Evolution of Micro-CT: Moving from 3D to 4D
Luke Hunter* and Jan Dewanckele TESCAN, 765 Commonwealth Drive, Suite 101, Warrendale, PA 15086 *
luke.hunter@
tescan.com
Abstract: For materials characterization, obtaining insight through 3D imaging has been extremely helpful in improving our understanding of complex systems. X-ray computed tomography (CT) has proven to be an extremely useful technique in this field, allowing for non-destructive interrogation of samples of all types across many length scales. In this article we look at how micro-CT is moving from static 3D imaging into the realm of 4D data collection through dynamic CT, where CT data are collected on samples undergoing change in an uninterrupted matter.
Keywords: micro-CT, in situ, computed tomography, 3D imaging, material characterization
Introduction Micro computed tomography, or micro-CT, has rapidly
evolved as a leading technique for 3D non-destructive micro- structural characterization. Simply put, micro-CT collects a series of images around a sample, using x-rays as the signal. Tose images are then combined into a virtual 3D recon- struction of the original sample that includes internal details. X-rays, as opposed to electrons or visible light used in surface characterization techniques, penetrate and interact with a wide range of materials, providing useful information about the internal features without the need to physically section the sample. While 2D x-ray imaging has been widely used for over 120 years, 3D techniques emerged in medical applica- tions about 60 years ago with higher resolution 3D imaging (that is, micro-CT) making its appearance in the early 1980s. Te first micro-CT systems obtained resolutions on the order of 50–100 μm. Since then, and especially in the last 20 years, the resolution has improved tremendously with some nano- CT systems reaching a resolution below 100 nm. Sub-micron (500–1000 nm) systems are becoming commonplace in aca- demia and industrial research departments to better under- stand material fundamentals, while “industrial” CT systems with resolution ranging from a few micrometers to millimeters are more prevalent in production settings. With micro-CT being recognized as an essential technique
and with wide commercial availability, it is natural for scien- tists and engineers to push the boundaries of 3D x-ray imaging. In situ imaging, where samples are subjected to some type of stimulus, is one clear example. Synchrotron facilities, where the available flux of x-ray photons can be a billion times higher than what is possible in the lab, have been at the forefront of imag- ing advancements. To this point, over the last 10+ years there has been substantial emphasis on imaging evolving structures, typically via in situ testing. Whether it’s loading of materials [1–2], heating of samples [3], fluid flow inside a sample [4], or examining the beat of an insect’s wing [5], speed of data collec- tion is critical. With some synchrotrons it is possible to collect hundreds of full tomography datasets per second, which is cru- cial for data collection in processes that involve rapid changes in metallic foams [6]. In the lab in situ work is certainly pos- sible, however, it has typically been limited to slow processes
28 doi:10.1017/S1551929521000651
(hours to days) or interrupted testing (for example, compressive testing where loading is performed step-wise and imaging is done while there is no change in loading) [7]. To bridge the gap between the synchrotron and the lab, TESCAN has developed a series of hardware and soſtware tools that make scan speeds on the order of seconds possible and enable dynamic CT capability in a laboratory setting [8]. Tere are some limitations related to sample size, resolution, and image noise, but these are practical issues that will be overcome as the technology moves forward. Te aim of this paper is to highlight recent examples where
the temporal resolution of CT imaging is pushed. We will dem- onstrate how dynamic CT in the lab can contribute to better understanding of processes in geosciences and material sciences.
Basics of Micro-CT With computed tomography, a tomogram is created by
the relative rotation of a sample between an x-ray source and detector while collecting a series of 2D radiographs at different angular locations and then reconstructing the data into a full 3D data set. In some cases, the sample is stopped at each step, called step-and-shoot, whereas in other cases the sample con- tinuously rotates, for example, continuous, or smooth CT. Te 2D radiographs are essentially grayscale images of the sample where the gray level at each pixel location corresponds to the overall x-ray attenuation of the material along that path. Once the radiographs are collected, they are processed through a reconstruction algorithm resulting in a stack of 2D slices, which make up a full 3D volume of the original sample [9]. Te basic system design and workflow are illustrated in Figure 1. Te resultant 3D data set provides insight about the internal features of the sample as well as relative density differences. Te primary influences on the quality of the data are the reso- lution of the system and the type and size of sample. Te spatial resolution of a system (Rsys x-ray source spot (Rsource (Rdect
) is predominantly a function of the ), the detector resolution (or pixel size)
), and the geometric relation of the sample (Geo Mag),
source, and detector (equation 1): R
sys =
RR GeoMag
+
22 source
dect (1) Although voxel size is oſten used in place of spatial resolu-
tion, it is important to note that voxel size is only a function of the geometric relation of the components and the detector pixel size, while for true spatial resolution the size of the x-ray spot must be taken into account. Te ability of x-rays to pen- etrate and interact with the sample is the other primary influ- ence on image quality. For a given x-ray energy, the attenuation of photons will
vary dramatically as the atomic weight of the sample material increases. Tis creates a limit on thickness (size) of a sample
www.microscopy-today.com • 2021 May
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