Abstract
Purpose. To characterize and quantify the induced radiofrequency (RF) electric (E)-fields and B 1+rms fields in patients undergoing magnetic resonance (MR) examinations; to provide guidance on aspects of RF heating risks for patients with and without implants; and to discuss some strengths and limitations of safety assessments in current ISO, IEC, and ASTM standards to determine the RF heating risks for patients with and without implants. Methods. Induced E-fields and B 1+rms fields during 1.5 T and 3 T MR examinations were numerically estimated for high-resolution patient models of the Virtual Population exposed to ten two-port birdcage RF coils from head to feet imaging landmarks over the full polarization space, as well as in surrogate ASTM phantoms. Results. Worst-case B 1+rms exposure greater than 3.5 μT (1.5 T) and 2 μT (3 T) must be considered for all MR examinations at the Normal Operating Mode limit. Representative induced E-field and specific absorption rate distributions under different clinical scenarios allow quick estimation of clinical factors of high and reduced exposure. B 1 shimming can cause +6 dB enhancements to E-fields along implant trajectories. The distribution and magnitude of induced E-fields in the ASTM phantom differ from clinical exposures and are not always conservative for typical implant locations. Conclusions. Field distributions in patient models are 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 19 ε2 revision of the ASTM F2182 standard, the major shortcomings of previous versions have been addressed by requiring that the relationship between ASTM test conditions and in vivo tangential E-fields be established, e.g. numerically. With this requirement, the principal methods defined in the ASTM standard for passive implants are reconciled with those of the ISO 10974 standard for active implantable medical devices.
Highlights
The potential hazards of magnetic resonance (MR) examinations have been acknowledged since the 1970s (Bottomley and Andrew 1978, Bottomley and Edelstein 1981, Bottomley et al 1985), and their extensive study has led to procedures and safety guidelines to mitigate potential adverse effects (ICNIRP 2004, IEC 2015, U.S Food and Drug Administration 2014)
Induced E-fields and B1+rms fields during 1.5 T and 3 T MR examinations were numerically estimated for high-resolution patient models of the Virtual Population exposed to ten two-port birdcage RF coils from head to feet imaging landmarks over the full polarization space, as well as in surrogate ASTM phantoms
Field distributions in patient models are condensed, visualized for quick estimation of risks, and compared to those induced in the ASTM phantom
Summary
The potential hazards of magnetic resonance (MR) examinations have been acknowledged since the 1970s (Bottomley and Andrew 1978, Bottomley and Edelstein 1981, Bottomley et al 1985), and their extensive study has led to procedures and safety guidelines to mitigate potential adverse effects (ICNIRP 2004, IEC 2015, U.S Food and Drug Administration 2014). The assessment of PIMD/AIMD compatibility with MR examinations is performed according to ASTM F2182 (ASTM 2019), which defines the RF-heating assessment of PIMDs by means of exposure in a homogeneous phantom; and ISO 10974 (ISO/ TS 2018), which describes a set of hybrid experimental/numerical approaches for AIMDs. The results may result in MR labeling indicating that implants are safe during scans at Normal Operating Mode, or requiring limitations on the SAR or B1 levels that the scanner may attain. The peak spatial SAR and whole-body averaged SAR inside different patient models have been compared (Murbach et al 2013), and the influence of different RF coil feed configurations on different anatomies has been investigated (Murbach et al 2013, Lucano et al 2018) None of these studies considered a wide range of clinical factors (patient models, imaging positions, RF transmit coil geometries, implant exposure conditions) which determine the in vivo induced RF fields. The latest edition of ASTM F2182 (ASTM 2019) includes the new section ‘Significance and Use’, recommending that implant evaluation in the ASTM phantom be supported with simulations using high-resolution patient models in realistic clinical settings—an approach that is considered essential to the methodologies described in ISO 10974
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