Intersubject variability in a comprehensive numerical assessment of operator electromagnetic exposure to TMS

TL;DR

This study assesses how operator anatomy affects electromagnetic exposure during TMS, showing that closer coil distances increase exposure and anatomical differences matter.

Contribution

The study introduces a numerical assessment of TMS operator exposure using anatomically detailed models to evaluate intersubject variability.

Findings

At 40 cm distance, all models complied with ICNIRP exposure limits.

Closer distances (12 cm) consistently exceeded occupational limits across all models.

Anatomical features like fat thickness and tissue discontinuities strongly influence field distribution.

Abstract

Transcranial magnetic stimulation (TMS) is increasingly used in clinical and research settings, often requiring the operator to remain in close proximity to the stimulation coil. While regulatory guidelines exist for patient safety, the workplace exposure assessment, particularly its dependence on operator anatomy, remains limited. This study aimed to comprehensively evaluate the electric field (E-field) induced in TMS operators under realistic working conditions, with a focus on how inter-subject anatomical variability affects compliance with international safety guidelines.

Numerical simulations were performed using four anatomically detailed virtual human models exposed to a circular TMS coil in clinically relevant positions. Two coil heights (chest and abdomen) and three coil-to-operator distances (12 cm, 22 cm, 40 cm) were analyzed. The induced E-field was computed using magneto-quasi-static solvers, and the results were compared with the ICNIRP basic restriction for occupational exposure (1.13 V/m) and with the experimental threshold for peripheral neurostimulation (4 V/m). Whole-body percentiles, localized distributions, and anatomical cross-sections were evaluated to characterize inter-subject variability.

At 40 cm, all models and exposure scenarios were compliant with ICNIRP limits. At 22 cm, most configurations remained compliant, though some models slightly exceeded the reference level, particularly in the abdominal region. At 12 cm, the induced E-field systematically exceeded the occupational limit across all models, while remaining below the neurostimulation threshold. Anatomical features, such as subcutaneous adipose tissue thickness and tissue discontinuities, were found to strongly influence field localization and intensity, especially at closer distances.

Operator exposure to TMS is influenced by both coil positioning and individual anatomical characteristics. While increasing distance ensures compliance and reduces variability, closer configurations may require tailored assessments or mitigation strategies. These findings highlight the importance of considering inter-subject variability in the evaluation of operator exposure and suggest that anatomical models may underestimate variability in real clinical contexts. A limitation of this study is the assumption of a static operator not manually holding the coil, a common condition in clinical practice that warrants further investigation.

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