title: Accuracy analysis for triangulation and tracking based on time-multiplexed structured light.
abstract: OBJECTIVE
  The authors' research group is currently developing a new optical head tracking system for intracranial radiosurgery. This tracking system utilizes infrared laser light to measure features of the soft tissue on the patient's forehead. These features are intended to offer highly accurate registration with respect to the rigid skull structure by means of compensating for the soft tissue. In this context, the system also has to be able to quickly generate accurate reconstructions of the skin surface. For this purpose, the authors have developed a laser scanning device which uses time-multiplexed structured light to triangulate surface points.
  
  
  METHODS
  The accuracy of the authors' laser scanning device is analyzed and compared for different triangulation methods. These methods are given by the Linear-Eigen method and a nonlinear least squares method. Since Microsoft's Kinect camera represents an alternative for fast surface reconstruction, the authors' results are also compared to the triangulation accuracy of the Kinect device. Moreover, the authors' laser scanning device was used for tracking of a rigid object to determine how this process is influenced by the remaining triangulation errors. For this experiment, the scanning device was mounted to the end-effector of a robot to be able to calculate a ground truth for the tracking.
  
  
  RESULTS
  The analysis of the triangulation accuracy of the authors' laser scanning device revealed a root mean square (RMS) error of 0.16 mm. In comparison, the analysis of the triangulation accuracy of the Kinect device revealed a RMS error of 0.89 mm. It turned out that the remaining triangulation errors only cause small inaccuracies for the tracking of a rigid object. Here, the tracking accuracy was given by a RMS translational error of 0.33 mm and a RMS rotational error of 0.12°.
  
  
  CONCLUSIONS
  This paper shows that time-multiplexed structured light can be used to generate highly accurate reconstructions of surfaces. Furthermore, the reconstructed point sets can be used for high-accuracy tracking of objects, meeting the strict requirements of intracranial radiosurgery.
authors.affiliation: Institute for Robotics and Cognitive Systems, University of Lübeck, 23562 Lübeck, Germany and Graduate School for Computing in Medicine and Life Sciences, University of Lübeck, 23562 Lübeck, Germany.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Wagner
authors.affiliation: Institute for Robotics and Cognitive Systems, University of Lübeck, 23562 Lübeck, Germany and Graduate School for Computing in Medicine and Life Sciences, University of Lübeck, 23562 Lübeck, Germany.
authors.forename: Patrick
authors.initials: P
authors.lastname: Stüber
authors.affiliation: Institute for Robotics and Cognitive Systems, University of Lübeck, 23562 Lübeck, Germany and Graduate School for Computing in Medicine and Life Sciences, University of Lübeck, 23562 Lübeck, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Wissel
authors.affiliation: Institute for Robotics and Cognitive Systems, University of Lübeck, 23562 Lübeck, Germany.
authors.forename: Ralf
authors.initials: R
authors.lastname: Bruder
authors.affiliation: Institute for Robotics and Cognitive Systems, University of Lübeck, 23562 Lübeck, Germany.
authors.forename: Achim
authors.initials: A
authors.lastname: Schweikard
authors.affiliation: Institute for Robotics and Cognitive Systems, University of Lübeck, 23562 Lübeck, Germany.
authors.forename: Floris
authors.initials: F
authors.lastname: Ernst
mesh_terms: D002138:Calibration; D004867:Equipment Design; D006257:Head / Q000601:surgery; D006801:Humans; D007834:Lasers; D016018:Least-Squares Analysis; D016014:Linear Models; D017711:Nonlinear Dynamics; D061848:Optical Imaging / Q000295:instrumentation* / Q000379:methods*; D016634:Radiosurgery / Q000295:instrumentation; D012371:Robotics; D025321:Surgery, Computer-Assisted / Q000295:instrumentation