Biomedical Engineering Reference
In-Depth Information
recorded and used to compute the registration. In case these points are not recorded
or set correctly, the registration might be not optimal which would result in
inexactness for coil placement and target localization.
For direct head tracking on the contrary, no additional marker and therefore no
marker registration is required. The tracking system directly tracks the shape of the
object or specific landmarks: In this case the shape of the head or facial landmarks.
As we have image data from any patient, this tracking data could be directly
registered to the three-dimensional head scan of the patient. Therefore, direct
tracking is more robust as it does not need a headband that could shift during the
application. Furthermore, the application becomes more ordinary as direct tracking
does not need a manual registration before each treatment session. This means a
plus in comfort for patient and operator.
In this chapter, we present different techniques that can be used for direct head
tracking. The different techniques are introduced and implementation ideas are
described. Furthermore, we present some first results for each technique showing
its capability. Note that direct head tracking is not limited to robotized TMS, but is
also applicable for pure neuro-navigated TMS.
8.2 FaceAPI
The FaceAPI (Seeing Machines, Braddon, Canberra, Australia) is a software that
allows to track human faces with standard webcams. It automatically calculates the
three-dimensional (3D) head pose (position and orientation)
of the face
by
detection of facial landmarks from the webcam images in real-time [
16
].
8.2.1 The FaceAPI's Main Principle
With current image processing algorithms, facial landmarks such as the eye
corners, the tip of the nose, or mouth corners, can be extracted robustly from two-
dimensional (2D) images [
18
].
The FaceAPI uses these facial landmarks to estimate the 3D pose of the head in
relation to the camera. In principle, it measures distances between these landmarks
and correlates these distances to specific values of a standard head. Using trian-
gulation, the spatial displacement and rotation of the head with respect to the
standard head can be calculated.
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