Enhanced Optical Head Tracking for Cranial Radiotherapy: Supporting Surface Registration by Cutaneous Structures

Type of publication:  Artikel
Publikationsstatus: Veröffentlicht
Zeitschrift: International Journal of Radiation Oncology, Biology, Physics
Band: 95
Nummer: 2
Jahr: 2016
Seiten: 810-817
DOI: 10.1016/j.ijrobp.2016.01.041
Abriss: Purpose Precise tumor localization is vital in cranial radiotherapy. Optical marker-less surface imaging is capable of rapidly and non-invasively locating the patient's head. However, to some extent any required registration is a non-convex optimization problem: Identifying correct correspondences between online scans and a reference can be prone to local minima due to deformation of the triangulated surface and spatial ambiguities. We propose to predict variations of tissue thickness across the forehead from backscattered near-infrared (NIR) light. Registration can then be supported by this structural information to minimize the risk for these ambiguities. Methods and Materials 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 5,000 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. Results 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 less than 0.2mm, irrespective of the skin type. With this additional information, the average registration error improved from 3.4mm to 0.48mm by factor 7. Misalignments of more than 1mm were almost thoroughly (98.9%) pushed below 1mm. Conclusions 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. Disclosure & conflict of interest statement: The study and the research reported in this article were funded by Varian Medical Systems, Inc. (Palo Alto, CA, USA) and supported by the Graduate School for Computing in Medicine and Life Science, German Excellence Initiative [grant DFG GSC 235/1].
Autoren: Wissel, Tobias
Stüber, Patrick
Wagner, Benjamin
Bruder, Ralf
Erdmann, Christian
Deutz, Christin-Sophie
Sack, Benjamin
Manit, Jirapong
Schweikard, Achim
Ernst, Floris
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