Explaining RF induced current patterns on implantable medical devices during MRI using the transfer matrix

Abstract

In this work a simulation study is performed to gain insights in the patterns of induced radiofrequency (RF) currents for various implant-like structures at 1.5 T. The previously introduced transfer matrix (TM) is used to determine why certain current patterns have a tendency to naturally occur. This can benefit current safety assessment techniques and may enable the identification of critical exposure conditions. Theory and Methods: The induced current on an elongated implant can be determined by multiplication of the incident electric field along the implant with its TM. The eigenmode spectrum of the TMs for various lengths and various types of implants are determined. The eigenvector with the highest eigenvalue describes the incident electric field pattern that induces the highest current which in turn will lead to highest heating. Subsequently, a statistical probability analysis is performed using a wide range of potential incident electric field distributions in a representative human subject model during a 1.5 T MR exam which are determined by means of electromagnetic FDTD simulations. These incident electric field distributions and the resulting induced current patterns are projected onto eigenvectors of the TM to determine which eigenmodes of the implant dominate the current patterns. Results: The eigenvectors of the TM of bare and insulated wires resemble sinusoidal harmonics of a string fixed at both ends similar to the natural-current distribution on thin antennas(1). The currents on implants shorter than 20 cm are generally dominated by the first harmonic (similar to half a sine wave). This is firstly because for these implant lengths (relative to the RF wavelength), the first eigenvalue is more than three times bigger than the second showing the ability of an implant to accommodate one eigenmode better than another. Secondly, the incident electric fields have a high likelihood (≳95,7%) to project predominantly on this first eigenmode. Conclusion: The eigenmode spectrum of the TM of an implant provides insight into the expected shape of induced current distributions and worst-case exposure conditions. For short implants, the first eigenvector is dominant. In addition, realistic incident electric field distributions project more heavily on this eigenvector. Both effects together cause significant currents to always resemble the dominant eigenmode of the TM for short implants at 1.

Rapid safety assessment and mitigation of radiofrequency induced implant heating using small root mean square sensors and the sensor matrix Qs

“…All results were achieved with simple mock implants. The use of simplified models or mock implants is, however, common practice in the literature on implant safety in MRI 25,28,32,33,37,78‐83 . It is undoubtedly possible to equip real‐life implants with sensors, eg, with interventional devices this is a well‐known common practice 50,84,85 .…”

“…The use of simplified models or mock implants is, however, common practice in the literature on implant safety in MRI. 25,28,32,33,37,[78][79][80][81][82][83] It is undoubtedly possible to equip reallife implants with sensors, eg, with interventional devices this is a well-known common practice. 50,84,85 The sensor approach is designed for wire-like implants and requires a priori knowledge of the hot spot location.…”

New Insights into MR Safety for Implantable Medical Devices

“…With such a standing wave formation, the dissipation of the electrical power at the tip of a conductor becomes larger than in the instance without a standing wave. 56 The extent of the increase is based on the difference in the impedance between the tissue and the IMD material. When the impedance of the IMD material is more conductive than the surrounding tissue, the concentration of the electric current, and thus the standing wave amplitude, becomes larger, resulting in higher SAR concentration induced by electric charge dissipation at the boundary between the material and tissue.…”