1,720,978 research outputs found
Assessing robustness and quantifying uncertainty in level set segmentation of the bowel lumen
No full textThis thesis explores various methods to assess the robustness of an algorithm used to determine the severeness of Crohn's disease, segmenting the bowel lumen by minimizing a total variation based energy functional similar to the Local Gaussian Distribution Fitting energy functional. By comparing the local solution of this minimization problem obtained using gradient descent with the global solution of a convex formulation of the optimization problem, the conclusion is made that it is definitely not the global solution that is searched for, which emphasizes the strong dependence of the algorithm on its initial condition. The dependence of the algorithm on its input parameters is also tested by slightly varying the input parameters or by artificially creating missing input data and measuring its influence on the output solution. Applications of the methods discussed in this thesis to test the robustness of the specific algorithm mentioned comprehends the justification to measure the input data less accurate, which is both cheaper and faster, the necessity of a centerline to serve as an initial condition, and a technique to determine the most important parts along the centerline, that is the parts that have the greatest influence on the output solution.Imaging Science & TechnologyApplied Science
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Prediction of response to immune checkpoint inhibitors in solid tumours using CT-based biomarkers
No full textBiomedical Engineering | Medical Physic
Dynamic B0 Inhomogeneity Correction at Low Field (50mT) using Interleaved B0 Mapping: Applied to Turbo Spin Echo and Magnetic Resonance Fingerprinting
No full textLow-field MRI is susceptible to temperature changes in magnet temperature, resulting in time-dependent inhomogeneities. This causes time-dependent phase shifts in the measured signal, leading to image distortions. In this project, a method is proposed to interleave short magnetic field (B0) mapping sequences in the target image acquisition for Turbo Spin Echo (TSE) and Magnetic Resonance Fingerprinting (MRF). Simulations show that the proposed correction method works in 2D. Experiment results show that interleaved B0 mapping can be done at relatively low spatial and temporal resolution, shortening the B0 acquisition time. Also, the distortion correction works for both TSE and MRF, though its advantages over static inhomogeneity correction are limited to acquisitions with a small bandwidth per voxel and large or very inhomogeneous frequency drifts.Biomedical Engineering | Medical Physic
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Application of Sophisticated Models to Conventional Diffusion-Weighted MRI Data
No full textThe brain’s white matter mainly consists of (myelinated) axons that connect different parts of the brain. Diffusion-weighted MRI (DW-MRI) is a technique that is particularly suited to image this white matter. The MRI signal in DW-MRI is sensitized to diffusion of water in the microstructure by introducing strong bipolar gradients in the MRI pulse sequence. By measuring the diffusion in different directions, the local diffusion profile of water molecules is obtained which reflects microstructural characteristics of the white matter.The focus of this thesis is on the analysis of conventional DW-MRI data acquired in the context of the Rotterdam Scan Study. This is a prospective population-based cohort study with more than 10.000 participants to investigate causes of neurological disease in elderly people. Conventional DW-MRI is defined as diffusion data acquired with a single diffusion-weighting factor and a small number of diffusion-sensitizing gradient orientations. The objectives of this thesis are (1) to enhance our insight in the relation between tissue structure and the DW-MRI signal from conventional DW-MRI sequences, and (2) to develop methods to quantify diffusion properties in the brain as accurately and precisely as possible based on conventional DW-MRI data.To gain insight into the relation between tissue structure and the DW-MRI signal, simulated DW-MRI signals based on Monte Carlo simulations of spins between randomly packed cylinders are compared to experimentally acquired data from a hardware phantom. The hardware phantom consists of solid fibers and acts as a model for the extra-axonal diffusion. The simulated DW-MRI signal is in good agreement with the experimentally acquired data. Furthermore, simulations show that the DW-MRI signal from spins between randomly packed cylinders is relatively independent of the cylinder diameter for b-values up to 1500 s/mm2. For b-values higher than 1500 s/mm2, substrates with a smaller cylinder diameter yield a larger attenuation of the diffusion-weighted signal (chapter 2).Conventional DW-MRI data is commonly analyzed with a technique known as diffusion tensor imaging. Here, thewater diffusion profile is modelled by a 3D Gaussian diffusion profile. However, in white matter structures in close proximity to the cerebrospinal fluid (CSF) the use of the single diffusion tensor model is inappropriate. A novel framework is introduced to analyze white matter structures adjacent to the CSF. In this framework a constrained two-compartment diffusion model is fit to the data in which the CSF is explicitly modeled with a free water diffusion compartment. The proposed diffusion statistics are shown to be relatively independent of partial volume effects with CSF and are applied to study ageing in the fornix, a small white matter structure bordering the CSF (chapter 3).A significant part of the white matter constitutes of ‘crossing fibers’, whereby two or more white matter tracts contribute to the DW-MRI signal in a voxel. The single diffusion tensor model cannot adequately describe the data in such voxels. To solve this issue a fiber orientation atlas and a model complexity atlas were used to analyze conventional DW-MRI data with a simple crossing fibers model, namely the ball-and-sticks model. It is shown that the application of a fiber orientation atlas and a model complexity atlas can significantly improve the reproducibility and sensitivity of diffusion statistics in a voxel-based analysis (chapter 4).Finally, a framework is proposed that aims to specifically improve the analysis of longitudinal DW-MRI data. In this framework the ball-and-sticks model is fit simultaneously to multiple scans of the same subject. The orientations of the sticks are constrained to be the same over different scans, while all other parameters are estimated separately for each scan. The use of this framework is shown to increase the precision of estimated ball-and-sticks model parameters in longitudinal DW-MRI studies (chapter 5).In conclusion, this thesis describes frameworks to enhance the accuracy or precision of estimated diffusion properties of the white matter by applying sophisticated diffusion models to conventional DW-MRI data. We anticipate that many diffusion MRI studies may benefit from the work described in this thesis.ImPhys/Quantitative Imagin
Analysis methods for abdominal imaging: Quantifying Crohn's desease activity
No full textImPhys/Quantitative Imagin
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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