title: Enhanced Optical Head Tracking for Cranial Radiation Therapy: Supporting Surface Registration by Cutaneous Structures.
abstract: To support surface registration in cranial radiation therapy by structural information. The risk for spatial ambiguities is minimized by using tissue thickness variations predicted from backscattered near-infrared (NIR) light from the forehead.
  
  
  
  In a pilot study we recorded NIR surface scans by laser triangulation from 30 volunteers of different skin type. A ground truth for the soft-tissue thickness was segmented from MR scans. After initially matching the NIR scans to the MR reference, Gaussian processes were trained to predict tissue thicknesses from NIR backscatter. Moreover, motion starting from this initial registration was simulated by 5000 random transformations of the NIR scan away from the MR reference. Re-registration to the MR scan was compared with and without tissue thickness support.
  
  
  
  By adding prior knowledge to the backscatter features, such as incident angle and neighborhood information in the scanning grid, we showed that tissue thickness can be predicted with mean errors of <0.2 mm, irrespective of the skin type. With this additional information, the average registration error improved from 3.4 mm to 0.48 mm by a factor of 7. Misalignments of more than 1 mm were almost thoroughly (98.9%) pushed below 1 mm.
  
  
  
  For almost all cases tissue-enhanced matching achieved better results than purely spatial registration. Ambiguities can be minimized if the cutaneous structures do not agree. This valuable support for surface registration increases tracking robustness and avoids misalignment of tumor targets far from the registration site.
authors.affiliation: Institute for Robotics and Cognitive Systems, University of Lübeck, Lübeck, Germany; Graduate School for Computing in Medicine and Life Science, University of Lübeck, Lübeck, Germany. Electronic address: wissel@rob.uni-luebeck.de.
authors.forename: Tobias
authors.initials: T
authors.lastname: Wissel
authors.affiliation: Institute for Robotics and Cognitive Systems, University of Lübeck, Lübeck, Germany; Graduate School for Computing in Medicine and Life Science, University of Lübeck, Lübeck, Germany.
authors.forename: Patrick
authors.initials: P
authors.lastname: Stüber
authors.affiliation: Institute for Robotics and Cognitive Systems, University of Lübeck, Lübeck, Germany; Graduate School for Computing in Medicine and Life Science, University of Lübeck, Lübeck, Germany.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Wagner
authors.affiliation: Institute for Robotics and Cognitive Systems, University of Lübeck, Lübeck, Germany.
authors.forename: Ralf
authors.initials: R
authors.lastname: Bruder
authors.affiliation: Institute for Neuroradiology, Universitätsklinikum Schleswig-Hostein, Campus Lübeck, Lübeck, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Erdmann
authors.affiliation: Clinic for Oral and Maxillo-Facial Surgery, Universitätsklinikum Schleswig-Hostein, Campus Lübeck, Lübeck, Germany.
authors.forename: Christin-Sophie
authors.initials: CS
authors.lastname: Deutz
authors.affiliation: Department of Neurology, Universitätsklinikum Schleswig-Hostein, Campus Lübeck, Lübeck, Germany.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Sack
authors.affiliation: Institute for Robotics and Cognitive Systems, University of Lübeck, Lübeck, Germany; Graduate School for Computing in Medicine and Life Science, University of Lübeck, Lübeck, Germany.
authors.forename: Jirapong
authors.initials: J
authors.lastname: Manit
authors.affiliation: Institute for Robotics and Cognitive Systems, University of Lübeck, Lübeck, Germany.
authors.forename: Achim
authors.initials: A
authors.lastname: Schweikard
authors.affiliation: Institute for Robotics and Cognitive Systems, University of Lübeck, Lübeck, Germany.
authors.forename: Floris
authors.initials: F
authors.lastname: Ernst
mesh_terms: D000328:Adult; D000368:Aged; D016371:Cranial Irradiation / Q000379:methods*; D005260:Female; D006257:Head; D006801:Humans; D008297:Male; D008875:Middle Aged; D010865:Pilot Projects; D011880:Radiotherapy Planning, Computer-Assisted; D012542:Scattering, Radiation; D012867:Skin / Q000033:anatomy & histology; D019265:Spectroscopy, Near-Infrared