title: Video-guided calibration of an augmented reality mobile C-arm.
abstract: OBJECTIVE
  The augmented reality (AR) fluoroscope augments an X-ray image by video and provides the surgeon with a real-time in situ overlay of the anatomy. The overlay alignment is crucial for diagnostic and intra-operative guidance, so precise calibration of the AR fluoroscope is required. The first and most complex step of the calibration procedure is the determination of the X-ray source position. Currently, this is achieved using a biplane phantom with movable metallic rings on its top layer and fixed X-ray opaque markers on its bottom layer. The metallic rings must be moved to positions where at least two pairs of rings and markers are isocentric in the X-ray image. The current "trial and error" calibration process currently requires acquisition of many X-ray images, a task that is both time consuming and radiation intensive. An improved process was developed and tested for C-arm calibration.
  
  
  METHODS
  Video guidance was used to drive the calibration procedure to minimize both X-ray exposure and the time involved. For this, a homography between X-ray and video images is estimated. This homography is valid for the plane at which the metallic rings are positioned and is employed to guide the calibration procedure. Eight users having varying calibration experience (i.e., 2 experts, 2 semi-experts, 4 novices) were asked to participate in the evaluation.
  
  
  RESULTS
  The video-guided technique reduced the number of intra-operative X-ray calibration images by 89% and decreased the total time required by 59%.
  
  
  CONCLUSIONS
  A video-based C-arm calibration method has been developed that improves the usability of the AR fluoroscope with a friendlier interface, reduced calibration time and clinically acceptable radiation doses.
authors.affiliation: Chair for Computer Aided Medical Procedures, Fakultät für Informatik, Technische Universität München, Munich, Germany.
authors.forename: Xin
authors.initials: X
authors.lastname: Chen
authors.affiliation: Chair for Computer Aided Medical Procedures, Fakultät für Informatik, Technische Universität München, Munich, Germany.
authors.forename: Hemal
authors.initials: H
authors.lastname: Naik
authors.affiliation: Chair for Computer Aided Medical Procedures, Fakultät für Informatik, Technische Universität München, Munich, Germany.
authors.forename: Lejing
authors.initials: L
authors.lastname: Wang
authors.affiliation: Chair for Computer Aided Medical Procedures, Fakultät für Informatik, Technische Universität München, Munich, Germany.
authors.forename: Nassir
authors.initials: N
authors.lastname: Navab
authors.affiliation: Chair for Computer Aided Medical Procedures, Fakultät für Informatik, Technische Universität München, Munich, Germany. fallavol@in.tum.de.
authors.forename: Pascal
authors.initials: P
authors.lastname: Fallavollita
mesh_terms: D002138:Calibration; D002983:Clinical Competence; D004867:Equipment Design; D005471:Fluoroscopy / Q000379:methods*; D006801:Humans; D019637:Orthopedic Procedures / Q000379:methods*; D019047:Phantoms, Imaging; D011856:Radiographic Image Enhancement / Q000379:methods; D011857:Radiographic Image Interpretation, Computer-Assisted / Q000379:methods; D014057:Tomography, X-Ray Computed / Q000379:methods; D014741:Video Recording
keywords: C-arm fluoroscopy; Calibration; Medical augmented reality; Multi modalities; Orthopedic and trauma surgery; Video guidance; Visualization