Optimal Coil Orientation for Transcranial Magnetic Stimulation

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.

[1]  H. S. Meyer,et al.  Cell Type–Specific Three-Dimensional Structure of Thalamocortical Circuits in a Column of Rat Vibrissal Cortex , 2011, Cerebral cortex.

[2]  A Pentland,et al.  Maximum likelihood estimation: The best PEST , 1980, Perception & psychophysics.

[3]  S. Boniface,et al.  Magnetic brain stimulation with a double coil: the importance of coil orientation. , 1992, Electroencephalography and clinical neurophysiology.

[4]  C. Westin,et al.  Resolving crossings in the corticospinal tract by two-tensor streamline tractography: Method and clinical assessment using fMRI , 2009, NeuroImage.

[5]  M. Hallett.  Transcranial Magnetic Stimulation: A Primer , 2007, Neuron.

[6]  Jacob Cohen.  Statistical Power Analysis for the Behavioral Sciences , 1969, The SAGE Encyclopedia of Research Design.

[7]  T Allison,et al.  Localization of functional regions of human mesial cortex by somatosensory evoked potential recording and by cortical stimulation. , 1996, Electroencephalography and clinical neurophysiology.

[8]  Alexander Opitz,et al.  Impact of the gyral geometry on the electric field induced by transcranial magnetic stimulation , 2011, NeuroImage.

[9]  P. Fox,et al.  Column‐based model of electric field excitation of cerebral cortex , 2004, Human brain mapping.

[10]  R J Ilmoniemi,et al.  Modeling of the stimulating field generation in TMS. , 1999, Electroencephalography and clinical neurophysiology. Supplement.

[11]  U. Ziemann,et al.  A practical guide to diagnostic transcranial magnetic stimulation: Report of an IFCN committee , 2012, Clinical Neurophysiology.

[12]  H. Topka,et al.  Motor thresholds in humans: a transcranial magnetic stimulation study comparing different pulse waveforms, current directions and stimulator types , 2001, Clinical Neurophysiology.

[13]  D. Ridder,et al.  rTMS for the treatment of tinnitus: The role of neuronavigation for coil positioning , 2010, Neurophysiologie Clinique/Clinical Neurophysiology.

[14]  Achim Schweikard,et al.  Planning and analyzing robotized TMS using virtual reality. , 2006, Studies in health technology and informatics.

[15]  R. Prost,et al.  Motor homunculus: passive mapping in healthy volunteers by using functional MR imaging--initial results. , 2009, Radiology.

[16]  P. Rossini,et al.  Consensus paper: Combining transcranial stimulation with neuroimaging , 2009, Brain Stimulation.

[17]  A. Schleicher,et al.  Two different areas within the primary motor cortex of man , 1996, Nature.

[18]  Jejo D. Koola,et al.  The Maximum-likelihood Strategy for Determining Transcranial Magnetic Stimulation Motor Threshold, Using Parameter Estimation by Sequential Testing Is Faster Than Conventional Methods With Similar Precision , 2004, The journal of ECT.

[19]  K. Sakai,et al.  Preferential activation of different I waves by transcranial magnetic stimulation with a figure-of-eight-shaped coil , 2006, Experimental Brain Research.

[20]  W. Paulus,et al.  Half sine, monophasic and biphasic transcranial magnetic stimulation of the human motor cortex , 2006, Clinical Neurophysiology.

[21]  Á. Pascual-Leone,et al.  Noninvasive human brain stimulation. , 2007, Annual review of biomedical engineering.

[22]  D. Avery,et al.  EEG and the Variance of Motor Evoked Potential Amplitude , 2006, Clinical EEG and neuroscience.

[23]  Satoru Miyauchi,et al.  Localizing the site of magnetic brain stimulation by functional MRI , 1998, Experimental Brain Research.

[24]  F. Awiszus.  TMS and threshold hunting. , 2003, Supplements to Clinical neurophysiology.

[25]  J. Karhu,et al.  Navigated transcranial magnetic stimulation , 2010, Neurophysiologie Clinique/Clinical Neurophysiology.

[26]  H. Alkadhi,et al.  Localization of the motor hand area to a knob on the precentral gyrus. A new landmark. , 1997, Brain : a journal of neurology.

[27]  P H Ellaway,et al.  Suppression of voluntary motor activity revealed using transcranial magnetic stimulation of the motor cortex in man. , 1994, The Journal of physiology.

[28]  B. Day,et al.  The effect of magnetic coil orientation on the latency of surface EMG and single motor unit responses in the first dorsal interosseous muscle. , 1994, Electroencephalography and clinical neurophysiology.

[29]  M. Hallett,et al.  Non-invasive differentiation of motor cortical representation of hand muscles by mapping of optimal current directions. , 1994, Electroencephalography and clinical neurophysiology.

[30]  Friedemann Awiszus,et al.  Fast estimation of transcranial magnetic stimulation motor threshold: is it safe? , 2011, Brain Stimulation.

[31]  Y. M. Kadah,et al.  Functional magnetic resonance imaging and transcranial magnetic stimulation: Effects of motor imagery, movement and coil orientation , 2005, Clinical Neurophysiology.

[32]  Daniela Balslev,et al.  Inter-individual variability in optimal current direction for transcranial magnetic stimulation of the motor cortex , 2007, Journal of Neuroscience Methods.

[33]  Lars Richter,et al.  Stimulus Intensity for Hand Held and Robotic Transcranial Magnetic Stimulation , 2013, Brain Stimulation.

[34]  Alexander Opitz,et al.  How the brain tissue shapes the electric field induced by transcranial magnetic stimulation , 2011, NeuroImage.

[35]  K. Sakai,et al.  Transcranial stimulation of the leg area of the motor cortex in humans , 1993, Acta neurologica Scandinavica.

[36]  P. Lachenbruch.  Statistical Power Analysis for the Behavioral Sciences (2nd ed.) , 1989 .

[37]  I. Kanazawa,et al.  Predominant activation of I1-waves from the leg motor area by transcranial magnetic stimulation , 2000, Brain Research.

[38]  M. Hallett,et al.  Optimal Focal Transcranial Magnetic Activation of the Human Motor Cortex: Effects of Coil Orientation, Shape of the Induced Current Pulse, and Stimulus Intensity , 1992, Journal of clinical neurophysiology : official publication of the American Electroencephalographic Society.

[39]  Franco Cauda,et al.  Discovering the somatotopic organization of the motor areas of the medial wall using low‐frequency bold fluctuations , 2011, Human brain mapping.

[40]  Alexander Schlaefer,et al.  Robust real‐time robot–world calibration for robotized transcranial magnetic stimulation , 2011, The international journal of medical robotics + computer assisted surgery : MRCAS.