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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)
Micro damage structures of fiber reinforced polymers (FRP) can be characterized nondestructively by X-ray refraction topography, which images inner surfaces. The damage accumulation after fatigue treatment of short glass fiber reinforced polybutylene terephthalate (PBT-GF) is correlated to the applied load and the number of cycles. Micro cracks and fiber/matrix debonding during fatigue can be detected separately due to their different direction of X-ray small angle scattering. The residual strength over the crack surface decreases by a linear decay as 30 %, 60 % and 90 % of the materials lifetime is passed.
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.
Non-destructive Inspection of Ceramic/Composite Materials using Non-classical X-Ray Techniques
(1999)
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.
A novel approach to strongly suppress artifacts in radiography and computed tomography caused by the effect of diffuse background Signals (backlight) of 2D X-ray detectors is suggested. Depending on the detector geometry the mechanism may be different, either based on the optical scattering of the fluorescent screen materials into optical detection devices or Compton or X-ray fluorescence scattering by the detector components. Consequently, these erroneous intensity portions result in locally different violations of Lambert Beers law in single projections (radiographs).
When used as input data for computed tomography these violations are directly observed via modulation of the projected mass as a function of the rotation Phase and the samples aspect ratio (dynamics). The magnitude of the diffuse Background signal depends on the detector area covered by the projected sample. They are more pronounced the smaller the shaded area and the stronger the total attenuation.
Moreover, the local intensity mismatch depends on the attenuation of the sample.
We present very basic reference data measured with multiple metal foils at a synchrotron radiation source. Beam hardening artifacts can be excluded due to the monochromatic radiation. The proposed correction procedure assumes a constant (non-local) scattering mechanism.
Enhanced Spatial Resolution in 2D CT-Reconstruction without Filtered Back Projection: DIRECTT
(2008)
In order to demonstrate the challenging tasks in current R&D applications in the field of renewable energy
sources experimental neutron and electron tomographic data sets of PEM fuel cell components are processed by
an advanced version of the DIRECTT algorithm. The neutron measurements of the water distribution suffer from
several intrinsic limitations such as massive position dependent focal smearing due to the large primary neutron
beam aperture. Electron (TEM) tomography allows characterisation of nanometre sized catalyst particles but is
hampered by the following restrictions: partial opacity, a limited sector of projections (missing wedge), very few
projections, samples exceeding the detector size (region of interest) and improper angular alignment. Beyond the
capabilities of other algorithms DIRECTT proves to overcome these essential reconstruction problems, in
comparison to FBP and SIRT. Nevertheless, careful data pre-processing is an inevitable requirement in order to
fully exploit the algorithm's potentials.
Incomplete tomographic data sets such as limited view (missing wedge) data represent a well-known challenge
for reconstruction algorithms, since they unavoidably lead to substantial image artefacts. Such data sets may
occur in industrial computed tomography of limited access (e.g. extended components, fixed objects), directional
opacity, limited sample life time or laminographic set-up. We present strategies to effectively suppress the
typical elongation artefacts (e.g. lemon-like deformed pores) by our iterative algorithm DIRECTT which offers
the opportunity to vary the versatile reconstruction parameters within each cycle. Those strategies are applied to
experimental data obtained from metallic foams as well as model simulations. Comparison is drawn to state-ofthe-
art techniques (filtered backprojection and algebraic techniques). Further reference is made to reconstructions
of complete data sets serving as gold standards. For quantitative assessment of the reconstruction
quality adapted techniques based on spatial statistics are introduced.
A novel approach to strongly suppress artifacts in radiography and computed tomography caused by the effect of
diffuse background signals ('backlighting') of 2D X-ray detectors is suggested. Depending on the detector
geometry the mechanism may be different, either based on the optical scattering of the fluorescent screen
materials into optical detection devices or Compton or X-ray fluorescence scattering by the detector components.
Consequently, these erroneous intensity portions result in locally different violations of Lambert Beers law in
single projections (radiographs) as function of the detector area coverage and the magnitude of the attenuation.
The absorption of multiple metal sheets is investigated by monochromatic synchrotron radiation, thus excluding
beam hardening. The proposed correction procedure simply requires the individual subtraction of one and the
same fraction of the primary and transmitted mean intensity, as a constant (non-local) scattering mechanism is
assumed.
The relevance of nano sized structures and interfaces in upcoming technologies requires new non-destructive Xray
techniques exploiting attenuation as well as scattering. In the course of driving the resolution and the contrast
of modern radiography into micro and nano metre dimension the inevitable scattering effects require increased
attention. Several radiologists interpret the observed edge artefacts simply by 'phase contrast' due to different
X-ray path lengths within the sample and others refer to Fresnel scattering. The ultra small angle scattering is of
high intensity and may reach the level of the primary beam. We present monochromatic synchrotron
measurements on rather simply shaped objects (such as plates and cylinders) which prove clearly the dominance
of refraction intensity over total reflection and coherent small angle scattering. The refraction conditions hold for
angles of incidence of several degrees to the surface, but typical deflection angles range from several seconds to
minutes of arc. Additionally to the concept of 'phase contrast' of the primary beam its deflection becomes the
essential issue. The appropriate removal of the attenuation contribution enables the imaging of inner surfaces and
interfaces by refraction scanning topography. The presented measurements of reference particles demonstrate the
reliability of laboratory refraction scanners applying X-ray tubes.
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.
X-ray compton tomography
(2014)
The potentials of incoherent X-ray Scattering (Compton) tomography are investigated. Imaging materials of very different density or atomic number at once is a perpetual challenge for X-ray tomography or radiography, in general. In a basic laboratory set-up for simultaneous perpendicular Compton Scattering and direct beam attenuation tomographic scans are conducted by single channel photon counting. This results in asymmetric distortions of the projection profiles of the scattering CT-data set. ln a first approach corrections of Compton scattering data by taking advantage of rotational symmetry yield tomograms without major geometric artefacts.
A cylindrical sample composed of PE, PA, PVC, glass and wood demonstrates similar Compton contrast for all the substances, while the conventional absorption tomogram only reveals the two high order materials.
Compärison to neutron tomography reveals astonishing similarities except for the glass component (without hydrogen). Therefore, Compton CT bears the potential to replace neutron tomography, which requires much more efforts.
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.
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.
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.
Computed tomography reconstructions of projected data usually assume an ideal point-like
focal spot. However, in practice, the actual finite focus size leads to blurred projections,
unavoidably. The elements (voxels) of the reconstruction array are smeared differently as a
function of their individual position relative to source and detector. Neglecting the focal
smearing generates considerably blurred reconstructions. Previous attempts were based on
directly deconvolving blurred projections from a constant kernel, which yields better results
but still exhibits artefacts. Iterative algorithms can be adopted to take into account focal
smearing as a function of position. However, this requires detailed prior knowledge of
quantitative smear functions. The DIRECTT (Direct Iterative Reconstruction of Computed
Tomography Trajectories) algorithm is a promising candidate to meet these requirements. It
has been demonstrated elsewhere that DIRECTT copes with limited data sets such as
limited view and region-of- interest data by tracing single sinusoidal-like trajectories in
Radon space, which are selected from the set of all possible trajectories by appropriate
criteria. Currently, position-dependent smearing is used in the projection part of iterations.
At the example of model reconstructions we demonstrate the gain of spatial resolution by
iterative variable desmearing according to the DIRECTT algorithm in comparison to the
standard filtered back-projection.
Grating interferometric set-ups have been established in the last decade. They are promising candidates to obtain enhanced image contrast from weakly absorbing micro and nano structures. They are based on X-ray refraction and near-field diffraction using the Talbot effect.
At the expense of taking multiple images, Talbot-Lau grating interferometry allows separating the absorption, refraction, and scattering contributions by analysing the disturbances of a phase grating interference pattern. Contrary to other refraction enhanced methods, this technique can be applied using conventional X-ray tubes (divergent, polychromatic source). This makes it attractive to solve typical non-destructive testing problems.
We investigated the efficiency of phase gratings, i.e. the visibility (the amplitude of oscillations) upon variation of propagation distance and phase grating rotation around an axis parallel to the grid lines. This grating rotation changes the grating shape (i.e. the distributions of phase shifts). This can yield higher visibilities than derived from rectangular shapes.
Our study includes experimental results obtained from synchrotron radiation, as well as simulations for monochromatic radiation. The advantages of Talbot-Lau interferometry are demonstrated at the example of glass capillaries.