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In many cone-beam CT (CBCT) designs (linac-mounted and C-arms), source and detector trajectories are reproducible but deviate from their nominal paths due to mechanical flex. Since accurate knowledge of the scan geometry is crucial for CBCT image quality, angle-dependent geometric calibration is required, generally using phantoms with bearing balls (BBs) at known positions. However, when tangentially offsetting the detector to increase the field-of-view, multi-BB calibration is not ideal because: (a) the same BBs are not visible in opposite views, negatively affecting the robustness in matching opposing projections, and (b) the calibration becomes more sensitive to deviations from nominal BB positions, e.g. from manufacturing tolerances or post-manufacturing deformation. We present a novel calibration method for scanners with significantly offset detector. Inspired by checkerboard camera calibration, it uses circuit boards with etched copper patterns, readily available to micrometre precision. The method offers full nine-degrees-of-freedom calibration, is robust to phantom assembly and positioning inaccuracies, and uses patterns that cover a large portion of the detector and extend across the overlap region to improve matching of opposing views. For quantitative evaluation was used the smallest sphere intersected by rays back-projected from a 360-degree scan of BBs 100 mm from the isocentre. For a linac-mounted CBCT, the average radius was decreased from 0.48 mm (with single-BB calibration) to 0.14 mm. Corresponding improvements were seen in off-centre slices of Catphan phantom reconstructions. We propose the method as an easy-to-manufacture, simple-to-use, robust alternative to multi-BB calibration for CBCT systems with offset detector, suitable also for noncircular orbits.
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