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

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.

Radiofrequency induced heating of biodegradable orthopaedic screw implants during magnetic resonance imaging

“…As such, it is important to note that the phantom measurements described by the ASTM standard generally provide an overestimation of the temperature increase due to the lack of blood perfusion, wet tissue convection, and blood vessel conduction effects, all of which would be found in vivo and in clinical situations. However, previous studies [ 57 ] have shown that although the standard implicitly assumes conservative measurements, the heating responses measured in the ASTM phantom can be greatly underestimated for certain situations which should further caution users of the standardised protocol.…”

Classification Scheme of Heating Risk during MRI Scans on Patients with Orthopaedic Prostheses

“…The use of an ASTM-like phantom opens the very sensitive problem about the relation between in vitro and in vivo results [ 35 ]. Previous work showed that the RF heating of the ASTM phantom is usually conservative with respect to the anatomical model, but sometimes, it greatly underestimates the thermal effects [ 36 ]. For the GC frequencies, Arduino et al [ 34 ], analysing two anatomical models with hip, knee, and shoulder prostheses, found that a phantom with ASTM gel thermal properties always underestimated the temperature increase with respect to the in vivo results.…”

Impact of MRI RF coil design on the RF-induced heating of medical implants: fixed B 1 + rms exposure versus normal operating mode

“…Previous studies have addressed the impact of the human body and RF coil feed configurations on the RF absorption in the human body (Murbach et al 2014, Lucano et al 2018. Previous work (Yao et al 2021) also summarized the potential + B 1 rms spread under normal operating mode for different RF coils. However, none of these studies quantified the impact of RF coil geometry and design on the in vivo EM fields that medical implants are exposed to during MRI scanning, and the end-point power deposition near the implant electrode.…”