Coil design considerations for deep transcranial magnetic stimulation.

Area of Science:

• Neuroscience
• Biomedical Engineering
• Medical Physics

Background:

• Deep transcranial magnetic stimulation (dTMS) offers therapeutic potential but requires optimized coil designs for effective and safe brain targeting.
• Understanding coil characteristics is crucial for advancing dTMS applications, particularly for deeper brain structures.

Purpose of the Study:

• To investigate the field characteristics and design tradeoffs of coils intended for deep transcranial magnetic stimulation (dTMS).
• To compare the performance of novel dTMS coil designs against existing commercial coils, including FDA-approved devices.

Main Methods:

• Finite element method simulations were used to parametrically analyze two dTMS coil designs on a spherical head model.
• Simulated coils were compared with five commercial TMS coils, including two FDA-approved devices for depression treatment.

Main Results:

• Smaller coils provide better focality but increase superficial cortical stimulation and energy requirements, with diminishing focality benefits at greater depths.
• Ferromagnetic cores enhance superficial targeting efficiency but are less effective for deeper targets due to magnetic saturation.
• Coil designs like the H1 coil, which distance windings, necessitate higher stimulation energy.

Conclusions:

• The double cone coil demonstrates a favorable balance of energy efficiency, stimulated volume, and superficial field strength among commercial options.
• Stimulating targets at depths of approximately 4 cm or greater with dTMS risks exceeding safety guidelines for superficial stimulation.
• Targeting depths around 6 cm with current dTMS technology is likely unsafe due to excessive superficial stimulation and activated brain volume.