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Abstract We present a method for measuring small, discrete features near the resolution limit of X‐ray computed tomography (CT) data volumes with the aim of providing consistent answers across instruments and data resolutions. The appearances of small features are impacted by the partial volume effect and blurring due to the data point‐spread function, and we call our approach the partial‐volume and blurring (PVB) method. Features are segmented to encompass their total attenuation signal, which is then converted to a volume, in turn allowing a subset of voxels to be used to measure shape and orientation. We demonstrate the method on a set of gold grains, scanned with two instruments at a range of resolutions and with various surrounding media. We recover volume accurately over a factor of 27 range in grain volume and factor of 5 range in data resolution, successfully characterizing particles as small as 5.4 voxels in true volume. Shape metrics are affected variably by resolution effects and are more reliable when based on image‐based caliper measurements than perimeter length or surface area. Orientations are reproducible when maximum or minimum axis lengths are sufficiently different from the intermediate axis. Calibration requires end‐member CT numbers for the materials of interest, which we obtained empirically; we describe a first‐principles calculation and discuss its challenges. The PVB method is accurate, reproducible, resolution invariant, and objective, all important improvements over any method based on global thresholds.
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Resolution-invariant measurements of small objects in polychromatic CT data
Features such as particles, pores, or cracks are challenging to measure accurately in CT data when they are small relative to the data resolution, characterized as a point-spread function (PSF). These challenges are particularly acute when paired with segmentation, as the PSF distributes some of the signal from a voxel among neighboring ones; effectively dispersing some of the signal from a given object to a region outside of it. Any feature of interest with one or more dimensions on the order of the PSF will be impacted by this effect, and measurements based on global thresholds necessarily fail. Measurements of the same features should be consistent across different instruments and data resolutions. The PVB (partial volume and blurring) method successfully compensates by quantifying features that are small in all three dimensions based on their attenuation anomaly. By calibrating the CT number of the phase of interest (in this case, gold) it is possible to accurately measure particles down to <6 voxels in data acquired on two instruments, 14 years apart, despite severe artifacts. Altogether, the PVB method is accurate, reproducible, resolution-invariant, and objective; it is also notable for its favorable error structure. The principal challenge is the need for representative effective CT numbers, which reflect not only the features of interest themselves, but also the X-ray spectrum, the size, shape and composition of the enclosing sample, and processing details such as beam-hardening correction. Empirical calibration is the most effective approach.
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- Award ID(s):
- 1762458
- PAR ID:
- 10164887
- Date Published:
- Journal Name:
- Developments in X-Ray Tomography XII
- Page Range / eLocation ID:
- 111130B
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1117/12.2532082
