Optimal Coil Orientation for Transcranial Magnetic Stimulation

Abstract

We study the impact of coil orientation on the motor threshold (MT) and present an optimal coil orientation for stimulation of the foot. The result can be compared to results of models that predict this orientation from electrodynamic properties of the media in the skull and from orientations of cells, respectively. We used a robotized TMS system for precise coil placement and recorded motor-evoked potentials with surface electrodes on the abductor hallucis muscle of the right foot in 8 healthy control subjects. First, we performed a hot-spot search in standard (lateral) orientation and then rotated the coil in steps of 10° or 20°. At each step we estimated the MT. For navigated stimulation and for correlation with the underlying anatomy a structural MRI scan was obtained. Optimal coil orientation was 33.1±18.3° anteriorly in relation to the standard lateral orientation. In this orientation the threshold was 54±18% in units of maximum stimulator output. There was a significant difference of 8.0±5.9% between the MTs at optimal and at standard orientation. The optimal coil orientations were significantly correlated with the direction perpendicular to the postcentral gyrus (). Robotized TMS facilitates sufficiently precise coil positioning and orientation to study even small variations of the MT with coil orientation. The deviations from standard orientation are more closely matched by models based on field propagation in media than by models based on orientations of pyramidal cells.

Impact of non-brain anatomy and coil orientation on inter- and intra-subject variability in TMS at midline

“…A significant portion of subjects do not follow the group average trend. This heterogeneity can be supported by previous recordings of lower-limb motor neuron activity by Di Lizzaro et al, which has shown specific individuals to have both much greater motor thresholds for the parallel orientation (145% perpendicular case) and much greater thresholds for the perpendicular orientation (127% the parallel case) (Di Lazzaro et al, 2001) (similar dissociations seen in Richter et al (Richter et al, 2013)), highlighting that although these group trends are important, they are not a hard rule for all subjects.…”

Spatial localization and distribution of the TMS-related ‘hotspot’ of the tibialis anterior muscle representation in the healthy and post-stroke motor cortex

“…Additionally, our reported measurements do not include variability in coil orientation. Current evidence suggests that adjustments in the coil orientation may induce variations in motor thresholds [24,27]. It is also important to note that we provide a rather simplified model of reporting our results in the XY plane and have not taken into consideration measurements in the Z plane.…”

An integrated framework for targeting functional networks via transcranial magnetic stimulation

“…Efforts to stimulate brain regions with externally observable responses readily demonstrate the importance of both coil location and orientation when attempting to maximize the response to stimulation (Balslev et al, 2007; Brasil-Neto et al, 1992; Opitz et al, 2013; Richter et al, 2013). For example, when stimulating motor cortices, the optimal coil orientation for generation of a motor evoked potential (MEP) can vary markedly from one individual to the next (Balslev et al, 2007; Opitz et al, 2013; Richter et al, 2013), even if the coil is properly located above a target area (e.g., hand knob).…”

“…For example, when stimulating motor cortices, the optimal coil orientation for generation of a motor evoked potential (MEP) can vary markedly from one individual to the next (Balslev et al, 2007; Opitz et al, 2013; Richter et al, 2013), even if the coil is properly located above a target area (e.g., hand knob). Unfortunately, when attempting to stimulate higher order multimodal association areas, such as dorsolateral prefrontal cortex (DLPFC), the challenges are two-fold.…”