In situ electric field dosimetry analysis for powerline frequency peripheral nerve magnetic stimulation

Abstract

Humans are exposed to environmental 60 Hz magnetic fields (MFs), inducing in our body electric fields (EFs) and currents, potentially stimulating the peripheral nervous system (PNS). Uncertainties exist regarding the 60 Hz MF PNS stimulation threshold. The spatially extended nonlinear node model (SENN) is used to help define international MF exposure guidelines and standards protecting workers and the general public. However, other models exist, particularly the McIntyre-Richardson-Grill (MRG) model, the new gold standard for electrostimulation. This study aims (1) to model a new extremely low frequency MF exposure system for the human leg and (2) to investigate the in situ EFs generated by the system at 60 Hz at the skin level and in the nerves of the leg using a realistic human body model with both the SENN and the MRG models. A Helmholtz like-coil system was designed to generate in situ EFs sufficient for nerve stimulation, modeled using Biot-Savart and Faraday laws. Sim4Life simulations assessed the induced EFs at skin and nerve levels using a detailed human body model and two nerve excitation frameworks: the SENN and MRG models. High EF intensities were observed in four sensory and sensory-motor nerves, with MRG-derived thresholds lower than SENN-derived thresholds. Results also highlight the significance of nerve orientation in EF induction. This study emphasizes the critical role of comprehensive modeling for the design and validation of MF exposure systems and underscores the need for experimental data to refine models, standards, and guidelines.

Keywords: Dosimetry; Extremely low-frequency magnetic fields; MRG; Powerline frequency; SENN.

Fig. 1

Dimensions and configuration of the theoretical coils assembled in a Helmholtz-like configuration. The average current loop is represented by the blue circles.

Fig. 2

Surface-based whole-body model used for the simulation of the MF. (A) JEDUK cV4.0 human model from IT’IS Foundation (Zurich, Switzerland) and the Virtual Population family (10.13099/ViP11017-04–0)—Reproduced with permission from the IT’IS Foundation under a CC BY license; (B) Helmholtz-like coil system positioned at the knee/popliteal fossa level. The figure was created using Sim4Life version 6.2 (Zurich MedTech AG, Zurich, Switzerland, https://zmt.swiss/).

Fig. 3

EF maps for four views (front, back, left and right) at the skin level on the left leg during a MF stimulation by the Helmholtz-like coil system. The colors represent the level of induced EF in the leg, with white indicating an EF equal to or greater than 6.15 V/m and black equal to 0 V/m. The MF vectors are represented by arrows, the dark ones corresponding to 0 T and the lighter ones to 0.7 T. The figure was created using Sim4Life version 6.2 (Zurich MedTech AG, Zurich, Switzerland, https://zmt.swiss/).

Fig. 4

Simulated principal leg’s nerve segments, with highest in situ EF. The colors represent the level of induced EF in the leg, with white indicating an EF equal to or greater than 6.15 V/m and black equal to 0 V/m. The two yellow circles represent the middle of each coil. The figure was created using Sim4Life version 6.2 (Zurich MedTech AG, Zurich, Switzerland, https://zmt.swiss/).

Fig. 5

EF orientation in relation to nerve trajectory. The figure illustrates the spatial distribution and orientation of the electric field (E(x,y,z,f0)) in V/m, with field intensity indicated by color and arrow size. The color scale ranges from 0 to 4.54 V/m. The yellow line represents the nerve pathway through the field, with markers showing specific points along its trajectory.