Evaluation of Robotized TMS: The Current System in Practice - Robotized Transcranial Magnetic Stimulation - page 54

sites for comparable stimulation results. The setup of the robotized system also
allows to position the coil around the head in any coil orientation. As shown with
our systematic analysis of head motion in Chap. 2 , the robotized TMS neglects
head

motion during stimulation to achieve high accuracy throughout the

stimulation.
However, while performing these studies, major deficits and problems with the
system have occurred which make it hardly applicable for the broad clinical
application in its present state:

• Time consuming calibration step:
A tracking system tracks the position of the head during application. To use the
head positions for the robot to move the TMS coil, a calibration between
tracking system and robot is mandatory (cf. Sect. 1.3.2.2 ) . Therefore, the TMS
coil at the robot's end effector is substituted for a marker. The robot's end
effector is now moved to a set of different poses and the marker poses are
measured by the tracking system. Based on this data the calibration matrix is
computed. After calibration, the marker is detached from the end effector and
the TMS coil is remounted. Typically, this calibration step requires roughly
10 m of setup time before the TMS session can start. As the tracking system
and/or the robot might have been moved, this calibration step must be performed
before each TMS session. If the calibration result is poor, due to inappropriate
end effector positions, the calibration step must be re-performed, which requires
additional time.
Assuming a TMS study with 20 patients or subjects that are stimulated on five
consecutive days, roughly 16 h of the operator's time are required just for
system setup.
• Trajectory planning and target accessibility:
In order to achieve a maximum of patient safety with the current system, the
permitted robot trajectories are strongly restricted. Any potentially dangerous
trajectory from the current robot pose (coil pose) to the coil target position on
the head is prohibited by the software control. In many cases, a manual robot
pre-positioning is therefore required. As this must be done with the robot
controller, it can only be achieved with experienced robot operators.
During our experiments with the robotized TMS system, for instance, 2 stim-
ulation sessions were postponed as the operator on that day was not able to
achieve a robot pre-positioning that allows for a safe trajectory to the stimula-
tion target. Additionally, for approximately 20 stimulation sessions the pre-
positioning for that stimulation has required more than 10 m.
• Coil positioning on the head:
For coil placement on the patient's head, the coil is first positioned roughly
10 mm above the target. Subsequently, the coil is moved on the head in steps of
1 mm until the patient confirms the coil on the head. This positioning approach
requires therefore the feedback of the patients and often results in suboptimal
coil positioning. Some patients wait until the coil strongly touches the head
before they confirm the coil on the head. This results in a heavy force on the