Novel Mechanistic Model and Computational Approximation for Electromagnetic Safety Evaluations of Electrically Short Implants

Ilaria Liorni, Esra Neufeld, Sven Kuhn, Manuel Murbach, Earl Zastrow, Wolfgang Kainz, and Niels Kuster, Physics in Medicine and Biology, accepted manuscript online 18 October 2018

In this paper, we address unresolved issues related to the safety of persons with conductive medical implants exposed to electromagnetic (EM) fields. Implants exposed to EM fields that are in compliance with currently accepted reference limits – particularly at frequencies <100 MHz – may enhance local fields in and energy absorption by tissues to values that are much higher than the basic restrictions currently considered safe. A mechanistic model based on transfer functions that was postulated for elongated active implants at magnetic resonance imaging (MRI) frequencies, has been used as a basis for standards that concern MRI implant safety. However, this mechanistic model is inconsistent with the behavior observed for electrically short implants, such as abandoned leads in MRI or active or passive implants under low-frequency exposure conditions, e.g., of wireless power transfer (WPT) technologies. In this paper, a new mechanistic model for electrically short implants is proposed that allows implant safety assessment to be decomposed into separate steps. For each tip shape, the model requires only a single simulation or measurement of the implant exposed under homogeneous or semi-homogeneous conditions. To validate the approach, predictions made with the mechanistic model were compared to the results of numerical simulations for electric- and magnetic-field exposures. The impact of parameters such as tissue properties, length, tip shape, and insulation thickness on safety- and compliance-relevant quantities was studied. Validation involving an anatomically detailed computational human body model with a realistic implant model at multiple locations under electric and magnetic exposures resulted in prediction agreement on the order of 7% (maximal deviation <15%). The approach was found to be applicable to electrical lengths up to 20% of the effective wavelength and can be used to derive suitable testing procedures as well as to develop safety guidelines and standards.

The scientific and technical impact of the study can be summarized as:

• We have developed a quantitative, generalized mechanistic model for electrically short implants to predict local EM energy deposition and field enhancements
• For insulated or partially insulated electrically short leads, the power deposited at the tip depends mainly on the voltage difference (electrical exposures) or the electromotive force (magnetic exposures) between the tips and is a simple function of the local tissue resistivities at the tips, only weakly dependent on tip shape; the insulation thickness is relevant only for short implants at frequencies high enough for capacitive coupling through the insulation to matter
• The simple steps used to apply the model are described – characterization of tip-related resistance and field distribution through simple simulations of homogeneous exposure in a homogeneous environment followed by prediction of local exposure metrics by combining these characterizing simulations with the precomputable in vivo incident field conditions in the absence of the implant and information about the tissues surrounding critical exposed implant locations; a third step, the use of the previous steps to identify potential worst-case lead trajectories, can provide valuable safety information, e.g., for regulatory purposes
• The main application of this new general model is to allow direct assessment of risk and/or compliance with the basic restrictions for patients with implants in the near field of strong low-frequency sources

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