Induced radiofrequency fields in patients undergoing MR examinations: insights for risk assessment

Aiping Yao; Manuel Murbach; Tolga Goren; Earl Zastrow; Wolfgang Kainz; Niels Kuster

doi:10.1088/1361-6560/ac212d

Induced radiofrequency fields in patients undergoing MR examinations: insights for risk assessment

Phys Med Biol. 2021 Sep 15;66(18). doi: 10.1088/1361-6560/ac212d.

Authors

Aiping Yao^{1

2}, Manuel Murbach¹, Tolga Goren¹, Earl Zastrow¹, Wolfgang Kainz³, Niels Kuster^{1

2}

Affiliations

¹ IT'IS Foundation, 8004 Zurich, Switzerland.
² Swiss Federal Institute of Technology (ETH) Zurich, 8092 Zurich, Switzerland.
³ US Food and Drug Administration (FDA), Center for Devices and Radiological Health (CDRH), Silver Spring, MD 20993, United States of America.

PMID: 34433143
DOI: 10.1088/1361-6560/ac212d

Abstract

Purpose. To characterize and quantify the induced radiofrequency (RF) electric (E)-fields andB_1+rmsfields 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. InducedE-fields andB_1+rmsfields 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-caseB_1+rmsexposure greater than 3.5μT (1.5 T) and 2μT (3 T) must be considered for all MR examinations at the Normal Operating Mode limit. Representative inducedE-field and specific absorption rate distributions under different clinical scenarios allow quick estimation of clinical factors of high and reduced exposure.B₁shimming can cause +6 dB enhancements toE-fields along implant trajectories. The distribution and magnitude of inducedE-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. InducedE-fields in patient models can significantly exceed those in the surrogate ASTM phantom in some cases. In the recent 19^ε2revision of the ASTM F2182 standard, the major shortcomings of previous versions have been addressed by requiring that the relationship between ASTM test conditions andin vivotangentialE-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.

Keywords: AIMD; ASTM phantom; PIMD; incident RF fields; induced RF fields; magnetic resonance imaging (MRI); radiofrequency (RF) exposure.

Creative Commons Attribution license.

Publication types

Research Support, Non-U.S. Gov't

MeSH terms

Humans
Magnetic Resonance Imaging* / adverse effects
Magnetic Resonance Spectroscopy
Phantoms, Imaging
Radio Waves* / adverse effects
Risk Assessment