title: An on-board surgical tracking and video augmentation system for C-arm image guidance.
abstract: OBJECTIVE
  Conventional tracker configurations for surgical navigation carry a variety of limitations, including limited geometric accuracy, line-of-sight obstruction, and mismatch of the view angle with the surgeon's-eye view. This paper presents the development and characterization of a novel tracker configuration (referred to as "Tracker-on-C") intended to address such limitations by incorporating the tracker directly on the gantry of a mobile C-arm for fluoroscopy and cone-beam CT (CBCT).
  
  
  METHODS
  A video-based tracker (MicronTracker, Claron Technology Inc., Toronto, ON, Canada) was mounted on the gantry of a prototype mobile isocentric C-arm next to the flat-panel detector. To maintain registration within a dynamically moving reference frame (due to rotation of the C-arm), a reference marker consisting of 6 faces (referred to as a "hex-face marker") was developed to give visibility across the full range of C-arm rotation. Three primary functionalities were investigated: surgical tracking, generation of digitally reconstructed radiographs (DRRs) from the perspective of a tracked tool or the current C-arm angle, and augmentation of the tracker video scene with image, DRR, and planning data. Target registration error (TRE) was measured in comparison with the same tracker implemented in a conventional in-room configuration. Graphics processing unit (GPU)-accelerated DRRs were generated in real time as an assistant to C-arm positioning (i.e., positioning the C-arm such that target anatomy is in the field-of-view (FOV)), radiographic search (i.e., a virtual X-ray projection preview of target anatomy without X-ray exposure), and localization (i.e., visualizing the location of the surgical target or planning data). Video augmentation included superimposing tracker data, the X-ray FOV, DRRs, planning data, preoperative images, and/or intraoperative CBCT onto the video scene. Geometric accuracy was quantitatively evaluated in each case, and qualitative assessment of clinical feasibility was analyzed by an experienced and fellowship-trained orthopedic spine surgeon within a clinically realistic surgical setup of the Tracker-on-C.
  
  
  RESULTS
  The Tracker-on-C configuration demonstrated improved TRE (0.87 ± 0.25) mm in comparison with a conventional in-room tracker setup (1.92 ± 0.71) mm (p < 0.0001) attributed primarily to improved depth resolution of the stereoscopic camera placed closer to the surgical field. The hex-face reference marker maintained registration across the 180° C-arm orbit (TRE = 0.70 ± 0.32 mm). DRRs generated from the perspective of the C-arm X-ray detector demonstrated sub- mm accuracy (0.37 ± 0.20 mm) in correspondence with the real X-ray image. Planning data and DRRs overlaid on the video scene exhibited accuracy of (0.59 ± 0.38) pixels and (0.66 ± 0.36) pixels, respectively. Preclinical assessment suggested potential utility of the Tracker-on-C in a spectrum of interventions, including improved line of sight, an assistant to C-arm positioning, and faster target localization, while reducing X-ray exposure time.
  
  
  CONCLUSIONS
  The proposed tracker configuration demonstrated sub- mm TRE from the dynamic reference frame of a rotational C-arm through the use of the multi-face reference marker. Real-time DRRs and video augmentation from a natural perspective over the operating table assisted C-arm setup, simplified radiographic search and localization, and reduced fluoroscopy time. Incorporation of the proposed tracker configuration with C-arm CBCT guidance has the potential to simplify intraoperative registration, improve geometric accuracy, enhance visualization, and reduce radiation exposure.
authors.affiliation: Department of Biomedical Engineering, Johns Hopkins University, Traylor Building, Room #726, 720 Rutland Avenue, Baltimore, MD 21205-2109, USA. sreaung1@jhu.edu
authors.forename: S
authors.initials: S
authors.lastname: Reaungamornrat
authors.forename: Y
authors.initials: Y
authors.lastname: Otake
authors.forename: A
authors.initials: A
authors.lastname: Uneri
authors.forename: S
authors.initials: S
authors.lastname: Schafer
authors.forename: D J
authors.initials: DJ
authors.lastname: Mirota
authors.forename: S
authors.initials: S
authors.lastname: Nithiananthan
authors.forename: J W
authors.initials: JW
authors.lastname: Stayman
authors.forename: G
authors.initials: G
authors.lastname: Kleinszig
authors.forename: A J
authors.initials: AJ
authors.lastname: Khanna
authors.forename: R H
authors.initials: RH
authors.lastname: Taylor
authors.forename: J H
authors.initials: JH
authors.lastname: Siewerdsen
mesh_terms: D054893:Cone-Beam Computed Tomography; D004867:Equipment Design; D005471:Fluoroscopy; D006801:Humans; D021621:Imaging, Three-Dimensional; D011856:Radiographic Image Enhancement / Q000295:instrumentation*; D011857:Radiographic Image Interpretation, Computer-Assisted / Q000379:methods; D025321:Surgery, Computer-Assisted / Q000295:instrumentation*; D014963:X-Ray Intensifying Screens