title: Contact-less stylus for surgical navigation: registration without digitization.
abstract: OBJECTIVE
  We present a laser-based, contact-less, stylus for the purpose of fiducial registration and digitization in the context of surgical navigation.
  
  
  METHODS
  We augmented a laser pointer with a spatial measurement device and used the laser beam as a means to locate a fiducial in 3D space. We developed a method for calibrating the orientation of the laser beam with respect to its attached tracking target. Digitization of a fiducial was formulated as a line intersection problem, and registration was formulated as a point-to-line registration problem.
  
  
  RESULTS
  We achieved an RMS fiducial localization error of 0.63 mm for 151 measurements of 12 fiducial markers. Mean TRE values of less than 1.5 mm over the entire surface of a lumbar vertebra were achievable using 4 fiducial markers. We found that contact-based rigid registration performed carefully under near-ideal conditions outperforms contact-less registration in terms of TRE.
  
  
  CONCLUSIONS
  An inexpensive contact-less stylus can be used to obtain accurate fiducial registration, which can be performed without explicit fiducial digitization.
authors.affiliation: Robarts Research Institute, Western University, London, ON, Canada. chene@robarts.ca.
authors.forename: Elvis C S
authors.initials: ECS
authors.lastname: Chen
authors.affiliation: Department of Electrical Engineering and Computer Science, York University, Toronto, ON, Canada.
authors.forename: Burton
authors.initials: B
authors.lastname: Ma
authors.affiliation: Robarts Research Institute, Western University, London, ON, Canada.
authors.forename: Terry M
authors.initials: TM
authors.lastname: Peters
mesh_terms: D000465:Algorithms; D002138:Calibration; D057918:Fiducial Markers; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000295:instrumentation* / Q000379:methods; D007834:Lasers; D008159:Lumbar Vertebrae / Q000000981:diagnostic imaging; D025321:Surgery, Computer-Assisted / Q000295:instrumentation* / Q000379:methods
keywords: Calibration; Fiducial localization error; Navigation; Point-to-line; Procrustes; Registration; Surgical guidance; Target registration error