title: 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 ([Formula: see text]). 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.
authors.affiliation: Institute for Robotics and Cognitive Systems, University of Lübeck, Lübeck, Germany. richter@rob.uni-luebeck.de
authors.forename: Lars
authors.initials: L
authors.lastname: Richter
authors.forename: Gunnar
authors.initials: G
authors.lastname: Neumann
authors.forename: Stephen
authors.initials: S
authors.lastname: Oung
authors.forename: Achim
authors.initials: A
authors.lastname: Schweikard
authors.forename: Peter
authors.initials: P
authors.lastname: Trillenberg
mesh_terms: D019054:Evoked Potentials, Motor / Q000502:physiology*; D005528:Foot / Q000502:physiology*; D006801:Humans; D008279:Magnetic Resonance Imaging; D050781:Transcranial Magnetic Stimulation / Q000295:instrumentation* / Q000379:methods*