Deep Transcranial Magnetic Stimulation Using Figure-of-Eight and Halo Coils

Abstract

Transcranial magnetic stimulation (TMS) is a technique for noninvasive stimulation of the human brain. Stimulation is produced by generating a brief, transient high intensity magnetic field by passing a brief, transient electric current through a magnetic coil placed upon the scalp. The induced electrical currents in the underlying cortical tissue produce a localized axonal depolarization [1-3]. TMS has become a major tool in brain research and, potentially, a promising treatment for various neurobehavioral disorders. Recently, interests in stimulating deeper cortical, subcortical and limbic areas have arisen and have become an active research topic in transcranial magnetic stimulation, because several studies show that activation of deeper prefrontal and limbic regions may increase the antidepressant effect [4-5]. Standard TMS is applied with an electromagnetic coil called a round, or figure-of-eight (Fo8) coil which induces stimulation in cortical regions mainly just superficially under the windings of the coil. These coils are not suitable for stimulating deep brain regions directly. As the fields induced by these coils decrease rapidly as a function of depth, only very high intensities would allow functional stimulation of deep brain regions and such intensities would lead to painful scalp stimulation. Furthermore, the risk of seizure is increased. These limitations have led to the development of novel coil designs suitable for deep transcranial magnetic stimulation (dTMS). The Halo-coil, a large circular coil being placed around the head was developed for dTMS. It was shown that the Halo-coil working with a typical round coil at the top of the head can increase the fields at depth in the brain [6]. The present study was to study the field characteristics of Fo8 coil which was placed at the left dorsolateral prefrontal cortex (DLPFC) working with a Halo-coil as shown in Fig. 1. The purpose of this study is to show the field penetration depth by Fo8 coil can be further increased under the help of the Halo coil.