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Standardizing TMS Intensity Across Different Coils Using Individualized Electric Field Modeling
Evgenii Kim1,2, Mohammad Daneshzand1,2, Keren Zhu1,2
1Athinoula A. Martinos Center for Biomedical Imaging, Department of Radiology, Massachusetts General Hospital, Boston, Massachusetts, USA.
Human Brain Mapping
|May 30, 2026
Summary
Resting motor thresholds (rMTs) for Transcranial Magnetic Stimulation (TMS) vary by coil. This study shows rMT reflects a consistent cortical electric field, enabling coil-independent intensity prediction and reducing patient discomfort.
Area of Science:
- Neuroscience
- Biomedical Engineering
- Medical Physics
Background:
- Transcranial Magnetic Stimulation (TMS) requires standardized intensity for therapeutic and research consistency.
- Current methods using resting motor thresholds (rMTs) are coil-dependent, necessitating re-thresholding and introducing variability.
- A fundamental question exists whether rMT reflects a consistent cortical electric field (E-field) magnitude irrespective of coil geometry.
Purpose of the Study:
- To test the hypothesis that rMT corresponds to a coil-invariant cortical E-field magnitude.
- To evaluate a computational method for predicting TMS stimulator output across different coils using a reference rMT.
- To compare the accuracy of E-field-based prediction using detailed and simplified head models against direct scaling.
Main Methods:
- Recruited thirteen healthy participants for Transcranial Magnetic Stimulation (TMS) using two figure-of-eight coils of different sizes.
- Simulated E-field distributions using a fast multipole boundary element method within personalized MRI-based head models.
- Compared rMT prediction accuracy between detailed (five-layer) and simplified (three-layer) models against direct rMT scaling.
Main Results:
- The personalized E-field-based approach significantly improved rMT prediction accuracy over direct scaling (p < 0.001).
- Root-mean-square error (RMSE) was 1.26-1.32% of maximum stimulator output (MSO) for E-field models, versus 6.1% MSO for direct scaling.
- Individual rMT was found to correspond to a constant cortical E-field magnitude ratio across coil types.
Conclusions:
- Individual rMT reflects a consistent cortical E-field magnitude, independent of coil geometry.
- E-field-based prediction provides a more accurate, coil-independent method for standardizing TMS intensity.
- This approach reduces the need for repeated thresholding, enhancing patient comfort and study consistency.
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