Introduction - Robotized Transcranial Magnetic Stimulation - page 16

1.3.2.3 Motion Compensation
Motion Compensation (MC) aims to keep the coil at its designated position when
the head moves. Besides keeping the coil at the stimulation target, MC also adapts
the robot trajectory when the head is moving during robot motion. At the target,
MC continuously queries the head position from the tracking system. This results
in updated robot end effector positions to keep the coil on the target and subse-
quently moves the robot. Due to computation time and system latencies, MC
cannot follow head movements instantaneously. The overall latency for robotized
TMS is roughly 200-300 ms [ 64 ]. Approximately 100 ms of this latency are due
to the robot inertia [ 62 ]. Even though it cannot compensate for fast head motion,
practical experiments have proven that it is sufficient for the purpose of TMS [ 46 ].
As presented in Chap. 2 , a systematic analysis of head motion during TMS reveals
that on average less than 5 % of the induced electric field strength is lost after
30 min of stimulation. In contrast, 32 % of the induced electric field strength is
lost on average after 30 min when not using motion compensation.
Notably, head motion is mainly spontaneous motion and is—in contrast to
cyclic breathing or cardiac motion [ 15 ]—not predictable. Therefore, motion pre-
diction cannot be used for TMS. Thus, motion compensation is the only applicable
technique.
1.4 Purpose of this Work
Robotized TMS in its present setup was already introduced in 2008 [ 46 ]. Beside
the detailed description of the system's setup and function, some basic tests were
performed showing the general functionality and behavior of the system. Never-
theless, a systematic analysis and a practical evaluation of the system is still
missing.
Therefore, we foremost systematically analyze the impact of head motion to the
accuracy of TMS, which fundamentally shows the importance of active motion
compensation for accurate TMS. During this analysis, we also present the
requirements for robotized TMS to compensate for head motion. Furthermore, we
evaluate the robotized TMS system (in its present state) during TMS studies. On
the one side, the studies and their outcomes support the special features of
robotized TMS for precise and accurate coil positioning. On the other side,
however, this practical evaluation shows the deficits of the present implementa-
tion: So far, only well-trained and experienced operators are able to purposefully
and effectively employ the robotized TMS system. Therefore, there is a need for an
easy, safe and clinically applicable system.
As primarily researchers, neuroscientists, physicians and medical staff are the
operators of TMS, we improve the robotized TMS system for