Robust Real-Time Robot/Camera Calibration - Robotized Transcranial Magnetic Stimulation - page 78

Table 4.4 Results for a TMS experiment with a head phantom: The stimulation coil is moved to
five different stimulation points based on a CT-scan
Target point QR24 Tsai-Lenz Online calibration
1 1.08 5.47 1.85
2 1.42 9.53 2.52
3 2.36 10.82 3.45
4 3.16 6.24 2.99
5 0.96 3.54 0.22
Mean 1.80 7.12 2.21
The differences from the actual coil position and the real target point are measured for the
different calibration methods. All values in mm
Fig. 4.12 Trend of the direction for the misplacement between target point and coil position.
Note that this figure only gives the directions and is not to scale. The distances can be found in
Table 4.4
suitable for use in the robotized TMS system. It achieves an overall positioning
accuracy of the robotized TMS of 2.2 mm. It is not as accurate as the currently
used QR24 algorithm [ 8 ], which results in an overall positioning accuracy of
1.8 mm. However, online calibration is more accurate than the standard hand-eye
calibration method proposed by Tsai and Lenz [ 25 ], which leads to an overall
positioning error of more than 7 mm.
Most importantly for robotized TMS, this robust real-time calibration method
features three main benefits:
1. It increases the system's usability and therefore its clinical acceptance.
2. It features additional safety to the system as it detects a movement of tracking
system or robot during the application.
3. It is easily adaptable for other (medical) robotized systems where at least one
link is always visible for the tracking device.