Evaluation of Robotized TMS: The Current System in Practice - Robotized Transcranial Magnetic Stimulation - page 43

Chapter 3
Evaluation of Robotized TMS:
The Current System in Practice

We evaluate the current robotized Transcranial Magnetic Stimulation (TMS)
system in practice in two brain research scenarios. For these studies, we take
advantage of the robotized TMS system for accurate coil positioning. We inves-
tigate the influence of coil orientation on the stimulation outcome for the stimu-
lation of the foot. Further, we study the impact of changes in the scalp-to-cortex
distance on the MEP amplitude and therefore on the stimulation intensity. These
studies show that robotized TMS is a powerful tool for brain research as it allows
for very precise coil positioning and rotating in small steps. Without robotized
TMS these studies are hardly possible with the same accuracy, repeatability and
comparability. However, these studies also show deficits of the current robotized
TMS system allowing only well-trained and experienced operators to effectively
employ the robotized TMS system.

3.1 Optimal Coil Orientation for TMS of the Lower Limb 1

For a figure-8 coil, the largest current density is obtained directly below the center of
the coil. Thus, when ignoring inhomogeneities of the conductivity of the tissue, the
position of the pyramidal cells that control a given muscle is indicated by the center
of the coil, if the threshold is minimal with respect to surrounding coil positions. In
addition to the coil position, the coil orientation also influences thresholds and
amplitudes in TMS. In clinical routine, brain research, and experimental treatments
with repetitive TMS, this is considered by recommending standard orientations [ 11 ],
such as posterior-lateral for the hand muscles [ 9 , 12 , 16 ] and perpendicular to the
interhemispheric cleft ( ¼ lateral) for foot muscles [ 28 ] (cf. Fig. 3.1 a).
For stimulation of the leg motor area, Terao et al. [ 27 ] have further investigated
the MEP intensities and latencies for different coil orientations with 45 steps.
They have reconfirmed that a lateral coil orientation was best as it produced the
highest MEP amplitudes and shortest latencies [ 27 ]. The recordings of the Motor

1

Parts of this section have been already presented in [ 22 , 32 ].