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- Beitrag zu einem Tagungsband (28) (entfernen)
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- Computed tomography (9)
- Reconstruction algorithm (4)
- X-ray refraction (4)
- Phase contrast (3)
- Soft matter radiography (3)
- Composites (2)
- DIRECTT (2)
- Diffuse background (2)
- Edge artefact (2)
- Electron tomography (2)
The combination of tomographic, microstructural data with other experimental techniques and with modeling is paramount, if we want to extract the maximum amount of information on material and component properties. In particular, quantitative image analysis, statistical approaches, direct discretization of tomographic reconstructions represent concrete possibilities to extend the power of the tomographic 3D representation to insights into the material and component performance. This logic thread holds equally for industrial and academic research, and valorizes expensive experiments such as those carried out at synchrotron sources, which cannot be daily repeated. We shall show a few examples of possible use of X-ray tomographic data for quantitative assessment of damage evolution and microstructural properties, as well as for non-destructive testing. We will also show how X-ray refraction computed tomography (CT) can be highly complementary to classic absorption CT, being sensitive to internal interfaces.
The emerging technology of generation and detection of Terahertz waves (1 THz = 1012
Hz) offers diverse potentials in non-destructive testing (NDT) regarding security as well
as safety aspects. Herein, we focus on the latter with emphasis on imaging techniques.
The THz range (0.1
10 THz, 3mm
30 µm) closes the technological gap between
ultra high frequency electronics and FIR optics in the electro-magnetic (EM) spectrum.
Electro-optical sampling provides straight access to the EM wave including its phase
(rather than intensity). THz waves are well suited to characterize non-metallic materials
since they penetrate paper, plastics, ceramics and certain composites (e.g. GFC).
We employ a commercial fibre-coupled THz time domain spectrometer (TDS) for
scanning the samples laterally through a focal spot. At each position the entire temporal
pulse train is recorded, which offers the opportunity to use various parameters derivable
for imaging.
Topographic measurements are performed as reflection set-up. The achievable spatial
resolution is diffraction limited at about 100 µm -300 µm (which allows for perception
of single defects on a sub-mm scale). In contrast to typical pulse echoed ultrasound
testing THz topography is a non-contact inspection tool without coupling agents.
Several reflections of subsequent concealed layers are detected instead of just the first
encountered one. Comparing to X-ray (radiology) the non-ionizing THz-waves generate
images of similar contrast regarding metal and plastic (organic) components, while
conventional radiography emphasizes one type of material depending on the energy preselected.
Moreover, the full temporal information is exploited to derive spectra in
certain selected time windows, i.e. separated spectral properties of each layer in
multilayered structures.
Tomographic measurements are performed in transmission mode. The sample is
mounted on a rotation stage in order to allow for lateral scanning under the different
projection angles. The reconstruction is typically performed by filtered backprojection,
which is widely used for X-ray CT. The adaption of this technique to the THz range
comes along with several experimental drawbacks such as considerable refraction and
scattering. According to these difficulties we investigate the occurring artefacts. The
THz-TDS provides the opportunity to calculate a separate backprojection of each
frequency interval as obtained by the Fourier transformation of the recorded time
resolved amplitude. This results in a spectrally resolved reconstruction of each voxel,
which is unique in computed tomography technology.
Quantitative X-ray refraction topography is applied for improved nondestructive evaluation of advanced high performance ceramics. The method is based on X-ray small angle scattering by refraction of X-rays due to density differences of microstructures in heterogeneous materials. It determines the amount of inner surfaces and interfaces within the range of µm to nm dimensions. As the scattered refraction intensity is proportional to the inner surface density of a sample, a reference standard gives a quantitative measure for analytical requirements. X-ray refraction scanning topography visualizes integral interface properties at 10 µm spatial resolution by two-dimensional computer images. Characteristic microstructure parameters, i.e. density fluctuation, porosity and pore size distribution of several examples of SiO[2]-and SiC ceramics at different stages of sintering are presented.
For the first time we present direct 2D imaging of refracted X-rays without any discrimination of the Primary radiation. X-refraction works in analogy to visible light optics: X-rays are entirely deflected at interfaces where discontinuities of (electron) density occur. This is demonstrated at the example of inner and outer surfaces of model samples of well-defined geometry (fibres, capillaries, and monodisperse micro particles). The samples are scanned through a 50 μm monochromatic (20 keV) pencil beam. In order to warrant a sufficient angular resolution a 2D detector (pixel size 7 μm) is placed 3 m downstream of the sample. Scanning the sample in micron steps allows for detecting local changes of interface / surface orientation directly in two dimensions. At the actual angular resolution of about 3 seconds of arc (scattering vector increments Δk = 10-3 nm-1) and 50 μm spatial resolution (scanning) the material’s inner surfaces (with nanometer separation) can be characterized even at sampling rates below 1 second per frame. Moreover, our technique is suited to directly determine particle diameters of up to 250 nm by means of diffraction fringes. Some potential applications to technical submicron structures are discussed.
Tomography data obtained from transmission electron microscopes are especially attractive due to their unrivaled spatial resolution in the nanometer range or even less, but they require enormous efforts in sample preparation and suffer from a diverse accumulation of experimental restrictions, which unavoidably result in fundamental reconstruction artifacts. These restrictions refer to: partial opacity, a limited view (limited angle or missing wedge), very few angles (with respect to the detector size), limited to a region of interest (ROI; due to the sample size), variable angular increments as well as sample degradation due the interactions with the electron beam. An advanced version of the DIRECTT (Direct Iterative Reconstruction of Computed Tomography Trajectories) algorithm proves to cope with most of these severe deviations from ideal CT measuring conditions. However, careful data preprocessing is required in order to exploit the capabilities of the algorithm.
Nanometer sized Ruthenium catalyst particles for fuel cell applications are 3D imaged at a few Ångström resolution in order to estimate their partial free surface on carbon black supports, which rule the efficiency of the catalytic activity.
Comparisons of DIRECTT reconstructions to the conventional filtered back projection, prove the significant improvements.
X-ray dark field imaging
(2014)
A new kind of enhanced contrast X-ray imaging of weakly absorbing materials in dark field mode is presented.
Samples such as plastics and biological tissue are a perpetual challenge for radiographic imaging. Recent innovative approaches such as Diffraction Enhanced Imaging (DEI), “phase contrast”, grating interferometry (Talbot-Lau), or Refraction Enhanced Imaging (REI) have in common that they yield enhanced contrast based on deflected X-rays.
Here, we introduce a REI modification, which works with a slightly bent single crystal reflecting in Bragg geometry. The samples are placed upstream of the thin crystal within a parallel synchrotron beam. The crystal’s curvature creates a dark-field stripe in the transmission image, similar to the inverse of a Darwin-Prins rocking curve of plane crystals. The reflection condition is met better or worse as a steady function of incidence position.
Refracted beam portions of sample interfaces appear bright on black stripes. The entire image is finally synthesized by multiple dark-field stripes. The technique is an alternative to the mentioned techniques. We demonstrate the technique’s advantage by examples of biological and technical microstructures.