Optimizing and assessing multichannel TMS focality

Area of Science:

• Neuroscience
• Biomedical Engineering
• Medical Physics

Background:

• Multichannel transcranial magnetic stimulation (mTMS) allows electronic steering of electric fields, overcoming limitations of single-channel TMS (sTMS).
• Optimizing input currents for focal stimulation in mTMS remains a challenge.
• Systematic comparisons of different mTMS designs under realistic constraints are lacking.

Purpose of the Study:

• To develop a user-centric framework for optimizing and evaluating mTMS focality.
• To introduce a generic optimization algorithm and meaningful focality metrics.
• To compare mTMS coil arrays against traditional sTMS for various cortical targets.

Main Methods:

• Developed a fast optimization framework with E-field constraints and current-rate limits.
• Utilized high-resolution finite-element models of nine individual brains.
• Compared 5-channel planar and 6/12-channel spherical mTMS systems with figure-of-eight sTMS coils.
• Quantified performance using Focality, Target2Max, and OverstimulatedArea metrics.

Main Results:

• mTMS achieved focality comparable to repositioned sTMS for all targets.
• sTMS showed slightly better focality for superficial targets.
• mTMS performed similarly for deeper cortical targets (>25mm depth).
• Increased channels improved focality but required stronger current-rate constraints.
• Planar mTMS design excelled for deeper targets; spherical designs improved with more channels.

Conclusions:

• mTMS systems offer performance comparable to sTMS, enabling efficient multi-target stimulation without coil repositioning.
• The developed open-source framework aids in designing and evaluating mTMS systems.
• This supports goal-directed mTMS development and application.