Measurement of sample stage error motions in cone-beam X-ray computed tomography instruments by minimization of reprojection errors

Ferrucci, Massimiliano; Dewulf, Wim; Dönmez, Alkan

doi:10.1016/j.precisioneng.2020.09.004

Cited by 10 publications

(7 citation statements)

References 8 publications

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“…from a tilt of the rotation axis of the rotary table. It is already known in the literature, that calibration procedures based on multi-sphere reference objects can extract such deviations [41].…”

Section: Resultsmentioning

confidence: 99%

Complex 3D scan trajectories for industrial cone-beam computed tomography using a hexapod

Butzhammer

Hausotte

2021

Meas. Sci. Technol.

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Industrial x-ray computed tomography (CT) represents an established measurement technique in the field of nondestructive testing and dimensional metrology. However, the measurement accuracy is sometimes limited by measurement artifacts that cannot be avoided using a standard circular scan trajectory. This problem can be addressed with the aid of flexible 3D trajectories, but up to date, the application of these is mainly restricted to special CT devices using robot arms. In this paper, we present results using a hexapod as an additional positioning system in a commercial industrial CT scanner. In addition to the 360∘ rotation, task-specific tilting of the part during the scan is possible in this way. We used and adapted geometry calibration procedures based on a multi-sphere reference object to enable reconstruction with high accuracy. Using a demonstrator test fixture with high absorbing elements, we show that severe metal and truncation artifacts can be avoided for a region-of-interest scan. Furthermore, cone-beam artifacts, which are inherent to circular scan trajectories, can be reduced significantly. Using measurement objects that can be measured well with a circular trajectory, we found that applying a 3D trajectory leads to dimensional measurement deviations in the same range or even lower than those of a circular trajectory. This suggests that the pose repeatability of the hexapod is sufficient to perform complex scan paths without general loss of accuracy. The obtained results could be relevant for end users of conventional CT scanners, as upgrading existing devices is in principle possible. The presented investigations form the basis for the application of trajectory optimization algorithms.

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Section: Resultsmentioning

confidence: 99%

Complex 3D scan trajectories for industrial cone-beam computed tomography using a hexapod

Butzhammer

Hausotte

2021

Meas. Sci. Technol.

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“…Bircher et al [11] applied multi-sphere-based geometry calibration using different magnifications to increase the accuracy for metrological CT. Similar methods were used by Ferrucci et al [12], who focused on determining sample stage error motions.…”

Section: Introductionmentioning

confidence: 99%

Calibration of 3D scan trajectories for an industrial computed tomography setup with 6-DOF object manipulator system using a single sphere

Butzhammer

Müller²,

Hausotte³

2022

Meas. Sci. Technol.

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In industrial X-ray computed tomography (CT), the application of more complex scan paths in comparison to the typical circular trajectory (360° rotation of the measurement object) can extend the potential of CT. One way to enable such 3D scan trajectories is to use a 6-degrees-of-freedom object manipulator system. In our case, a hexapod is mounted on top of the rotary table of a commercial CT scanner. This allows for adaptive tilting of the measurement object during the scan. For high accuracy, the geometry calibration of such setups is typically done using the X-ray projections of a calibrated multi-sphere object. Contrary to this, here, we demonstrate a procedure that is based on only a single sphere and can therefore experimentally be implemented with low effort. Using the intrinsic geometry parameters of the CT device as prior information, the hexapod coordinate system with respect to the CT machine coordinate system is determined by means of a one-step optimization approach. The resulting parameters are used to calculate projection matrices that enable the volume reconstruction for 3D scan trajectories. The method is validated using simulated X-ray images and experimental investigations including dimensional measurements. For the used setup, geometric measurement results for 3D scan trajectories that are calibrated with the presented method show in sum increased errors compared to the circular scans. A limited pose accuracy of the manipulator system is discussed as a potential cause. The results nevertheless indicate that the presented method is generally feasible for dimensional CT measurements provided that the pose accuracy is sufficient. The calibration procedure can therefore be a low-cost and easier to implement alternative compared to trajectory calibration methods based on multi-sphere objects, but with a tendency towards lower measurement accuracy. The methodology can in principle be transferred to different setups with 6-DOF manipulator systems, e.g. C-arm CT devices with a robot arm.

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“…The underlying concept is to minimize the reprojection error, which is a measure for the difference between the measured 2D coordinates of the projected sphere midpoints in the detector image and the calculated ones setting up an analytical formula for the forward projection. Investigations that are based on this methodology can be found in [3][4][5][6]. Geometry calibration based on spherical markers is also relevant for CT systems that have a high degree of geometric freedom and thus are able to perform 3D scan trajectories.…”

Section: Introductionmentioning

confidence: 99%

“…While the Deng correction according to (1) was applied e.g. in [3,4], it was not used in [5] as no clear improvements were observed. One problem of equation ( 1) is that it is quite sensitive to errors from the ellipse detection, especially for small values of c. Lüthi et al [6] and Kang et al [7] used equation ( 2) that is more stable but needs the sphere radius and the axial distance from the source x 0 as additional input.…”

Section: Introductionmentioning

confidence: 99%

CT scan trajectory calibration based on projected metal spheres: When and how should errors from elliptical distortion be corrected?

Butzhammer¹,

Braun²,

Hausotte³

2023

eJNDT

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The majority of geometry calibration procedures for industrial X-ray Computed Tomography (CT) are based on determining projected sphere centres in X-ray projections. The centres are typically derived from the elliptical contours that arise for cone beam setups. However, the ellipse centre is not identical with the projected sphere centre. A correction method (also known as Deng correction) exists, but is only used in some instances in literature. To this extent, it is an open question for which cases the correction leads to a significant improvement. For specific cases, it can even lead to larger errors. In this contribution, we therefore analyse the parameter range for which the correction is suitable. This is done by means of a simulation parameter study. Additionally, we propose an adapted correction formula that can be used for 3D scan trajectory calibration where the source-detector-distance is known. It is further proposed to keep the ellipse orientation in the ellipse fitting routine fixed. The results from simulation are selectively compared with experimental findings. Overall, the adapted correction leads to superior results compared with the original Deng correction. Whether a correction leads to an improvement at all strongly depends on the ratio between the axial source-object-distance and the sphere radius and on the position of the projected sphere centre on the detector. A theoretically derived decision criterion was largely confirmed. The presented results therefore can give a guidance for potential users who want to decide whether a correction is advantageous for a given sphere radius and CT system geometry.

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Measurement of sample stage error motions in cone-beam X-ray computed tomography instruments by minimization of reprojection errors

Cited by 10 publications

References 8 publications

Complex 3D scan trajectories for industrial cone-beam computed tomography using a hexapod

Complex 3D scan trajectories for industrial cone-beam computed tomography using a hexapod

Calibration of 3D scan trajectories for an industrial computed tomography setup with 6-DOF object manipulator system using a single sphere

CT scan trajectory calibration based on projected metal spheres: When and how should errors from elliptical distortion be corrected?

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