Optimizing and assessing multichannel TMS focality

Area of Science:

• Neuroscience and Biomedical Engineering
• Computational Electromagnetics

Background:

• Multichannel transcranial magnetic stimulation (mTMS) allows electronic steering of electric fields to multiple brain targets without repositioning.
• Optimizing input currents for focal stimulation in mTMS and comparing different systems under hardware constraints remain challenging.

Purpose of the Study:

• Develop a user-centric framework to optimize and assess mTMS focality.
• Introduce a generic optimization algorithm and quality metrics for mTMS.
• Compare mTMS coil arrays against single-channel TMS (sTMS) for cortical stimulation.

Main Methods:

• Developed a fast optimization framework integrating target electric field constraints and realistic current-rate limits.
• Utilized high-resolution finite-element models of nine individual brains for E-field simulations.
• Compared two mTMS designs (5-channel planar, 6/12-channel spherical) with sTMS figure-of-eight coils using Focality, Target2Max, and OverstimulatedArea metrics.

Main Results:

• mTMS achieved focality comparable to repositioned sTMS across targets in silico.
• sTMS showed slightly better performance for superficial targets; mTMS was similar for deeper targets (>25 mm).
• Increased mTMS channels improved focality but required stronger current-rate constraints; planar design excelled for deeper targets.

Conclusions:

• mTMS systems demonstrate performance comparable to standard TMS for efficient, multi-target stimulation without repositioning.
• An open-source framework is provided for designing and evaluating mTMS systems.
• This framework supports goal-directed mTMS development and application.