250 research outputs found

    Modifications of iterative reconstruction algorithms for the reduction of artefacts in high resolution X-ray computed tomography

    Get PDF
    X-ray Computed Tomography is a non destructive technique which allows for the visualization of the internal structure of complex objects. Most commonly, algorithms based on filtered backprojection are used for reconstruction of the projection data obtained with CT. However, the reconstruction can also be done using iterative reconstructions methods. These algorithms have shown promising results regarding the improvement of the image quality. An additional advantage is that these flexible algorithms can be modified in order to incorporate prior knowledge about the sample during the reconstruction, which allows for the reduction of artefacts. In this paper some of these advantages will be discussed and illustrated: the incorporation of an initial solution, the reduction of metal artefacts and the reduction of beam hardening artefacts

    Temporary employment and employability: training opportunities and efforts of temporary and permanent employees.

    Get PDF
    The rise of temporary employment contributes to the fact that people can no longer count on life time employment with one employer. The conclusion that life time employment within the same organisation is no longer a prerogative for all, inspires the search for new career concepts. 'Life time employability' is often put forward as an alternative to 'life time employment'. A successful career is, then, believed to be assured by having and obtaining the appropriate capacities for being continuously employable on the internal and external labour market during one's working life. At first sight, temporary employment relations and employability go hand in hand. For temporary employment is less dramatic when it is linked to a higher employability. The career opportunities of temporary workers are safeguarded by their employability. Opponents, however, add some critical observations to this statement and claim that contractual flexibility and employability enhancement are at odds. In this article, we deal with this question. If temporary employment and employability enhancing activities are at odds, temporary employees get less facilities to expand their employability. This can have important implications for the career opportunities of temps. We compare the employability enhancing activities of temporary and permanent employees. We study one central employability enhancing activity, namely training. Firstly, we have a look at the capacity and the willingness of temporary and permanent employees to participate in training in order to enhance their employability. Secondly, we also study the training opportunities that are offered by employers to temporary and permanent employees. The results indicate that, although temps do largely take responsibility for their own training, they get less opportunities to enhance their employability than permanent employeEmployment;

    3D image analysis of a volcanic deposit

    Get PDF
    During the last decades, X-ray micro CT has become a well established technique for non-destructive testing in a wide variety of research fields. Using a series of X-ray transmission images of the sample at different projection angles, a stack of 2D cross-sections is reconstructed, resulting in a 3D volume representing the X-ray attenuation coefficients of the sample. Since the attenuation coefficient of a material depends on its density and atomic number, this volume provides valuable information about the internal structure and composition of the sample. Although much qualitative information can be derived directly from this 3D volume, researchers usually require more quantitative results to be able to provide a full characterization of the sample under investigation. This type of information needs to be retrieved using specialized image processing software. For most samples, it is imperative that this processing is performed on the 3D volume as a whole, since a sequence of 2D cross sections usually forms an inadequate approximation of the actual structure. The complete processing of a volume consists of three sequential steps. First, the volume is segmented into a set of objects. What these objects represent depends on what property of the sample needs to be analysed. The objects can be for instance concavities, dense inclusions or the matrix of the sample. When dealing with noisy data, it might be necessary to filter the data before applying the segmentation. The second step is the separation of connected objects into a set of smaller objects. This is necessary when objects appear to be connected because of the limited resolution and contrast of the scan. Separation can also be useful when the sample contains a network structure and one wants to study the individual cells of the network. The third and last step consists of the actual analysis of the various objects to derive the different parameters of interest. While some parameters require extensive calculations, others can be obtained easily. The different parameters which can be obtained are related to the size, shape and orientation of the objects. Additionally, the connectivity of a network can be analysed by comparing the set of objects before and after separation. The size of each object can be characterized by its volume, equivalent diameter and the diameter of the maximum inscribed sphere. The surface can be determined by extracting a polygonal mesh from the volume data. Calculation of Feret’s diameter reveals information about the objects elongation. Additionally, the moments of inertia can be calculated to obtain the axes of an equivalent ellipsoid. This data can be used to determine the main axis and therefore the orientation of the object within the sample. Feret’s diameter and the equivalent ellipsoid are representative for the basic shape of the object. Additionally, using a routine that fills concave regions, the convex hull of an object can be retrieved to quantify the convexity. Different ratios can be defined, which compare the surface area with the volume of the object (sphericity) or the volume of the convex hull. These ratios and the convexity characterize the objects roughness and shape. The described parameters are used to characterize volcanic deposits found in the area west of Lac Pavin (lake in Auvergne, France). The samples are taken from the most recent ‘red scoria’ layer, which is believed to be the result of the latest eruption in Western-Europe. There is however, ambiguity on the origin of the layer in terms of age and placement. The aim is to fingerprint this layer in such a way that the various eruptions in the area can be distinguished from one another. Measurements of the vesicle density, volume and connectivity of the investigated deposits provide information about the intensity of the eruption. Additionally, vesicle geometry can be related to the magmatic permeability, which is essential to the dynamics of the eruption

    Latest developments in 3D analysis of geomaterials by Morpho+

    Get PDF
    At the Centre for X-ray Tomography of the Ghent University (Belgium) (www.ugct.ugent.be) besides hardware development for high-resolution X-ray CT scanners, a lot of progress is being made in the field of 3D analysis of the scanned samples. Morpho+ is a flexible 3D analysis software which provides the necessary petrophysical parameters of the scanned samples in 3D. Although Morpho+ was originally designed to provide any kind of 3D parameter, it contains some specific features especially designed for the analysis of geomaterial properties like porosity, partial porosity, pore-size distribution, grain size, grain orientation and surface determination. Additionally, the results of the 3D analysis can be visualized which enables to understand and interpret the analysis results in a straightforward way. The complementarities between high-quality X-ray CT images and flexible 3D software are opening up new gateways in the study of geomaterials

    Strategies for autonomy used by people with cervical spinal cord injury:A qualitative study

    Get PDF
    Purpose. To identify strategies used by people with high cervical spinal cord injury (SCI) to function autonomously. A multidimensional concept of autonomy was used, with four dimensions: independence, self-determination, participation and identification. Methods. Qualitative methods were used, involving literature study and semi-structured interviews with eight individuals with high SCI who had been discharged from the rehabilitation centre for several years and were members of a sports club. Results. Strategies for independence included making independent functioning a personal challenge and learning from others with SCI. Strategies for self-determination included keeping oneself informed, setting personal goals and being assertive. Strategies for participation were making challenges out of barriers, planning and organizing, asking and accepting help, and dealing with reactions from others. Strategies for identification involved taking life as it comes and focussing on positive aspects of life. Conclusions. Different strategies are necessary for different dimensions of autonomy. Some strategies seem contradictory in terms of their effects on different dimensions of autonomy. Patients can be made aware of strategies for autonomy during the rehabilitation phase
    • …