Time-multiplexed structured light for head tracking

Introduction: Image-guided intracranial radiosurgery relies on highly accurate tracking of the patient’shead. This can be achieved by using the rigid skull structure for registration between a reference imagingdataset and imaging datasets that are acquired during the treatment. Accuray Inc. (Sunnyvale, CA, USA)developed the 6D Skull Tracking [1] for their CyberKnife system. This method uses stereoscopic X-rayimages in which the rigid bone structure of the head is segmented and registered to a planning CT.Experiments with an anthropomorphic skull phantom revealed that this method achieves sub-millimetertracking accuracy. The On-board Imager (OBI) by Varian Medical Systems Inc. (Palo Alto, CA, USA) usescone-beam CT for target localisation regarding bone structures. According to [2], experiments with ananthropomorphic head phantom showed that the OBI system achieves sub-millimeter tracking accuracy.The high accuracy of X-ray based tracking methods is achieved at the cost of tracking speed. This is dueto the fact that X-ray imaging has to be carried out at low frame rates to guarantee acceptable additionaldose levels.By contrast, optical head tracking offers much faster scan rates. Here, visible or infrared light is actively orpassively used to reconstruct the surface of the skin. However, this method is highly inaccurate since theregistration process is influenced by deformable structures which are given by the skin and the muscles.In order to overcome this problem, we are currently developing a system which utilises infrared laser lightto measure features of the soft tissue on the patient’s head [3]. These features are intended to offerregistration with respect to the rigid skull structure by means of compensating for the soft tissue. Thisabstract describes the reconstruction of the skin surface using time-multiplexed structured light. To verifythe applicability of the reconstruction method for head tracking, it was used for tracking a head phantomand a human head. In this context, the accuracy of the head tracking was determined by using a robot tocalculate the ground truth. For this purpose, the laser scanning device was mounted to the end-effector ofthe robot. In future developments, this scanning device is intended to be enhanced by the aforementionedfeatures of the soft tissue.