Simulation-aided investigation of beam hardening induced errors in CT dimensional metrology

Abstract

Industrial x-ray computed tomography (CT) systems are being increasingly used as dimensional measuring machines. However, micron level accuracy is not always achievable, as of yet. The measurement accuracy is influenced by many factors, such as the workpiece properties, x-ray voltage, filter, beam hardening, scattering and calibration methods (Kruth et al 2011 CIRP Ann. Manuf. Technol. 60 821–42, Bartscher et al 2007 CIRP Ann. Manuf. Technol. 56 495–8, De Chiffre et al 2005 CIRP Ann. Manuf. Technol. 54 479–82, Schmitt and Niggemann 2010 Meas. Sci. Technol. 21 054008). Since most of these factors are mutually correlated, it remains challenging to interpret measurement results and to identify the distinct error sources. Since simulations allow isolating the different affecting factors, they form a useful complement to experimental investigations. Dewulf et al (2012 CIRP Ann. Manuf. Technol. 61 495–8) investigated the influence of beam hardening correction parameters on the diameter of a calibrated steel pin in different experimental set-ups. It was clearly shown that an inappropriate beam hardening correction can result in significant dimensional errors. This paper confirms these results using simulations of a pin surrounded by a stepped cylinder: a clear discontinuity in the measured diameter of the inner pin is observed where it enters the surrounding material. The results are expanded with an investigation of the beam hardening effect on the measurement results for both inner and outer diameters of the surrounding stepped cylinder. Accuracy as well as the effect on the uncertainty determination is discussed. The results are compared with simulations using monochromatic beams in order to have a benchmark which excludes beam hardening effects and x-ray scattering. Furthermore, based on the above results, the authors propose a case-dependent calibration artefact for beam hardening correction and edge offset determination. In the final part of the paper, the investigations are expanded with experiments of a new set-up that includes non-cylindrical features; the effectiveness of the proposed calibration artefact is also studied.

A simulation-based study on the influence of beam hardening in X-ray computed tomography for dimensional metrology

“…Suppes et al [11] reasoned the grey value inhomogeneity caused by beam hardening influenced the position of the determined surface; experimental data showed correcting for beam hardening reduced both errors in form and length measurements. The group of Dewulf et al [12][13][14][15] have shown that changes in X-ray path length and the coefficients of a linearisation polynomial can influence dimensional measurements. All the aforementioned results support the premise that beam hardening influences dimensional measurements; however, an explanation as to how this influence occurs has yet to be given.…”

Evaluation of the geometric accuracy of computed tomography and microcomputed tomography of the articular surface of the distal portion of the radius of cats

“…36 Effects are more pronounced in bone because of the high calcium content 37 and subsequent absorption of x-rays. 38 Attempts to optimize corrections for beam hardening exist in clinical applications 37 and in industrial metrological applications. [38][39][40] Investigators of another study 41 compared results for CT LO , oblique CT, and CT TO images of a human ankle prosthesis implanted in a stifle joint of a pig.…”

“…38 Attempts to optimize corrections for beam hardening exist in clinical applications 37 and in industrial metrological applications. [38][39][40] Investigators of another study 41 compared results for CT LO , oblique CT, and CT TO images of a human ankle prosthesis implanted in a stifle joint of a pig. In that study, 41 measurements for CT TO images were less accurate than measurements for CT LO images.…”

A Review of Common Beam Hardening Correction Methods for Industrial X-ray Computed Tomography

“…The final step is a reconstruction of the image by using this effective energy to generate monochromatic projection data (Thomsen et al 2015). This method if chosen to be applied requires knowledge about the detector efficiency and the energy spectrum MC simulation which has been extensively used for investigation of beam hardening artifact in many studies such as Arunmuthu et al (2013), Lifton et al (2013), Ramakrishna et al (2006), Sahebnasagh et al (2012), Tan et al (2014), Thomsen et al (2015) and Yan et al (2000). It has high precision and is very flexible regarding changes in the beam geometry, sample composition, objects geometry and densities.…”