Direct Head Tracking - Robotized Transcranial Magnetic Stimulation - page 135

As the coordinate system of a TMS coil is well defined (cf. Sect. 1.2.2 ), we can
easily transfer the reference image into the origin of the scanner coordinate frame.
Note that scanning of the coil is not time critical as the coil registration is done
in an additional calibration step before the TMS session starts. During the TMS
session the position of the coil is obtained with the robot's forward calculation [ 7 ]
and the computed coil registration. Thus, no further scans of the coil are needed for
the application.
8.3.1.3 Registration of Head-Scan to 3D Laser Scanner and Head
Tracking
In accordance with coil registration and calibration, we must also employ a ref-
erence image for head tracking with the laser scanning system. As we have an
MRI-scan, or other medical head scans, for each patient, we use the virtual head
outline generated from that data as the reference image. To transform this refer-
ence image into the laser scanner coordinate frame, we use three manually selected
landmarks: the tip of the nose, the left, and the right outer eye corners. We place
the tip of the nose in the origin of the laser scanner. The two landmarks at the eye
corners are used to span the coordinate system.
Due to the tracking of the head with the laser scanner via the reference image,
the robot is able to move the coil to the desired target position at the real head.
Therefore, we use the transform from coil to robot's end effector that has been
calculated in the coil registration step. The needed transforms for the head navi-
gation with the laser scanner are illustrated in Fig. 8.4 .
Fig. 8.4 Head tracking and coil placement at the head. For placing the coil at the head, the
transform C T H is given. Receiving the head position from the laser scanner, the robot has to
move the coil in a way that the coil has the desired position relative to the head. Again, a
reference image H Ref is used for tracking