Direct Head Tracking - Robotized Transcranial Magnetic Stimulation - page 143

(a) (b)

Fig. 8.10 Head contour generation from 3D laser scan. a From the scanned data points (red)a
smooth head contour is computed. b The head contour is computed with the PowerCrust method [ 2 ]

8.4.2 Comparison to Manual Contour Generation

A pointer tracked by a Polaris tracking camera can be also used to generate a
manual head contour. For this purpose, the pointer is continuously tracked while
moved on the head surface. Typically, with this method 500-1000 surface points
are collected. Using again the PowerCrust algorithm [ 2 ] a head contour can be
generated.
We therefore generate such a manual head contour of the head phantom based
on roughly 1,000 surface points. Subsequently, we overlay this contour with the
surface points from a 3D laser scan. Figure 8.11 visualizes this overlay. We clearly
see that the manual contour provides quite good results in the top area of the head.
However, in the facial area it performs quite poor whereas the laser scan provides
full information also in the facial area.

8.4.3 Application in Robotized TMS Studies

We have successfully applied head contours based on 3D laser scans for two
ongoing TMS studies with roughly 20 subjects [ 1 ]. For both studies, a motor
cortex mapping has been performed. A stimulation hot-spot for right foot and for
left hand has been identified for the first and second experiment, respectively.
Once the hot-spot has been found, the coil is positioned exactly at the hot-spot