44. Medical device lead coupling and heating induced by MRI radiofrequency

Abstract

Introduction MRI is nowadays an indispensable technique with its excellent resolution achieved without ionizing radiations. Active implantable medical devices exist (such as pacemakers) which are MRI-compatible and do not show any hazardous local heating when interacting with the MRI machine radiofrequency, but multiple devices, as when two leads are side by side, were studied [1] , [2] but are not know enough to allow the implanted patients in these cases to undergo an MRI scan. The work presented here focuses on the coupling effects and the resulting heating between two leads, by simulation and experiments. Material and methods An ASTM [3] phantom inside a radiofrequency “birdcage” antenna was modeled with the CST Microwave Studio software (CST GmbH, Darmstadt, Germany). Two simples metallic cables, isolated and 5 mm bare at one tip were inserted in the phantom, spaced with 2.5 mm. One of them was also isolated at the upper extremity (referred to as “capped”) while the other one had its two tips bare (referred to as “uncapped”). An electromagnetic simulation was made and its results were used to launch a thermal simulation. With the latter, the temperature variations versus time were collected at both cables’ tips. The same process was repeated for each cable alone. Corresponding experiment were made with a Signa HDx 1.5 T MRI machine (GE Healthcare Technologies, Milwaukee, WI), an ASTM phantom filled with gel and cables made to match the simulations. An FSE sequence was used, generating high radiofrequency heating measured by optical temperature probes. Results Simulations and experiments show a coupling between the two cables: it is indeed observed that the “capped” cable heating is halved in presence of the “uncapped” cable. It is also observed that alone, the “capped” cable heats more that the “uncapped” one, but once together the higher heating is at the “uncapped” cable’s tip (see the appendix for the curves). Conclusion This study highlights the existence of lead coupling, such as it could occur in a patient’s body. This coupling could be explained by a theoretical model of two antennas subjected to radiofrequency, including mutual inductance and capacitance between the cables. Moreover, experiments on implant leads such as pacemaker leads could allow concluding on the occurrence of the phenomenon in more concrete cases. In the end, understanding this kind of interactions will allow a evolution in the design of medical devices, for each patient to be allowed to undergo an MRI scan safely, whatever his or her implants. Acknowledgement The authors thank the Region Lorraine and FEDER for financial support. Download : Download high-res image (188KB) Download : Download full-size image