title: Fiducial marker-based correction for involuntary motion in weight-bearing C-arm CT scanning of knees. II. Experiment.
abstract: OBJECTIVE
  A C-arm CT system has been shown to be capable of scanning a single cadaver leg under loaded conditions by virtue of its highly flexible acquisition trajectories. In Part I of this study, using the 4D XCAT-based numerical simulation, the authors predicted that the involuntary motion in the lower body of subjects in weight-bearing positions would seriously degrade image quality and the authors suggested three motion compensation methods by which the reconstructions could be corrected to provide diagnostic image quality. Here, the authors demonstrate that a flat-panel angiography system is appropriate for scanning both legs of subjects in vivo under weight-bearing conditions and further evaluate the three motion-correction algorithms using in vivo data.
  
  
  METHODS
  The geometry of a C-arm CT system for a horizontal scan trajectory was calibrated using the PDS-2 phantom. The authors acquired images of two healthy volunteers while lying supine on a table, standing, and squatting at several knee flexion angles. In order to identify the involuntary motion of the lower body, nine 1-mm-diameter tantalum fiducial markers were attached around the knee. The static mean marker position in 3D, a reference for motion compensation, was estimated by back-projecting detected markers in multiple projections using calibrated projection matrices and identifying the intersection points in 3D of the back-projected rays. Motion was corrected using three different methods (described in detail previously): (1) 2D projection shifting, (2) 2D deformable projection warping, and (3) 3D rigid body warping. For quantitative image quality analysis, SSIM indices for the three methods were compared using the supine data as a ground truth.
  
  
  RESULTS
  A 2D Euclidean distance-based metric of subjects' motion ranged from 0.85 mm (±0.49 mm) to 3.82 mm (±2.91 mm) (corresponding to 2.76 to 12.41 pixels) resulting in severe motion artifacts in 3D reconstructions. Shifting in 2D, 2D warping, and 3D warping improved the SSIM in the central slice by 20.22%, 16.83%, and 25.77% in the data with the largest motion among the five datasets (SCAN5); improvement in off-center slices was 18.94%, 29.14%, and 36.08%, respectively.
  
  
  CONCLUSIONS
  The authors showed that C-arm CT control can be implemented for nonstandard horizontal trajectories which enabled us to scan and successfully reconstruct both legs of volunteers in weight-bearing positions. As predicted using theoretical models, the proposed motion correction methods improved image quality by reducing motion artifacts in reconstructions; 3D warping performed better than the 2D methods, especially in off-center slices.
authors.affiliation: Department of Radiology, Stanford University, Stanford, California 94305 and Department of Mechanical Engineering, Stanford University, Stanford, California 94305.
authors.forename: Jang-Hwan
authors.initials: JH
authors.lastname: Choi
authors.affiliation: Department of Radiology, Stanford University, Stanford, California 94305.
authors.forename: Andreas
authors.initials: A
authors.lastname: Maier
authors.affiliation: Department of Radiology, Stanford University, Stanford, California 94305.
authors.forename: Andreas
authors.initials: A
authors.lastname: Keil
authors.affiliation: Biomedical Engineering Department, California Polytechnic State University, San Luis Obispo, California 93407.
authors.forename: Saikat
authors.initials: S
authors.lastname: Pal
authors.affiliation: Department of Radiology, Stanford University, Stanford, California 94305.
authors.forename: Emily J
authors.initials: EJ
authors.lastname: McWalter
authors.affiliation: Musculoskeletal Research Laboratory, VA Palo Alto Health Care System, Palo Alto, California 94304.
authors.forename: Gary S
authors.initials: GS
authors.lastname: Beaupré
authors.affiliation: Department of Radiology, Stanford University, Stanford, California 94305.
authors.forename: Garry E
authors.initials: GE
authors.lastname: Gold
authors.affiliation: Department of Radiology, Stanford University, Stanford, California 94305.
authors.forename: Rebecca
authors.initials: R
authors.lastname: Fahrig
mesh_terms: D000465:Algorithms; D016477:Artifacts; D002138:Calibration; D003198:Computer Simulation; D057918:Fiducial Markers; D056973:Four-Dimensional Computed Tomography / Q000295:instrumentation / Q000379:methods*; D006801:Humans; D007717:Knee / Q000000981:diagnostic imaging*; D007866:Leg / Q000000981:diagnostic imaging; D008954:Models, Biological; D009038:Motion; D010363:Pattern Recognition, Automated; D019047:Phantoms, Imaging; D011187:Posture; D011829:Radiation Dosage; D011856:Radiographic Image Enhancement / Q000379:methods; D016683:Supine Position; D016474:Weight-Bearing