title: On-demand calibration and evaluation for electromagnetically tracked laparoscope in augmented reality visualization.
abstract: OBJECTIVE
  Common camera calibration methods employed in current laparoscopic augmented reality systems require the acquisition of multiple images of an entire checkerboard pattern from various poses. This lengthy procedure prevents performing laparoscope calibration in the operating room (OR). The purpose of this work was to develop a fast calibration method for electromagnetically (EM) tracked laparoscopes, such that the calibration can be performed in the OR on demand.
  
  
  METHODS
  We designed a mechanical tracking mount to uniquely and snugly position an EM sensor to an appropriate location on a conventional laparoscope. A tool named fCalib was developed to calibrate intrinsic camera parameters, distortion coefficients, and extrinsic parameters (transformation between the scope lens coordinate system and the EM sensor coordinate system) using a single image that shows an arbitrary portion of a special target pattern. For quick evaluation of calibration results in the OR, we integrated a tube phantom with fCalib prototype and overlaid a virtual representation of the tube on the live video scene.
  
  
  RESULTS
  We compared spatial target registration error between the common OpenCV method and the fCalib method in a laboratory setting. In addition, we compared the calibration re-projection error between the EM tracking-based fCalib and the optical tracking-based fCalib in a clinical setting. Our results suggest that the proposed method is comparable to the OpenCV method. However, changing the environment, e.g., inserting or removing surgical tools, might affect re-projection accuracy for the EM tracking-based approach. Computational time of the fCalib method averaged 14.0 s (range 3.5 s-22.7 s).
  
  
  CONCLUSIONS
  We developed and validated a prototype for fast calibration and evaluation of EM tracked conventional (forward viewing) laparoscopes. The calibration method achieved acceptable accuracy and was relatively fast and easy to be performed in the OR on demand.
authors.affiliation: Sheikh Zayed Institute for Pediatric Surgical Innovation, Children's National Health System, 111 Michigan Avenue NW, Washington, DC, 20010, USA.
authors.forename: Xinyang
authors.initials: X
authors.lastname: Liu
authors.affiliation: IGI Technologies, Inc., 387 Technology Drive #3110D, College Park, MD, 20742, USA.
authors.forename: William
authors.initials: W
authors.lastname: Plishker
authors.affiliation: IGI Technologies, Inc., 387 Technology Drive #3110D, College Park, MD, 20742, USA.
authors.forename: George
authors.initials: G
authors.lastname: Zaki
authors.affiliation: Sheikh Zayed Institute for Pediatric Surgical Innovation, Children's National Health System, 111 Michigan Avenue NW, Washington, DC, 20010, USA.
authors.forename: Sukryool
authors.initials: S
authors.lastname: Kang
authors.affiliation: Sheikh Zayed Institute for Pediatric Surgical Innovation, Children's National Health System, 111 Michigan Avenue NW, Washington, DC, 20010, USA.
authors.forename: Timothy D
authors.initials: TD
authors.lastname: Kane
authors.affiliation: Sheikh Zayed Institute for Pediatric Surgical Innovation, Children's National Health System, 111 Michigan Avenue NW, Washington, DC, 20010, USA. rshekhar@childrensnational.org.
authors.forename: Raj
authors.initials: R
authors.lastname: Shekhar
mesh_terms: D002138:Calibration; D055590:Electromagnetic Phenomena; D004867:Equipment Design; D006801:Humans; D020706:Laparoscopes; D010535:Laparoscopy; D019047:Phantoms, Imaging; D014584:User-Computer Interface
keywords: Augmented reality; Camera calibration; Electromagnetic tracking; Laparoscopic procedure; Laparoscopic visualization