that could be potentially caused to the patient. It is stated that the factors to be taken into consideration should include the following: device dimensions and resonant effects, device geometry, device components, device configuration, surface properties, and device construction. 
  5. The number of configurations subject to testing should be determined by the manufacturer on the basis of the overall number of configurations possible, and also the characteristics of the device and its components. 
  6. Upon finalizing the list of configurations subject to testing, the manufacturer shall conduct an assessment in order to identify the worst-case combinations for each scan condition based on the intended use of the device. According to the guidance, such an assessment could be carried out in the form of: in vitro temperature measurements according to ASTM F2182, computer modeling to determine temperature, computer modeling to determine local SAR, or a combination of the methods of the aforementioned methods. The authority additionally emphasizes that computer testing results should be subject to validation in accordance with the standardized methods, for instance, ASTM F2182. In the course of the assessment, the location of the maximum heating should be determined. 
  7. Provide an estimate of the accuracy of the results. In particular, the medical device manufacturer shall carry out an uncertainty analysis with regard to the results achieved in the course of the assessment conducted as described above. Moreover, it is also necessary to provide validation data for any and all computational models. 
  8. Should the in vitro heating identified in the course of the assessment exceed the pre-defined acceptance criterion, the manufacturer shall provide the estimates of the expected heating in order to confirm that the device could be used safely in the environment in which it is intended to be used. The authority acknowledges that the in vitro heating results could sometimes significantly exceed the actual heating. In any case, when conducting such an assessment, the manufacturer shall consider the worst-case scenarios based on the primary patient population and conditions in which the device should be used. 

In summary, the present FDA guidance provides recommendations regarding how medical device manufacturers shall assess compliance with the applicable safety requirements in terms of RF-induced heating. The document describes the methodologies to be used to reduce the scope of assessment, and also the particular way such an assessment should be performed. 

Sources:

https://www.fda.gov/regulatory-information/search-fda-guidance-documents/assessment-radiofrequency-induced-heating-magnetic-resonance-mr-environment-multi-configuration 

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