Threshold for Transcranial Magnetic Stimulation of the foot: precise control of coil orientation with a robotized system


Introduction: For transcranial magnetic stimulation (TMS) precise positioning of the stimulation coil is required. In clinical routine ‘standard’ coil orientations are used. Studies investigating the optimal coil orientation have been very coarse. The most detailed studies addressed the hand region, with rotation steps as large as 45°. For the stimulation of the right leg a left-to-right coil orientation is the standard orientation. We show that this orientation is not optimal.

Materials: We used a MC-B70 Butterfly coil and a MagPro X100 stimulator with MagOption (MagVenture A/S, Farum, Denmark) for stimulation. Recording of motor evoked potentials of the abductor hallucis muscle of the right foot was accomplished using a 2 channel DanTec Keypoint Portable (Alpine Biomed Aps, Skovlunde, Denmark) with surface electrodes. For placing and holding the coil precisely, we used a robotized system based on an Adept Viper s850 serial six joint robot (Adept Technology, Inc., Livermoore, CA, USA) and a Polaris infrared stereo-optical tracking system (Northern Digital Inc., Waterloo, Ontario, Canada). The motor threshold was determined with a maximum likelihood algorithm. So far, n=5 healthy male subjects, aged 24–30 years, were investigated.

First, we used a grid with a distance of 1cm for hot spot search. Then we placed the coil at the hot spot again and rotated the coil to orientations of 0°,20°,30°,40°,50°,60° and 80° where we performed the motor threshold estimation.

Results: We found that the mean lowest motor threshold was at 47° rotation (standard deviation 7.5°), rotating the coil clockwise starting from the typical left-to-right orientation. The mean difference of the motor threshold at the standard orientation to the optimal coil rotation for each subject was 7% with a SD of 5% of maximum stimulator output. In most cases the threshold varies smoothly with the orientation.

Conclusion: Robotized TMS facilitates precise coil positioning and orientation to study even small variations of the motor threshold with coil orientation. These changes might directly reflect the orientation of the biological structures influenced by TMS. Alternatively, they could be a consequence of the variations of conductivity within the target volume.

Acknowledgements: This work was partially supported by the Graduate School for Computing in Medicine and Life Sciences funded by Germany's Excellence Initiative [DFG GSC 235/1]
Original languageEnglish
JournalKlinische Neurophysiologie
Issue number01
Pages (from-to)280-280
Number of pages1
Publication statusPublished - 2011


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