Technique Principles

X-ray tomographic microscopy is a powerful technique used to visualize and study the three dimensional (3D) internal structure and material properties of a variety of optically opaque samples in a nondestructive manner. It was introduced in 1973 by Houndsfield (1) and initially applied mostly in the medical field, where it is now well established. With time it became clear that X-ray tomographic microscopy is an excellent tool for the investigation of a wider range of specimens. Nowadays, most academic and industrial research departments in many fields such as materials, food, pharmaceuticals, environmental and Earth sciences, are equipped with laboratory based microCT systems, which are routinely used for the nondestructive analysis of the internal structure of the most diverse samples.

In absorption contrast tomographic microscopy, radiographic projections are acquired, showing the selective attenuation of the X-ray beam traveling through the sample. For a given beam energy, the number of absorbed photons depends on the sample material (Z-number and electron density): the larger these parameters, the more photons absorbed. In phase contrast imaging, the diffraction of the beam and the resulting interference phenomena are exploited. This means that low absorbing or similar Z-number materials can still utilize the distinct capabilities available at a synchrotron. Radiographic projections, however, only provide 2D cumulative information of the structure along the beam path. 3D internal structural details can be ascertained by taking radiographs at different sample orientations and combining those using sophisticated algorithms for tomographic reconstructions based on Fourier analysis (e.g., Filtered back-projection) or iterative methods.

Knowledge provided by X-ray tomographic microscopy on the interior of optically opaque objects is immense. In addition to the mere visualization of the 3D internal structural details, extraction of quantitative information is now possible thanks to the ever increasing computational power. Number, size, shape, orientation, spatial distribution, connectivity and packing of features of interest in the analyzed volume are just a few of the possible quantitative metrics which can be extracted from tomographic datasets. If adequate calibration measurements are performed, X-ray tomographic microscopy can also provide insight into composition (e.g., bone mineralization).
At third generation synchrotrons, thanks to a very intense and coherent beam, X-ray imaging has experienced a true revolution. The tremendous photon density reached by these novel sources brings huge advantages with respect to traditional X-ray laboratory instruments. The high brilliance of synchrotron light provides increased spatial and temporal resolution: detection of details as small as 1 micron in millimeter-sized samples is routinely possible within only a few minutes. In addition, the monochromaticity of the X-ray beam makes quantitative measurements of material properties possible and vastly simplifies the identification of different phases, since beam hardening artifacts, distinctive for laboratory setups, can be avoided. Increased contrast and reduced noise are also promoted by the monochromatic beam and the high photon flux. Moreover the almost parallel beam geometry usual at tomographic microscopy endstations at synchrotron sources permits the accurate reconstruction of tomographic volumes without cone beam artifacts. The combination of these factors contributes to the astonishing quality of the resulting images.

Finally the latest detector generation based on CMOS technology coupled with the highly brilliant synchrotron radiation has made sub-second temporal resolution a reality. In this way, previously unimaginable new science and experiments, where fast dynamic processes can be captured for the first time in 3D through time (2), are now possible. Notes

1: G. N. Hounsfield, Computerized transverse axial scanning (tomography): Part 1. Description of system, Br J Radiol, 46, 1016-1022 (1973). pdf

2: R. Mokso, D. A. Schwyn, S. M. Walker, M. Doube, M. Wicklein, T. Müller, M. Stampanoni, G. K. Taylor, and H. G. Krapp, "Four-dimensional in vivo X-ray microscopy with projection-guided gating", Scientific Reports 5, 8727 (2015) DOI: 10.1038/srep08727