Introduction - Robotized Transcranial Magnetic Stimulation - page 9

4 (Nextstim Oy, Helsinki, Finland). Even though these systems differ in the
implementation and in some features, the general setup is identical. The overall
accuracy of these systems is roughly in the range of 5-6 mm [ 73 ]. The main
principles of neuro-navigated TMS are briefly introduced:
1.2.1 Head Registration and Tracking
Commonly, stereo-optic infrared tracking systems are used for tracking. As direct
head tracking is not possible with these systems, a marker, which is visible for the
tracking device, is attached to the head. Either the marker is integrated in a
headband or clipped to spectacles (eyeframe). As the head must be tracked, instead
of the marker, a registration to the 3D head scan is essential. To this end, a 3D
contour of the head is computed (Fig. 1.4 a). Now, a pointer with a marker is used
to record distinct anatomical landmarks on the patient's head. Typically, the
nasion and the corner of the eyes are taken. The landmark positions are then
recorded in relation to the head marker. The same points are selected in the virtual
3D head contour. Using a landmark based registration, the head marker can now be
registered to the virtual head. Figure 1.4 b shows the recorded landmarks and the
selected landmarks in the contour after the registration. It is important that at least
three distinct landmarks are available for such a registration. To improve the
accuracy of the registration, more than three landmarks can be used. Commonly, a
set of additional surface points is recorded with the pointer on the patient's head
(Fig. 1.4 c). Using an iterative registration, e.g., the Iterative Closest Point (ICP)
Fig. 1.4 Head registration for navigated (and robotized) TMS. a First, a 3D head contour is
generated—commonly from (f)MRI-scans. b Second, anatomical landmarks are recorded on the
patient's head (brighter spheres) and registered to selected landmarks in the contour (darker
spheres). c To improve the registration, surface points on the head are recorded (lines of small
dots) and an iterative surface registration is performed