Within the last decade, X-ray computed microtomography (µCT) has become an important tool for investigation in materials science [7,8,9], biology, and medicine [10], since it is a non-destructive method which provides three-dimensional information.
In principle, X-ray microtomography is similar to the conventional scanners used in medical applications, except that it provides images with higher spatial resolution. Getting higher spatial resolution in reasonable acquisition times requires intense X-ray beams. For this purpose, the use of X-ray beams extracted from the synchrotron radiation is particularly well suited. In addition, synchrotron radiation offers the possibility to select monochromatic X-ray beams (within a small energy bandwidth) with an energy optimized for the sample under investigation, while at the same time maintaining a high enough photon flux rate for efficient imaging. Monochromaticity is of great interest in tomography since it avoids beam hardening artifacts, which occur when the low energy radiation of the beam is absorbed by the sample.
The samples were imaged on the 3-D µCT setup developed on beamline ID 19 at the ESRF. The system uses a large monochromatic parallel beam and a 2-D area detector. The sample to be imaged is mounted on a translation/rotation stage allowing precise alignment in the beam. The acquisition consists of recording radiographic images of the sample for different angular positions. After conversion to light by a fluorescent screen, the radiographic images are digitized using a Frelon Camera [11], which consists of a 2-D CCD (Charge Couple Device) array with 1024 elements x 1024 elements, each 19 µm by 19 µm, and offers a dynamic range of 14 bits. The CCD camera is mounted perpendicularly to the X-ray beam in order to avoid direct interactions which cause noise in the recorded images. An optical magnification is used resulting in a pixel size of 6.65 µm by 6.65 µm for the recorded image. A 3-D filtered backprojection algorithm is then used to reconstruct a 3-D image of the sample from the series of 2-D projections [12].
In this experiment, the specimens were scanned using 25 keV X-rays. For each sample, 900 radiographic images were acquired over a field of view of 180 degrees. The 3-D reconstruction was limited to a 256 x 256 x 256 region of interest (ROI) within the sample.