Biomedical Engineering Reference
method [ 6 , 11 ], the surface points can now be matched to the virtual head contour.
The general scheme of head registration for TMS can be found in [ 53 ]. Note that
for Fig. 1.4 , the Voreen, 1 an open source software package, is used to render and
segment the head. Furthermore, the ICP method included in the PCL 2
is used for
registration [ 9 ].
1.2.2 Coil Tracking
Beside tracking the head, the coil must be also tracked to guide the user to the
stimulation target. This also requires a visible marker attached to the coil. The
marker must be registered to the coil. Different approaches exist to perform this
registration. A geometrical approach can be used: By using a pointer, the origin of
the coil, the x- and the y-axis are recorded in relation to the marker (Fig. 1.5 a).
Now the coordinate system of the coil can be calculated based on these points.
Another approach uses a calibration board with attached markers. The coil is
mounted to the board with a specific location as shown in Fig. 1.5 b. By tracking
the marker on the calibration board and the coil marker, the registration of coil to
coil marker is performed as the location of the coil in relation to the board marker
is known. Furthermore, a third approach may use the known geometry of the coil
and position of the marker by construction.
Now, with registration of head marker to real head and of coil to coil marker,
the tracking system can continuously track the pose of head and coil. For user
guidance, a Graphical User Interface (GUI) displays the position of the coil in
Fig. 1.5 Coil registration for navigated TMS. a Geometrical coil registration approach: By using
a pointer with marker spheres (C) the coil's (B) origin and x- and y-axis can be measured
relatively to the coil marker (A). This way, the coil can be registered to the marker. b A coil
calibration board (A) with board marker (B) by ANT (ANT Neuro B.V., Enschede, The
Netherlands) with a Magstim Air Film coil (C) (Magstim Ltd., Whitland, Wales, UK) with
attached coil marker (D) 2012 ANT Neuro B.V., with friendly permission