Geometric calibration of a mobile C-arm for intraoperative cone-beam CT

M. J. Daly, J. H. Siewerdsen, Y. B. Cho, D. A. Jaffray, J. C. Irish

Research output: Contribution to journalArticle

Abstract

A geometric calibration method that determines a complete description of source-detector geometry was adapted to a mobile C-arm for cone-beam computed tomography (CBCT). The non-iterative calibration algorithm calculates a unique solution for the positions of the source (Xs, Ys, Zs), detector (Xd, Yd, Zd), piercing point (Uo, Vo), and detector rotation angles (φ,θ,η) based on projections of a phantom consisting of two plane-parallel circles of ball bearings encased in a cylindrical acrylic tube. The prototype C-arm system was based on a Siemens PowerMobil modified to provide flat-panel CBCT for image-guided interventions. The magnitude of geometric nonidealities in the source-detector orbit was measured, and the short-term (∼4 h) and long-term (∼6 months) reproducibility of the calibration was evaluated. The C-arm exhibits large geometric nonidealities due to mechanical flex, with maximum departures from the average semicircular orbit of Δ Uo =15.8 mm and Δ Vo =9.8 mm (for the piercing point), ΔX and ΔY=6-8 mm and ΔZ=1 mm (for the source and detector), and Δφ∼2.9°, Δθ∼1.9°, and Δη∼0.8° (for the detector tilt/rotation). Despite such significant departures from a semicircular orbit, these system parameters were found to be reproducible, and therefore correctable by geometric calibration. Short-term reproducibility was <0.16 mm (subpixel) for the piercing point coordinates, <0.25 mm for the source-detector X and Y, <0.035 mm for the source-detector Z, and <0.02° for the detector angles. Long-term reproducibility was similarly high, demonstrated by image quality and spatial resolution measurements over a period of 6 months. For example, the full-width at half-maximum (FWHM) in axial images of a thin steel wire increased slightly as a function of the time (Δ) between calibration and image acquisition: FWHM=0.62, 0.63, 0.66, 0.71, and 0.72 mm at Δ=0 s, 1 h, 1 day, 1 month, and 6 months, respectively. For ongoing clinical trials in CBCT-guided surgery at our institution, geometric calibration is conducted monthly to provide sufficient three-dimensional (3D) image quality while managing time and workflow considerations of the calibration and quality assurance process. The sensitivity of 3D image quality to each of the system parameters was investigated, as was the tolerance to systematic and random errors in the geometric parameters, showing the most sensitive parameters to be the piercing point coordinates (Uo, Vo) and in-plane positions of the source (Xs, Ys) and detector (Xd, Yd). Errors in the out-of-plane position of the source (Zs) and detector (Zd) and the detector angles (φ,θ,η) were shown to have subtler effects on 3D image quality.

Original languageEnglish (US)
Pages (from-to)2124-2136
Number of pages13
JournalMedical physics
Volume35
Issue number5
DOIs
StatePublished - 2008

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Keywords

  • C-arm
  • Cone-beam CT
  • Flat-panel detector
  • Geometric calibration
  • Image-guided interventions
  • Imaging performance
  • Intraoperative imaging

ASJC Scopus subject areas

  • Biophysics
  • Radiology Nuclear Medicine and imaging

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