Comparing Image Quality in Phase Contrast subμ\mu X-Ray Tomography -- A Round-Robin Study

How to evaluate and compare image quality from different sub-micrometer (sub$\mu$) CT scans? A simple test phantom made of polymer microbeads is used for recording projection images as well as 13 CT scans in a number of commercial and non-commercial scanners. From the resulting CT images, signal and noise power spectra are modeled for estimating volume signal-to-noise ratios (3D SNR spectra). Using the same CT images, a time- and shape-independent transfer function (MTF) is computed for each scan, including phase contrast effects and image blur ($\mathrm{MTF_{blur}}$). The SNR spectra and MTF of the CT scans are compared to 2D SNR spectra of the projection images. In contrary to 2D SNR, volume SNR can be normalized with respect to the object's power spectrum, yielding detection effectiveness (DE) a new measure which reveals how technical differences as well as operator-choices strongly influence scan quality for a given measurement time. Using DE, both source-based and detector-based sub$\mu$ CT scanners can be studied and their scan quality can be compared. Future application of this work requires a particular scan acquisition scheme which will allow for measuring 3D signal-to-noise ratios, making the model fit for 3D noise power spectra obsolete

Estimation of the optimal parameters for K-edge subtraction imaging with PixiRad-2/PixieIII photon counting detector on a conventional laboratory X-ray micro-tomograph

Photon Counting Detectors (PCDs) open new opportunities in X-ray imaging. Pixie III is a PCD using simultaneously two energy thresholds. This enables to acquire images from two distinct energy bins in a single exposure. This is particularly suited to perform K-Edge Subtraction (KES) imaging with laboratory sources. In that context, one has however to deal with an energy bin optimization: narrow energy bins leads to high KES signal, at the expense of higher noise, while wider energy bins leads to poor KES signal but better statistics. This work presents a model that aims at finding the optimal thresholds and source voltage in order to retrieve the best Contrast to Noise Ratio (CNR) for a given sample. The model also optimizes the configurations for conventional absorption modality and compares both modalities. We noticed that the input flux and the energy difference between the thresholds influence the noise on image. We included this in the model using phenomenological laws. The model is then compared to empirical optimization by experimental screening of the parameters on model materials composed of barium, iodine and water. Finally, a study of the predicted CNRs has function of the sample composition is presented as an example of usage of the model

Understanding the Interdependence of Penetration Depth and Deformation on Nanoindentation of Nanoporous Silver

International audienceA silver-based nanoporous material was produced by dealloying (selective chemical etching) of an Ag 38.75 Cu 38.75 Si 22.5 crystalline alloy. Composed of connected ligaments, this material was imaged using a scanning electron microscope (SEM) and focused ion-beam (FIB) scanning electron microscope tomography. Its mechanical behavior was evaluated using nanoindentation and found to be heterogeneous, with density variation over a length scale of a few tens of nanometers, similar to the indent size. This technique proved relevant to the investigation of a material's mechanical strength, as well as to how its behavior related to the material's microstructure. The hardness is recorded as a function of the indent depth and a phenomenological description based on strain gradient and densification kinetic was proposed to describe the resultant depth dependence

Effets des hétérogénéités de microstructure sur la rupture et la ténacité des céramiques poreuses

Nous utilisons des simulations discrètes pour étudier les effets de la microstructure, à l'échelle des particules, sur les propriétés mécaniques des céramiques poreuses. Pour ces matériaux, le compromis est difficile à trouver entres propriétés fonctionnelles, qui imposent une porosité substantielle, et propriétés mécaniques. Les simulations numériques sont un outil intéressant pour proposer de nouvelles architectures microstructurales qui tentent de résoudre cette contradiction entre fonction et mécanique. Nous montrons en particulier qu'il peut être intéressant d'introduire des hétérogénéités locales pour améliorer, à fraction de porosité donnée, les propriétés mécaniques. Pour la partie expérimentale de ce travail, nous utilisons des billes de verre comme matériau modèle en raison de la taille relativement importante des billes (50-200 microns). Ceci permet un accès direct au nombre de coordination et aux tailles des ponts solides entre particules par tomographie aux rayons X . Les modules d'Young sont déterminés par ondes ultrasonores et la ténacité par des essais de flexion trois points