The coil orientation dependency of the electric field induced by TMS for M1 and other brain areas

Abstract

BackgroundThe effectiveness of transcranial magnetic stimulation (TMS) depends highly on the coil orientation relative to the subject’s head. This implies that the direction of the induced electric field has a large effect on the efficiency of TMS. To improve future protocols, knowledge about the relationship between the coil orientation and the direction of the induced electric field on the one hand, and the head and brain anatomy on the other hand, seems crucial. Therefore, the induced electric field in the cortex as a function of the coil orientation has been examined in this study.MethodsThe effect of changing the coil orientation on the induced electric field was evaluated for fourteen cortical targets. We used a finite element model to calculate the induced electric fields for thirty-six coil orientations (10 degrees resolution) per target location. The effects on the electric field due to coil rotation, in combination with target site anatomy, have been quantified.ResultsThe results confirm that the electric field perpendicular to the anterior sulcal wall of the central sulcus is highly susceptible to coil orientation changes and has to be maximized for an optimal stimulation effect of the motor cortex. In order to obtain maximum stimulation effect in areas other than the motor cortex, the electric field perpendicular to the cortical surface in those areas has to be maximized as well. Small orientation changes (10 degrees) do not alter the induced electric field drastically.ConclusionsThe results suggest that for all cortical targets, maximizing the strength of the electric field perpendicular to the targeted cortical surface area (and inward directed) optimizes the effect of TMS. Orienting the TMS coil based on anatomical information (anatomical magnetic resonance imaging data) about the targeted brain area can improve future results. The standard coil orientations, used in cognitive and clinical neuroscience, induce (near) optimal electric fields in the subject-specific head model in most cases.Electronic supplementary materialThe online version of this article (doi:10.1186/s12984-015-0036-2) contains supplementary material, which is available to authorized users.

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“…However, this assumption is not universally held in the field. In particular, several authors have suggested that the normal component of the electric field (Fox et al, 2004; Janssen et al, 2015; Laakso et al, 2014) is the determinant of the stimulation area, not the field strength. These models differ somewhat in the prediction of stimulation areas, with absolute field strength predicting strongest stimulation at the gyral crowns (Opitz et al, 2011; Thielscher et al, 2011), where the normal component model predicting peak stimulation deeper in the sulci (Fox et al, 2004).…”

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“…However, this is not necessarily true: Recent studies have proposed that a subcomponent of the electric field that is induced by TMS can best predict stimulation outcomes in the motor cortex (Laakso et al 2014; Janssen et al 2015). This subcomponent is perpendicular to and directed into the cortical surface and, for TMS, maximally affects neurons situated in the sulcal wall.…”

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“…The RMT was defined as the minimum stimulus intensity capable of inducing MEPs > 50 μV peak-to-peak amplitude in at least 5 out of 10 consecutive trials in the estimated hot-spot of the relaxed muscle (Rossini et al, 1994, Julkunen et al, 2009). The coil orientation was kept stable (according to its orientation at the hot-spot) during the whole mapping procedure of a given body part representation and was perpendicular to the respective sulcus (Janssen et al, 2015, Raffin et al, 2015). For mapping of the tongue area, some nTMS sessions required voluntary pre-innervation to reduce excitability thresholds and thus to prevent direct nerve stimulation causing discomfort and short-latency potentials.…”