Methodology for the evaluation of CT image quality in dimensional metrology

In the last years, x-ray computed tomography (CT) has gained more a nd more importance in metrology. However, computed tomography is a complex and indirect measurement procedure. Many factors contribute to the measurem ent r sult, which makes it difficult for the user to relate cause and effect. For examp le, the userset scan parameters significantly influence the measurement result. However, th selection of scan parameters is still based on the experience of the CT user, while the influence of the parameter choice on the measurement result cannot be quan tified. This leads to diverging and non-optimal scan results. The quality of th e CT scan can only be evaluated afterwards, e.g. by visual inspection of the reconstr ucted volume. Taking into account that the process chain of CT measurements is hig hly complex, the very first step is the generation of reliable high quality projections that can then be fed into the reconstruction. The quality of these projections can be d escribed quantitatively by image quality measures. Thus, in this paper, an approach regarding projection based evaluation of CT image quality in micro computed tomograp hy is presented . By performing a set of experiments, the influence of projection image quality on the reconstructed volume and respectively on the measurement res ult is evaluated. A derived model relates the projection quality measures to the actual measurement error of the CT scan. Hence, the proposed approach defin s a reliable, combined quality measure, which is based on a small number of projections acquired before the actual scan. An algorithm evaluates the quality of those projection for every workpiece that has to be m easured. Based on the evaluation, it varies the scan parameters, until an optimal projectio n quality is reached and a reduced measurement error in the CT scan is a hieved.