title: Self-Calibration of Cone-Beam CT Geometry Using 3D-2D Image Registration: Development and Application to Task-Based Imaging with a Robotic C-Arm.
abstract: OBJECTIVE
  Robotic C-arm systems are capable of general noncircular orbits whose trajectories can be driven by the particular imaging task. However obtaining accurate calibrations for reconstruction in such geometries can be a challenging problem. This work proposes a method to perform a unique geometric calibration of an arbitrary C-arm orbit by registering 2D projections to a previously acquired 3D image to determine the transformation parameters representing the system geometry.
  
  
  METHODS
  Experiments involved a cone-beam CT (CBCT) bench system, a robotic C-arm, and three phantoms. A robust 3D-2D registration process was used to compute the 9 degree of freedom (DOF) transformation between each projection and an existing 3D image by maximizing normalized gradient information with a digitally reconstructed radiograph (DRR) of the 3D volume. The quality of the resulting "self-calibration" was evaluated in terms of the agreement with an established calibration method using a BB phantom as well as image quality in the resulting CBCT reconstruction.
  
  
  RESULTS
  The self-calibration yielded CBCT images without significant difference in spatial resolution from the standard ("true") calibration methods (p-value >0.05 for all three phantoms), and the differences between CBCT images reconstructed using the "self" and "true" calibration methods were on the order of 10-3mm-1. Maximum error in magnification was 3.2%, and back-projection ray placement was within 0.5 mm.
  
  
  CONCLUSIONS
  The proposed geometric "self" calibration provides a means for 3D imaging on general non-circular orbits in CBCT systems for which a geometric calibration is either not available or not reproducible. The method forms the basis of advanced "task-based" 3D imaging methods now in development for robotic C-arms.
authors.affiliation: Department of Biomedical Engineering, Johns Hopkins University, Baltimore, MD USA 21205.
authors.forename: S
authors.initials: S
authors.lastname: Ouadah
authors.affiliation: Department of Biomedical Engineering, Johns Hopkins University, Baltimore, MD USA 21205.
authors.forename: J W
authors.initials: JW
authors.lastname: Stayman
authors.affiliation: Department of Biomedical Engineering, Johns Hopkins University, Baltimore, MD USA 21205.
authors.forename: G
authors.initials: G
authors.lastname: Gang
authors.affiliation: Department of Computer Science, Johns Hopkins University, Baltimore, MD USA 21205.
authors.forename: A
authors.initials: A
authors.lastname: Uneri
authors.affiliation: Siemens Healthcare, Forcheim Germany.
authors.forename: T
authors.initials: T
authors.lastname: Ehtiati
authors.affiliation: Department of Biomedical Engineering, Johns Hopkins University, Baltimore, MD USA 21205 ; Department of Computer Science, Johns Hopkins University, Baltimore, MD USA 21205.
authors.forename: J H
authors.initials: JH
authors.lastname: Siewerdsen
keywords: cone-beam CT; image-guided interventions; imaging task; task-based imaging