Impact of simultaneous exposure to RF and gradient electromagnetic fields on implant MR safety labeling

Abstract

Purpose: To investigate whether heating contributions produced by radiofrequency (RF) and gradient fields superpose sufficiently at the worst-case locations to justify their simultaneous consideration in magnetic resonance imaging (MRI) implant safety labeling.

Theory and methods: Six implant models were positioned in an ASTM phantom and realistically implanted in two anatomical human models, and exposed to gradient and RF fields at 64 MHz and 128 MHz. The simulations with the anatomical body models considered different axial exposure landmarks inside the RF and gradient body coils. The exposures were scaled to represent two sets of scenarios: either limited by the implant's MR conditional labeling to a fixed peak temperature rise, or representing an EPI or TrueFISP examination with clinically relevant parameters, where the implant label is not limiting.

Results: The temperature enhancement due to the combined RF and gradient sources, evaluated with respect to the maximum values obtained separately, depends on the implant, pulse sequence, and exposure landmark. A maximum relative enhancement of about 65% was found in the ASTM phantom, and maximum absolute enhancements above 0.3 K were found in anatomical models with realistic pulse sequences.

Conclusion: There are clinically relevant MR examination scenarios where the maximum heating contributions produced by RF and gradient fields combine, enhancing the local peak temperature increase beyond that obtained from either assessment alone. The results prove to be useful for defining safety margins on the maximum allowable temperature increase, avoiding the requirement of a combined gradient coil and RF test.

Keywords: MRI safety; gradient heating; implant safety; radiofrequency heating.

FIGURE 1

Range of exposure landmarks for different body/implant configurations. The figure depicts the extreme body positions within which the body models were moved in 7 cm steps. The pink transparent box and the black quoted outline represent the RF and GC, respectively (A) Glenn with the hip implant (B) Glenn with the knee implant (C) YoonSun with the shoulder implant (D) Glenn with the ankle plate (E) Glenn with the cranial plate (F) Glenn with the can of the active implant.

FIGURE 2

Peak temperature increase, in kelvin, on the six implants exposed to different MR pulse sequences as a function of multiple axial landmarks. Results refer to 1.5 T exposure to the RF EM field alone (red line), GC EM field alone (green line), and simultaneous application of RF and GC EM fields (blue line). The gray area highlights the enhancement of the peak temperature increase due to the simultaneous exposure to RF and GC EM fields with respect to the maximum between the peak temperature increase due to RF or GC alone.

FIGURE 3

Peak temperature increase, in kelvin, on the six implants exposed to different MR pulse sequences as a function of multiple axial landmarks. Results refer to 3 T exposure to the RF EM field alone (red line), GC EM field alone (green line), and simultaneous application of RF and GC EM fields (blue line). The gray area highlights the enhancement of the peak temperature increase due to the simultaneous exposure to RF and GC EM fields with respect to the maximum between the peak temperature increase due to RF or GC alone.