Induced Radiofrequency Fields in Patients Undergoing MR Examinations: Insights for Risk Assessment

Aiping Yao, Manuel Murbach, Tolga Goren, Earl Zastrow, Wolfgang Kainz, and Niels Kuster, Physics in Medicine and Biology, Volume 66, Article Number 185014, September 2021, online 26 August 2021; doi: 10.1088/1361-6560/ac212d

The aims of this study were to: (i) characterize and quantify the induced radiofrequency (RF) electric (E-) fields and B_1+rms fields in patients undergoing magnetic resonance imaging (MRI) examinations; (ii) provide guidance on aspects of RF heating risks for patients with and without implants; and (iii) discuss some strengths and limitations of safety assessments in current standards of the International Organization for Standardization (ISO), the International Electrotechnical Commission (IEC), and the American Society for Testing and Materials (ASTM) for assessment of the RF heating risks in patients with and without implants. Induced E-fields and B_1+rms fields during 1.5T and 3T MRI examinations were estimated numerically in Fats and Thelonious, two of the high-resolution anatomical models of the Virtual Population (ViP), exposed to 10 two-port birdcage RF coils at imaging landmarks from head to foot over the full polarization space, as well as in surrogate ASTM phantoms. Worst-case B_1+rms exposure greater than 3.5 µT (1.5T) and 2 µT (3T) must be considered for all MRI examinations at the normal operating mode limit. Representative induced E-field and specific absorption rate (SAR) distributions under different clinical scenarios allow quick estimation of clinical factors of high and reduced exposure. B₁ shimming can cause enhancements of +6 dB to E-fields along implant trajectories. The distribution and magnitude of E-fields induced in the ASTM phantom differ from those detected during clinical exposures and are not always conservative for typical implant locations. Field distributions in patient models were condensed, visualized for quick estimation of risks, and compared to those induced in the ASTM phantom. Induced E-fields in patient models can significantly exceed those in the surrogate ASTM phantom in some cases. In the recent revision of the ASTM F2182 standard (ASTM F2182-19^e2) the major shortcomings of previous versions were addressed by requiring that the relationship between ASTM test conditions and in vivo tangential E-field (E_tan) be established, e.g., numerically. With this requirement, the principal methods defined in the ASTM standard for passive implantable medical devices are reconciled with those of the ISO10974 standard for active implantable medical devices.

The scientific and technical impact of the study can be summarized as:

• Visualization of the E-field distributions in the Fats and Thelonious ViP models and comparison to those determined in the ASTM phantom indicate that in vivo E_tan based on the ASTM phantom may be underestimated by a factor of about 3, and the local SAR/average E-field (E_avg) by a factor of about 7
• Maximum B_1+rms exposure and the magnitude of E_tan along representative splines plotted as a function of imaging landmark, either in circular polarizing (CP) mode or with shimming, in both ViP models in the normal operating mode indicate that such plots of maximum E-fields, B₁₊ fields, and typical E_tan performed for small and large patients can help practitioners understand exposure scenarios for the purpose of risk assessment
• The significance of the ASTM F2182-19^e2 standard, requiring that the relationship between ASTM test conditions and in vivo E_tan be established, is demonstrated through a comparison of worst-case E-fields numerically determined in ViP models to the results found in the ASTM phantom

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