A short introduction to X-ray microscopy
History
The penetrative power of X-rays, discovered by German scientist Wilhelm Conrad Röntgen, makes them ideally suited to obtain non-destructive information about the internal structure of materials. In the early 1970s, the first X-ray computed tomography (CT) systems were developed, allowing users to obtain three-dimensional (3D) information on metre-sized to nanometre-sized samples.
First applications of this new 3D imaging technique were in the field of meteorite science (Arnold et al. 1983), palaeontology (e.g., Conroy and Vannier, 1987) and soil science (e.g., Petrovic et al.,1982). Initially, medical CT scanners were used, where the sample remains stationary, while the X-ray source and detector rotate. Although medical CT scanners have been improved over the years, their spatial resolution remained limited to several hundreds of micrometres – insufficient to image the often-delicate structures of Earth materials.
In the 1980s, a new research field emerged in high-resolution X-ray tomography, referred to as micro-CT. This method used X-ray tubes (e.g., Sato et al., 1981) and synchrotron radiation (e.g., Grodzins, 1983) as X-ray source. In the years since, both synchrotron-based and laboratory-based micro-CT have developed rapidly. The high brilliance of synchrotron radiation results in a clear superiority in terms of achievable spatial resolution and signal-to-noise ratio (Baruchel et al., 2006). Consequently, extremely fast (even sub-second) micro-CT imaging has become possible at synchrotron facilities.
However, the restricted accessibility of synchrotron light sources limits the number of experiments that can be performed. The much smaller X-ray flux in laboratory-based systems bounds the time resolution which can be attained at these facilities. Nevertheless, progress is being made to improve the quality of measurements performed on the sub-minute timescale (Bultreys et al., 2016). Recent developments in terms of X-ray optics make laboratory-based micro-CT comparable to synchrotron-based micro-CT in terms of spatial resolution, although with the trade-off of increased measurement time.
As the research field in X-ray, micro-CT imaging is continuously evolving, the wide-spread implementation of laboratory-based X-ray micro-CT scanners has revolutionized both experimental and numerical research in the field of Earth materials. While challenges do persist in this field, the next frontier in laboratory-based micro-CT imaging is to routinely execute in-situ, time-resolved imaging of dynamic processes with temporal resolutions on the order of several seconds.
‘Great discoveries are made accidentally less often than the populace likes to think.’
Wilhelm Röntgen
How it works
X-rays are considered as a light source and exhibit wavelengths between that of UV rays and Gamma rays. X-rays can readily penetrate many solid substances, which makes them ideal for analysing internal features within solid materials. CT images are generated by directing an X-ray at an object from multiple orientations and measuring the decrease in energy intensity along a series of linear paths. The decrease in intensity can be described by Beer’s Law, which describes the intensity decrease as a function of X-ray energy, path length, and material linear attenuation coefficient. Each time the X-ray source makes a full rotation around the sample, specialized computer algorithms are used to reconstruct the distribution of X-ray attenuation and generate cross-sectional images, or “slices”. By repeating this process for the entire sample and stacking these images, a 3D model of the sample is generated.
References
Arnold, J.R., Testa, J.P., Friedman, P.J., & Kambic, G.X. (1983). Computed tomographic analysis of meteorite inclusions. Science, 219 (4583), 383-384.
Baruchel, J., Buffiere, J.-Y, Cloetens, P., Di Michiel, M. Ferrie, E. Ludwig, W. Maire, E. Salvo, L. (2006). Advances in synchrotron radiation microtomography. Scripta Materialia, 55, 41-46.
Bultreys, T., Boone, M. A., Boone, M. N., De Schryver, T., Masschaele, B., Van Hoorebeke, L., & Cnudde, V. (2016). Fast laboratory-based micro-computed tomography for pore-scale research: Illustrative experiments and perspectives on the future. Advances in water resources, 95, 341-351.
Conroy, G.C., Vannier, M.W. (1987). Dental development of the taung skull from computerized-tomography. Nature, 329 (6140), 625–627.
Grodzins, L. (1983). Optimum energies for x-ray transmission tomography of small samples: Applications of synchrotron radiation to computerized tomography I, Nuclear Instruments and Methods in Physics Research,206(3), 541-545.
Petrovic, A.M., Siebert, J.E., Rieke, P.E. (1982). Soil bulk-density analysis in 3 dimensionsby computed tomographic scanning. Soil Science Society of America Journal 46 (3),445–450.
Sato, T., Ikeda, O., Yamakoshi, Y., Tsubouchi, M. (1981). X-ray tomography for microstructural objects. Applied Optics, 20 (22):3880-3883.
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