Optimizing TMS dosimetry: evaluating the effective electric field as a novel metric

Area of Science:

• Neuroscience
• Biophysics
• Medical Physics

Background:

• Transcranial Magnetic Stimulation (TMS) is a non-invasive brain stimulation technique.
• Accurate numerical dosimetry is crucial for optimizing TMS efficacy and safety.
• Current dosimetry methods may not fully capture the complex interaction between the electric field and neuronal orientation.

Purpose of the Study:

• Introduce the effective electric field (E_eff) as a novel observable for TMS numerical dosimetry.
• Evaluate the correlation of E_eff with TMS-induced muscle responses (Motor Evoked Potentials - MEPs).
• Compare E_eff's predictive utility against conventional electric field measures.

Main Methods:

• Calculated E_eff using custom TMS dosimetry software, integrating anisotropic head models with white and gray matter tractography data.
• Conducted a proof-of-concept experiment with seven TMS coil rotations targeting the primary motor area in a healthy subject.
• Recorded Motor Evoked Potentials (MEPs) to assess muscle response to varying TMS parameters.

Main Results:

• The E_eff trend closely mirrored MEP responses across different coil orientations, showing a clear peak and decline.
• E_eff demonstrated a strong correlation with MEPs (r=0.8).
• Conventional measures like E-field magnitude, tangential (E_tan), and normal (E_norm) components showed weaker correlations (r<0.5).

Conclusions:

• E_eff, derived from white and gray matter tractography, inherently accounts for axonal organization and local neuronal orientation.
• E_eff is a promising observable for TMS dosimetry, showing superior correlation with physiological responses compared to traditional metrics.
• This novel approach has the potential to significantly enhance the precision and predictability of TMS applications.