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- Englisch (22) (entfernen)
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- Computed tomography (22) (entfernen)
A novel approach is suggested to strongly suppress artifacts in radiography and computed tomography caused by the effect of diffuse background signals 'backlighting' of 2D X-ray detectors. Depending on the detector geometry the mechanism may be different. Either based on the optical scattering of the fluorescent screen materials into the 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 Beer's law in single projections (radiographs) as a 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.
Analyser-based imaging expands the performance of X-ray imaging by utilizing not only the absorption properties of X-rays but also the refraction and scatter rejection (extinction) properties. In this study, analyser-based computed tomography has been implemented on imaging an articular cartilage sample, depicting substructural variations, without overlay, at a pixel resolution of 3.6 µm.
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)
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.