Biomedical Engineering Reference
relation to the 3D head in real-time. Also, coil position and orientation of stim-
ulation points can be stored in the software.
In this procedure it is essential for accurate tracking that both, head marker and
coil marker, do not shift after registration.
1.3 Robotized TMS: Combining Neuro-Navigation
As holding the coil by hand for a stimulation sequence of up to 30 min is an
exhausting task, commonly a rigid holder or mechanical arm retains the TMS coil
after positioning [ 12 ]. In this way, the coil stably maintains its position during
stimulation. However, the stimulation point will not necessarily be stable over
time as the patient's head may move. The easiest and most used way is to ask the
patient to keep the head as still as possible while maintaining contact to the coil.
Another solution is using a head resting frame (chin rest) where the patient puts the
chin in a mold and presses the forehead against a frame [ 12 , 60 ]. The aim of such a
frame is additional head stabilization [ 81 ]. Obviously, a rigid head fixation, like in
radiation therapy, would bring head motion to a minimum [ 86 ]. However, it cannot
generally be used for TMS as it leads to serious discomfort for the patient and
results in stress and increased excitability.
Therefore, coil-handling devices must be improved [ 43 ]. Robotized systems for
TMS are combining the benefits of neuro-navigation with automation and are on
the rise for exact stimulation [ 32 , 40 ]. For robotized TMS, the magnetic coil is
placed directly on top of the patient's head by a robot [ 46 ]. With permanent head
position tracing at any time, the target position is known. As the shape of the head
is known from 3D images, the robot positions the TMS coil automatically at the
stimulation site in an orientation tangential to the cranium [ 47 ]. Once the target
point is reached, Motion Compensation (MC) is activated. This compensates
changes in the position of the stimulation point with appropriate robot movements
to keep high positioning accuracy during treatment, as first suggested in [ 76 , 77 ].
With robotic TMS systems an overall TMS coil positioning accuracy with a
positioning error smaller than 2 mm is achievable [ 46 , 63 ].
Currently, different engineering approaches for development of a robotic TMS
system exist: Either a specialized but limited application-orientated robot is
designed or a common flexible design is used and adapted to the TMS specifications.
1.3.1 Specialized Setup
In [ 42 , 61 , 89 ] a custom-built approach for a TMS robot is presented. The c-shaped
robot, with its non-standard kinematics, provides coil placement around the upper
half of the patient's head. It consists of three subsystems having in total seven