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 , detector, piercing point , and detector rotation angles 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 and long-term ( 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 and (for the piercing point), and and (for the source and detector), and , , and (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 (subpixel) for the piercing point coordinates, for the source-detector and , for the source-detector , and 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 between calibration and image acquisition: , 0.63, 0.66, 0.71, and at , , 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 and in-plane positions of the source and detector. Errors in the out-of-plane position of the source and detector and the detector angles were shown to have subtler effects on 3D image quality.
The C-arm prototype was developed in collaboration with Siemens Medical Systems, Special Products Division (Erlangen, Germany), with the scientific and engineering support of Dr. R. Graumann, Dr. K. Hermann, Dr. D. Ritter, and Dr. M. Mitschke. The authors express their gratitude to Dr. D. J. Moseley and Dr. S. M. Kim (University Health Network, Toronto ON) for assistance with implementing the calibration algorithm. Thanks also to S. Ansell and G. Wilson (University Health Network) for expertise with software components of the imaging system. Ongoing collaboration with surgeons and scientists in the Guided Therapeutics (GTx) Program at the University Health Network is gratefully acknowledged, including: Dr. M. Fehlings, Dr. M. Jewett, Dr. W. Kucharczyk, Dr. J. Trachtenberg, Dr. R. Weersink, and Dr. B. Wilson. The research was supported by the Princess Margaret Hospital Foundation and the National Institutes of Health Grant No. R01-CA-127444-01.
II. MATERIALS AND METHODS
II.A. Mobile isocentric C-arm for flat-panel cone-beam CT
II.B. Image quality evaluation
II.C. Geometric calibration
II.C.1. System geometry
II.C.2. Calibration phantom
II.C.3. Calibration algorithm
II.D. Geometric reproducibility
II.D.1. Short-term and long-term reproducibility
II.D.2. Effect of C-arm position
II.E. Calibration sensitivity and tolerance
II.E.1. Sensitivity: Parameter knockout
II.E.2. Calibration comparison: Full, single-BB, and semicircular
II.E.3. Tolerance: Systematic and random perturbations
III.A. Geometric reproducibility
III.A.1. Short-term and long-term reproducibility
III.A.2. Effect of C-arm position
III.B. Calibration sensitivity and tolerance
III.B.1. Sensitivity: Parameter knockout
III.B.2. Calibration comparison: Full, single-BB, and semicircular
III.B.3. Tolerance: Systematic and random perturbations
IV. DISCUSSION AND CONCLUSIONS
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