Biomedical Engineering Reference
Direct Head Tracking
Robotized TMS as a further development of neuro-navigated Transcranial
Magnetic Stimulation (TMS) requires accurate tracking data of the patient's head
a stable and accurate registration of the patient's head to medical head scans, e.g.
Magnetic Resonance Imaging (MRI)-scans, is mandatory (see Sect. 1.2.1 ).
8.1 Direct Versus Indirect Tracking
Currently, stereo-optic infrared tracking systems such as the Polaris system are
state-of-the-art for medical (head) tracking [ 4 , 11 ]. They are easy to install and
provide stable and accurate tracking results in the sub-millimeter range with a
tracking frequency of 30-60 Hz which is suitable for most medical applications.
Nevertheless, these systems have one disadvantage: They only provide indirect
tracking. This means that the object to be tracked cannot be measured directly. An
additional marker must be attached to the object. This marker can then be tracked
with the tracking system. Thus, a registration of a marker to the object is required
to provide the position of the object. Therefore, it is mandatory that the marker is
rigidly attached to the object. For TMS, this is done by attaching the marker to a
headband that the patient wears during operation or the marker is clamped to a
For exact head tracking, we must therefore assure that the headband does not
shift. This could happen when the patient moves the headband or the headband
loosens. In this case, the system must be stopped and the registration has to be
re-performed before the system can be restarted. Note that the same problem
occurs with use of a marker clamped to spectacle frame instead of a head band.
Furthermore, the tracking accuracy strongly depends on the registration quality. As
Parts of this chapter have been already published in [ 9 , 14 , 15 ].