Evaluation of Robotized TMS: The Current System in Practice - Robotized Transcranial Magnetic Stimulation - page 51

For each session we have estimated a distinct hot-spot in reference coil ori-
entation. With the coil rotation the best stimulation site might slightly shift. If this
shift has any effect on the MT it would result in a decrease of the MT for coil
rotations different to the reference orientation. Due to time constraints during
stimulation, we have not been able to additionally perform a hot-spot search for
each coil rotation. However, our results are universally valid as an upper limit.
Although the optimal coil orientation in our recordings differs between subjects,
the inter-individual variability is essentially smaller than reported by Balslev et al.
for the hand region. In our study the variability is 30 whereas Balslev et al. have
reported a variability of 63 [ 4 ]. In contrast to their study, we use precise coil
orientations in small steps. Balslev at al. used the Principle Component Analysis
(PCA) to compute optimal coil orientations for each subject based on recordings
with coarse rotation steps of 45 . Even though most of their results are convincing,
their PCA results for their subject 11 in session 2 are questionable as the optimal
orientation calculated by PCA was 88 : 5 whereas a local maximum has been at
90 . Therefore, we question the larger inter-individual variability (63 ) in optimal
current direction presented in that study. In contrast, we assume a smaller inter-
individual variability of roughly 30 which is supported by our measurements.
For clinical diagnosis and routine, the coil is commonly held by hand and
neuro-navigation is typically not used. Therefore, a slight posterior rotation of the
coil will be sufficient.
However, for TMS applications in (cognitive) neuroscience, brain research and
even treatments, the findings of this study should be taken into account to achieve
optimal and stable TMS results. E.g., for treatment of chronic tinnitus the coil is
commonly orientated vertically (superior-to-inferior) to target the Primary Audi-
tory Cortex (PAC) [ 13 ]. Applying the simulations [ 30 ] and our results, a horizontal
coil orientation (posterior-to-anterior) would be rather beneficial to induce currents
perpendicular to the gyrus crowns of PAC.
It needs further investigations to find out whether standard coil orientations for
other regions, e.g., 45 for the hand [ 6 ], are also not optimal. The robotized TMS
system is a powerful and sufficient tool for this purpose as it can rotate the coil
very precisely and in small steps while keeping the coil in a tangential orientation
to the head.

3.2 Coil-to-Scalp/Cortex Distance 2

Analytical computations have shown that the induced currents decrease quasi-
exponentially with depth [ 5 ]. Recordings with realistic phantoms performed by
TMS coil manufacturers confirm this decrease [ 14 , 15 ]. In contrast, recent in vivo
experiments have reported a linear decrease of motor threshold with increasing

2

Parts of this section have been already presented in [ 7 , 31 ].