FT-Control - Robotized Transcranial Magnetic Stimulation - page 93

Hence, we analyze the positioning time needed for a hot-spot search by moving
the coil to a grid of stimulation points to find the optimal stimulation site. Com-
monly, about 8-10 target points are used for hot-spot search in a standard TMS
application (cf. [ 9 ]or Sect. 1.2 ) . We compare the time needed using hand-assisted
positioning with using the robotized TMS system. To this end, we use a standard
figure-of-eight coil (MCF-B65). The hot-spot search is performed using a head
phantom. Note that, recording of MEPs is not required as the pure positioning time
is of interest.
We therefore investigate three different setups:
• For the first setup, the coil has the required orientation and is closely placed to
the target (roughly 10 cm).
• For the second setup, the coil is positioned approximately 25 cm away from the
target with a slight rotation (roughly 15 ). This setup is chosen such that there is
still a safe robot trajectory from starting point to target.
• In contrast, we place the coil for setup 3 such that there is no possible safe
trajectory from starting point to target for the robotized TMS system. In this
case, a manual robot pre-positioning is required. Therefore, we position the coil
approximately 50 cm away from the target with a rotation larger than 90 .
For all setups, we primarily conduct a fast hot-spot search using hand-assisted
positioning. Therefore, we move the coil to nine distinct stimulation positions in
the target region. We assess and store a random MEP to each stimulation point and
virtually display the MEP on the head surface in the TMS software. After the hot-
spot search is completed with the hand-assisted positioning method, we move the
coil back to its starting point. Subsequently, we perform the hot-spot search again
using the same points with the control software of the robotized TMS system
(without using FT control).
We measure the duration for positioning the coil from starting point to the ninth
stimulation point for each setup. For each scenario, an experienced user performs
five runs.
To evaluate the time needed for a manual robot pre-positioning, we ask an
inexperienced user to perform the pre-positioning for setup three. The user only
gets a short introduction into the robotized system. We measure the time for three
runs of setup three. For comparison, the inexperienced user also conducts a coil
positioning for setup three using hand-assisted positioning.
5.2.4.3 Latency of Contact Pressure Control
The latency of the pure contact pressure control cycle is of interest as it not only
shows the time needed to respond to head motion, but also indicates the time
needed until the robot is stopped by the software in case of an error or collision.
Furthermore, we are also highly interested in the maximum update frequency. This
sample rate refers to the ability to detect even fast and short-term impacts to the
coil.