Evaluation of Robotized TMS: The Current System in Practice - Robotized Transcranial Magnetic Stimulation - page 52

scalp-to-coil distance [ 8 , 25 , 26 ]. In these studies separators made of plastic have
been used to change the scalp-to-coil distance. The coil was placed and held by
hand during the experiments, and steps of 5 mm were mostly used set by the
separators. The typical maximum scalp-to-cortex distance that was reached and
investigated in these studies was 10 mm.
In order to systematically investigate the discrepancy between the previous
in vivo experiments and the induced electric field measurements, we utilize the
robotized TMS system to precisely and comparably repeat the in vivo measurements.
With the robotized TMS system we are able to increase the scalp-to-coil distance
with a constant orientation and position. We measure the MT at scalp-to-coil dis-
tances spaced only at 2 mm to detect even small deviations from a linear relation. To
achieve a maximum of scalp-to-coil distance at which the motor threshold is still
determinable, again we use the 'power mode' of the stimulator (see also above).
3.2.1 TMS Recordings
Ten healthy subjects participate in this study. According to the above described
study ( Sect. 3.1 ), we stimulate the Abductor hallucis muscle (AHM) of the right foot
and estimate a hot-spot first. Subsequently, we change the scalp-to-coil distance in a
random order in steps of 2 mm starting with 0 mm distance. At each distance, we
estimate the MT with the threshold hunting algorithm (see Sect. 1.1.4.3 ) .
We employ the robotized TMS system—in its current state—to accurately
position the coil and to precisely change the coil-to-scalp distance. The active
motion compensation ensures the correct distance of coil to scalp throughout the
experiment, even though the coil has no contact to the scalp. By using the
robotized TMS system, we are able to increase the coil-to-scalp distance for large
distances. In this way, we can measure the maximum distance for each subject at
which an MT can be estimated. Thus, we can detect even small deviations from a
linear relation by maximizing the recordings.
Furthermore, we conduct this experiment with two different coils: The MCF-
B65 (MagVenture AS, Skovlunde, Denmark) is a standard figure-of-eight coil and
the MC-B70 (MagVenture AS, Skovlunde, Danmark) is a figure-of-eight coil with
a slight bent for more focal 'depth' stimulation, see also Sect. 1.1.3 .
3.2.2 Measured Motor Thresholds and Distances
The robotized TMS system enables us to measure MTs up to a maximum scalp-to-
coil distance of 24 mm with the MC-B70 coil and up to 22 mm with the MCF-B65
coil. Due to precise positioning and distance adjustment throughout the experi-
ment, we record clear threshold curves. For both coils, an exponential fit of the MT
in relation to the scalp coil distance is better than a linear fit. For the MCF-B65 coil