Veerle Cnudde

Comparison of different nano- and micro-focus X-ray computed tomography set-ups for the visualization of the soil microstructure and soil organic matter

This study explores the potential of different X-ray computed tomography (X-ray CT) set-ups for the discrimination of soil mineral matter, soil organic matter (OM) and the pore phase. Different X-ray sources, detectors and filtering methods were investigated. Use of a low-energy detector as well as a Cu-filter decreased the potential for phase segmentation in X-ray CT images of an artificial sand-OM mixture. A mid-range X-ray detector showed to hold more potential. Results obtained for an artificial sand-OM sample showed that an attenuation coefficient (AC) grey-value histogram-based single threshold was unsuitable for automated phase segmentation. A dual thresholding approach enabled a better separation of the different phases. Secondly, the minimal measurable pore size class in a clay loam soil aggregate was compared using micro-focus and state-of-the-art nano-focus X-ray CT. The resolution of the scanned images depends on the spot size of the X-ray source, the resolution of the detector and the magnification used. Reliable discrimination of pore and solid phases was expected to be limited by the X-ray tubes focal spot size to [emailxa0protected], in contrast to the voxel size of [emailxa0protected] actually obtained using nano-focus CT. Although this study was limited in its extent, indications were found that more fine porosity is visible at higher resolutions and that large connected pore spaces may be observed. This fine porosity seems to be very locally autocorrelated. Further fundamental research into AC grey-value automated segmentation of OM from the mineral and pore phases, as well as the truly achievable minimal pore size class using artificial calibration samples, is necessary.

Virtual histology by means of high-resolution X-ray CT

Micro‐CT is a non‐destructive technique for 3D tomographic investigation of an object. A 3D representation of the internal structure is calculated based on a series of X‐ray radiographs taken from different angles. The spatial resolution of current laboratory‐used micro‐CT systems has come down over the last years from a few tens of microns to a few microns. This opens the possibility to perform histological investigations in 3D on a virtual representation of a sample, referred to as virtual 3D histology. The advantage of micro‐CT based virtual histology is the immediate and automated 3D visualization of the sample without prior slicing, sample preparation like decalcification, photographing and aligning. This not only permits a drastic reduction in preparation time but also offers the possibility to easily investigate objects that are difficult to slice. This article presents results that were obtained on punch biopsies of horse skin, (dental) alveolus of ponies and chondro‐osseous samples from the tarsus of foals studied with the new high resolution micro‐CT set‐up (HRXCT) at the Ghent University (Belgium) (http://www.ugct.ugent.be). This state‐of‐the‐art set‐up provides a 1 micron resolution and is therefore ideally suited for a direct comparison with standard light microscopy–based histology.

Strain Monitoring in Thermoplastic Composites with Optical Fiber Sensors: Embedding Process, Visualization with Micro-tomography, and Fatigue Results:

This study investigates the possibility of using optical fibers with Bragg gratings for measurements under fatigue loading conditions. Detailed information is given on the principle of optical fiber measurements, the embedding process, and the fatigue tests. To verify the strain derived from the optical fiber, the strain is compared with extensometer measurements. A special design of the blades of the extensometer is presented, since the standard blades suffer from a loss of grip on the surface of the specimen. Furthermore, X-ray micro-tomography is discussed and used for the visualization of the optical fibers and damage in the composite material. The material used for this study is a carbon fiber-reinforced polyphenylene sulfide. It can be concluded that the optical fiber survives over half a million loading cycles, without de-bonding of the fiber. Furthermore, the resolution of the microtomography is high enough to visualize not only the optical fiber, but also damage in the material.

Integration of X-ray micro tomography and fluorescence for applications on natural building stones

X-ray computed tomography (CT) is an excellent, non-destructive analysis tool for characterising many different materials. In geosciences, 3D visualisation is becoming of prime importance in characterising internal structures of various rock types. It enables new approaches in petrophysical research of rock components, including pore and mineral distribution. Although CT provides a lot of information, this technique is limited concerning information on chemical element distribution. X-ray fluorescence (XRF) on the other hand is an excellent technique to obtain the missing information on chemical properties. At the recently established Centre for X-ray Tomography of Ghent University (UGCT) a micro- and nanoCT scanner has been constructed. It is expected that by combination of high-resolution CT and XRF it will be possible to characterise the spatial mineral and element distribution. The combination of both techniques has been applied on natural building stones, in order to get a better insight into some geological parameters (porosity, pore structure, mineral distribution, colour, grain orientation, etc.). Afterwards, the integration of the Morpho+ software tool provides us a 3D quantification of the resulting data.

Correcting phase contrast artefacts in X-ray CT imaging

When going to higher resolution in X-ray transmission CT, one of the biggest problems is the appearance of phase contrast. Achieving high resolution usually means scanning very small and thus very low absorbing objects, especially in medical and biological applications. Since the absorption signal for such samples becomes very small, the contribution of the phase contrast signal to the projection image is no longer negligible. This phase signal, which is due to small angle refraction of the X-rays in the sample, results in severe artifacts in the reconstructed slices when using conventional reconstruction algorithms for transmission CT. The appearance of such phase artifacts can be prevented by using a method called Bronnikov Aided Correction, which applies a filtering operation on the projection images that almost completely removes the phase signal. This method is used to reconstruct a CT scan of a horse biopsy. Results are compared with those of a standard reconstruction.

Exploring the potential of X-Ray tomography in microstructural studies of cementitious systems

Durability of cementitious systems is to a large extent depending on the transport of potentially aggressive substances within the microstructure of the material. A fundamental evaluation of durability on the ‘engineering level’ should thus be realised by investigating the microstructure on the ‘materials science level’. Traditionally, mercury intrusion porosimetry (MIP) is giving very valuable results concerning the pore system of cementitious material. However, it is not a direct visual method, and shows some major drawbacks due to problems related to the basic assumptions made for the interpretation of the results. Based on MIP results, it is very difficult to validate advanced numerical simulation methods devoted to the description of the microstructural properties. A more direct, visual investigation of the pore system of cementitious materials can be realised by means of X-ray tomography. X-ray computed tomography (CT) provides non-destructive three-dimensional visualisation, creating images that map the variation of X-ray attenuation within objects. The X-ray transmission through an object is function of the material composition (effective atomic number), density and thickness. X-ray tomography produces a stack of 2Dshadow images of complete internal 3D-structures by reconstructing a matrix of X-ray attenuation coefficients. These images can then be volume-rendered to provide 3D-information. This contribution focuses on the possibilities of X-ray CT applied on cementitious systems. Interesting images of the air-void system can be obtained using micro-tomography, with a resolution of about 10 microns. However, for a better view of the microstructure, CT-scans with higher resolution have to be realised, using nano-tomography.

UGCT: the new CT facility of the Ghent University (Belgium)

TheUGCTX-ray tomography facility is a cooperation between theRadiation Physics research group (Department of Subatomic and Radiation Physics, Ghent University) and the Sedimentary Geology and Engineering Geology research group (Department of Geology and Soil Science, Ghent University). The facility operates a number of setups offering a wide range of spatial resolutions, X-ray energies and sample sizes. First there is a state-of-the-art transmission typeX-ray tubewith sub-micron focal spot size (900 nm) for extreme-high resolution CT with resolutions down to 1micron for samples up to 4 mm diameter. Secondly a high-power water-cooled X-ray tube is available with an energy between 30 and 160 keV for regular micro-CT applications with resolutions down to 3micron. For large and/or heavy samples up to 40 cm diameter, a dedicated beamline is available at the linear electron accelerator with high-energy X-rays up to 10MeV. The setups are built in room-size bunkers, allowing flexible experimental conditions such as conditioned environments or experimental equipment for real-time sample conditioning. In addition the facility also operates a desktop micro-CT scanner from Skyscan (model 1072) and a medical CT scanner from Philips (Tomoscan SR5000) for particular applications. The facility has several detectors available which are suited for the various applications. For nanoandmicro-CT imaging, a high-resolution 16 bit CCD camera (4K x 2.7K 9μ pixels, 1:1 fiber-optic coupling) and a 12 bit flat panel CMOS detector (1024x512 50μ pixels) are available. For high-speed applications a 6’’ large-field image intensifier can be used. For high-energy applications, a 1Kx1K CCD camera is used which is lens-coupled to a large scintillator that is optimized for high energies. m26.o01