Comparison of the induced fields using different coil configurations during deep transcranial magnetic stimulation

Abstract

Stimulation of deeper brain structures by transcranial magnetic stimulation (TMS) plays a role in the study of reward and motivation mechanisms, which may be beneficial in the treatment of several neurological and psychiatric disorders. However, electric field distributions induced in the brain by deep transcranial magnetic stimulation (dTMS) are still unknown. In this paper, the double cone coil, H-coil and Halo-circular assembly (HCA) coil which have been proposed for dTMS have been numerically designed. The distributions of magnetic flux density, induced electric field in an anatomically based realistic head model by applying the dTMS coils were numerically calculated by the impedance method. Results were compared with that of standard figure-of-eight (Fo8) coil. Simulation results show that double cone, H- and HCA coils have significantly deep field penetration compared to the conventional Fo8 coil, at the expense of induced higher and wider spread electrical fields in superficial cortical regions. Double cone and HCA coils have better ability to stimulate deep brain subregions compared to that of the H-coil. In the mean time, both double cone and HCA coils increase risk for optical nerve excitation. Our results suggest although the dTMS coils offer new tool with potential for both research and clinical applications for psychiatric and neurological disorders associated with dysfunctions of deep brain regions, the selection of the most suitable coil settings for a specific clinical application should be based on a balanced evaluation between stimulation depth and focality.

An Update on Repetitive Transcranial Magnetic Stimulation for the Treatment of Major Depressive Disorder

“…Various types of stimulating coils have been designed, each of which produce different magnetic field patterns and achieve different depths of the cortex. A circular coil (round coil) was the original design, but although the induced electric field was quite broad, it was not very deep . Other types of coils have been designed to generate more focal and deeper stimulation .…”

“…Most rTMS depression studies have used flat figure‐of‐eight‐shaped coils (a.k.a. butterfly coil) that produce relatively focused but also relatively superficial cortical stimulation, inducing the current in an estimated area of 2 cm 2 and extending ~ 1–4 cm into the cortex . The figure‐of‐eight coil can be angled to conform to the patient's head shape, potentially providing more focused and deeper stimulation.…”

“…The figure‐of‐eight coil can be angled to conform to the patient's head shape, potentially providing more focused and deeper stimulation. The double‐cone coil has the same figure‐of‐eight coil design, but its format with a more closed angle has enabled stimulation of deeper brain regions without losing focality …”

“…The size and geometry of the coil determine the depth, direction, and focality of the electromagnetic field that will penetrate the scalp and skull to activate cortical neurons during rTMS. 17 Various types of stimulating coils have been designed, each of which produce different magnetic field patterns and achieve different depths of the cortex. A circular coil (round coil) was the original design, but although the induced electric field was quite broad, it was not very deep.…”

“…17 Other types of coils have been designed to generate more focal and deeper stimulation. 17 Most rTMS depression studies have used flat figure-of-eight-shaped coils (a.k.a. butterfly coil) that produce relatively focused but also relatively superficial cortical stimulation, inducing the current in an estimated area of 2 cm 2 and extending ~ 1-4 cm into the cortex.…”