CT geometry determination using individual radiographs of calibrated multi-sphere standards

Abstract

Determining the absolute CT machine geometry is crucial for performing accurate measurements. A common method to achieve this is recording a set of radiographs of calibrated multi-sphere standards under different angles of rotation and subsequently minimising reprojection errors of the sphere positions. Here, we tested methods using individual radiographs and sets of radiographs under accurately measured displacements. We focused on source-object and source-detector distances that, if not accurately known, introduce scale errors. However, it is emphasised that all geometrical parameters (e.g. detector and rotation axis angular deviations) were determined. Using individual radiographs enabled stage error motions and drifts to be determined. The sensitivities of the proposed method were evaluated by displacing the CT’s motion axes on specified trajectories, and were below one micrometre for displacements parallel to the detector and about 10−4 for deviations in the source-detector distance. Using sets of radiographs recorded during one revolution enabled average parameters of the CT geometry to be determined and increased the accuracy of the method. First, the dependence on the number of radiographs, i.e. sphere positions, was investigated. Next, sets of radiographs of the multi-sphere standard were recorded at different source-object distances, the relative displacements of which were accurately measured by interferometry. Using this information improved the relative accuracy to determine displacements in the source-object distance to below 10−5.