Transcranial magnetic stimulation of deep brain regions: Principles and methods

Repetitive transcranial magnetic stimulation (rTMS) is an established technique for noninvasive brain stimulation and widely used in basic and clinical neurophysiology. Yet, brain stimulation using traditional rTMS systems is limited to superficial, i.e. mainly cortical brain sites laying at the outer cerebral or cerebellar convexity and deeper structures are only modulated by transsynaptic effects primarily stimulated regions exert. Here we report recent developments in extending rTMS to deep brain regions. The Hesed coils (H-coils) are a novel development in rTMS, designed to achieve effective stimulation of deeper neuronal regions without inducing unbearable fields cortically, thus broadly expanding the potential feasibility of TMS for research and treatment of various neuropsychiatric disorders. The construction principles and design of the H-coils and phantom measurements and clinical studies are presented comparing the penetration depth of the H-coils and traditional rTMS coils. Using this approach, transcranial stimulation of subcortical white matter tracts, neurons in the mesial temporal lobe and the ventromedial prefrontal cortex together with the adjacent cingulate gyrus will become available. Moreover, the threshold for neuronal activation depends on the duration of the TMS perturbation through a strength-duration curve. Thus, it may theoretically be possible to exploit the temporal characteristics of the neuronal response, in order to improve dramatically the efficacy and focality of the stimulation of deep brain structures, potentially enabling focused stimulation of deep regions with no activation of cortical brain regions. These considerations will be of particular interest for future treatment options in affective disorders, schizophrenia and drug addiction among others.