Measurement of the X-ray computed tomography instrument geometry by minimization of reprojection errors—Implementation on experimental data

Abstract

A procedure for measuring the geometry of X-ray computed tomography (CT) instruments is applied to an experimental CT instrument. In this study, the geometrical measurement procedure is implemented with the CT 2 reference object, comprising steel spheres with known center positions in a local coordinate frame affixed to a cylindrical carbon fiber framework. The procedure can be implemented with other sphere-based reference objects, provided the sphere center coordinates are known. The effects of number of acquired projections and rotation mode (stepped or continuous) on the quality of measured geometrical parameters are studied. Finally, the output of the geometrical measurement procedure is used to inform the physical adjustment of the experimental CT instrument to its ideal alignment. The effectiveness of the measurement procedure to correctly determine the instrument geometry is demonstrated from dimensional measurements performed on a tomographically reconstructed validation object from radiographs acquired under initial (misaligned) and adjusted (aligned) instrument geometry.

Geometric errors of CT scanners and their estimation by imaging a reference object

“…The repeatability contribution to the method uncertainty is thus urep = 0.0026 mm, which accounts for only 0.6% of the total variation calculated from the experimental data. Comparing with the variable SDD, the average range is 0.004 mm for SOD and 0.014 mm for SDD; the reason for this lower variability needs further analysis, but coupling between solvable parameters, reported by Ferrucci et al [8,10], may be a plausible explanation. The average chart exhibits most of the averages outside the narrow limits defined by the measurement method.…”

Traceable determination of non-static XCT machine geometry: New developments and case studies

“…The procedure (Figure 2) relies on recording radiographs of a traceably calibrated structure with fiducial markers, usually 3D multi-sphere standards [4,5] or 2D grid structures [7]. Next, the calibrated fiducial marker positions ( , , ) are forward projected to the detector plane ( , ), which can be achieved by parametrising the XCT geometry by means of a projection matrix [8]:…”

“…In parallel, the actual sphere, or feature, positions are located in the radiographs, usually by either detecting the edges of the markers or the grey value centre of gravity. Next, an optimisation algorithm that adjusts the XCT geometry to minimise the deviation between the calibrated forward-projected and actual sphere positions provides the best estimate of the XCT geometry during acquisition of the radiographs [3][4][5].…”

“…Since the geometrical arrangement of the XCT machine directly influences the scale and can introduce geometrical distortions, it is essential to accurately determine and account for it [2]. Methods to determine static XCT machine geometries are established [3][4][5], however, determining and accounting for non-static geometries has not yet been widely applied.…”

Geometric qualification for robot CT with flexible trajectories

“…This can be well calibrated via an extra scan of a dedicated phantom. Offline geometric calibration for CT equipment has been widely investigated [1][2][3][4]. However, current designs are mainly for a circular trajectory, which is not enough for a robot CT system with flexible trajectories.…”