Abstract
X-ray computed tomography (XCT) is a non-destructive imaging technique that has recently gained interest as a tool to measure surface topography. Large acquisition times are a major shortcoming of XCT. One contributing factor to the acquisition time is that a measurement can require the acquisition of thousands of radiographic projections. This paper explores the combined effects of undersampling, i.e. taking fewer radiographic projections and sinogram interpolation, i.e. estimating the missing radiographic projections by interpolation. Different degrees of sinogram interpolation are investigated through the measurement of the surfaces of a metal, additively manufactured part. The quality of the measurement result is assessed via the analysis of the reconstructed volumes, through the computation of quantitative indicators of spatial resolution and noise, and via the analysis of surface topographies extracted from the reconstructed volumes. The quality of the surfaces is assessed through the use of statistical models designed to estimate repeatability errors in the reconstruction, and through the computation of surface texture parameters. Results obtained with no undersampling and no sinogram interpolation are taken as reference. It is shown that noise in the volumetric reconstruction increases with respect to the reference with larger degrees of undersampling, but the increment can be partly compensated by sinogram interpolation with the effects on spatial resolution more difficult to interpret. The computation of surface texture parameters results in similar values for all but one case, the largest undersampling. The topography of the reconstructions indicate that the repeatability error remains similar in all experimental conditions, excluding the case of largest undersampling. Overall, the results indicate that a reduction of the acquisition time of XCT topography measurement is feasible. However, the obtained surface topographies suffer, and large undersampling creates unrecoverable negative effects on the spatial resolution, the noise characteristics and the obtained topographies, even when sinogram interpolation is used.
Highlights
X-ray computed tomography (XCT) is a non-destructive, volumetric imaging technique
The statistical topography models allow for one fundamental consideration, based on simple visual inspection: the amount of noise is reduced when increasing the number of projections from which the interpolation starts
This paper explored the possibility to reduce the number of acquired projections in XCT measurements of surfaces, by applying sinogram interpolation
Summary
X-ray computed tomography (XCT) is a non-destructive, volumetric imaging technique. While XCT originated in the medical sector over fifty years ago, the technology has transitioned into the nondestructive engineering sector as a metrological tool for both dimensional and surface metrology [1,2,3].The volumetric nature of XCT has led to an interest in applying it on additively manufactured (AM)parts [4], which often feature complex internal geometries. XCT has a complex measurement chain, and can be relatively slow when imaging metallic or other highly X-ray attenuating parts. Low photon flux due to high attenuation or low photon flux of the X-ray tube is often associated with an increase in the noise of the reconstructed volume [5], which in turn can cause errors in surface determination [6]. The concept of losing resolution for a reduction of noise is not limited to digital filters. Techniques such as increasing the focal spot size in return for a higher X-ray flux reduce the spatial resolving power of the X-ray system [3]. It is often necessary to find ways to decrease noise while maintaining a good resolution, which is often achieved by sacrificing the measurement speed
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