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 B1+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 B1+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 B1+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 F2182-19e2) the major shortcomings of previous versions were addressed by requiring that the relationship between ASTM test conditions and in vivo tangential E-field (Etan) 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: