1,720,971 research outputs found
Enhancement of the Spatial Resolution for the Temperature Sensing System of the 7 Tesla Magnetic Resonance Imaging Scanner
No full textThe MRI scanner with an ultrahigh magnetic field of 7T not only increases the image resolution but it also increases the Specific Absorption Rate (SAR) of the patient. In other words, the body temperature of the patient increases due to the absorption of heat produced by the 7T MRI scanner. This is dangerous for the health of the patient. In order to ensure that the SAR level of the patient does not exceed the acceptable limit, the body temperature of the patient should be monitored during the scan with a spatial resolution as small as possible. This way safety measures can be taken immediately if the body temperature increases. In order to monitor the temperature during the MRI scan, fiber optic sensors (FOS) can be used. The fiber optic sensors (FOS) are immune from electromagnetic interference and there is no electrical connection to the patient and thus it is safe to monitor the temperature during an MRI scan by using FOS [1]. However, the FOS may have a spatial resolution which is not acceptable for medical purposes. This study focuses on methods to increase the spatial resolution of an existing fiber optic temperature sensing system of a 7T MRI scanner. To increase the spatial resolution of the existing temperature sensing system two methods are evaluated, namely the total variation deconvolution method and the blind deconvolution method. This study shows that the total variation deconvolution method gives the best results for the input temperature estimate. The blind deconvolution method strongly depends on the initial guess of the impulse response of the temperature sensing system, which is difficult to find. Therefore the results of the input temperature and the impulse response are less reliable when using the blind deconvolution method. Also it is shown that the machine resolution gets worse when increasing the spatial resolution by interpolating the input temperature in the Fourier domain.Electrical Engineering, Mathematics and Computer ScienceCircuits and SystemsElectrical Engineerin
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
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
Accelerating Diffusion-Weighted Chemical Shift Imaging using Compressed Sensing with Parameter Mapping
No full textDiffusion-weighted chemical shift imaging (DW-CSI) is a recently developed MRI modality that enables radiologists to reveal the diffusion properties of small molecules that act in metabolic reactions in-vivo. In order to extract this diffusion information from a patient, DW-CSI requires approximately one hour of scan time. This extensive scan time makes DW-CSI currently inapplicable for the clinical setting. This thesis describes the closely intertwined implementation of compressed sensing with parameter mapping (CS-PM) in the DW-CSI processing pipeline to accelerate its acquisition. The CS-PM algorithm enables DW-CSI to acquire less measurements (sample under Nyquist) and subsequently reconstruct the missing samples with the use of a custom designed, model-based sparsifying dictionary. As proof of concept, CS-PM was evaluated on the water signal of a non-water-suppressed DW-CSI scan. The results of the integration of CS-PM in the DW-CSI processing pipeline already indicates a feasible acceleration factor of 1.5 (scan time reduction of ca. 20 minutes) along with valuable insight to further improve the performance of DW-CSI in combination with CS-PM.Electrical Engineering, Mathematics and Computer ScienceMicroelectronicsCircuits and Systems Grou
Optimizing a RF Coil for Prostate Imaging at 7 Tesla MRI
No full textUltra-high ?eld prostate MR imaging (?7T) becomes increasingly difficult due to insufficient radio frequency ?eld excitation penetration. RF coils used at ultra-high ?elds are adapted from lower ?eld strengths but their performance is suboptimal. These coils are designed as near-?eld antennas; however, better signal penetration can be obtained using far-?eld concept, speci?cally in the case of prostate which is located deep in the body. In this project the optimization of an RF coil for prostate imaging at 7T Magnetic Resonance system is investigated. This RF coil was designed following far-?eld-based concept recently proposed by Raaijmakers et al., who has demonstrated that higher transmit B1+ at depth can be obtained; thus increasing the transmit performance necessary to acquire good images. The RF coil consists of a fractionated dipole antenna mounted on an acrylic substrate, and it is used as part of a surface array. The optimization is evaluated in terms of safety (local SAR), transmit performance (B1+), and coupling measurements. A parameters study was performed to improve the performance of the RF coil. Numerical simulations were used to evaluate new designs obtained from the parameters study. From the simulated results, three designs were constructed for validation purposes. Two different measurements were performed to validate the simulations: B1 measurements in the 7 Tesla MR scanner and coupling measurements with a network analyzer; these measurements were performed on a phantom with tissue electrical properties. Finally, an in vivo measurement using two identical sinusoidal-shaped snake antennas (s20) aligned on the pelvis was performed to image a healthy prostate. Both, simulations and measurements in the snake antennas demonstrated that despite the improvement gained in safety with lower levels of SAR, the radiation ?eld penetration pattern is less homogeneous affecting the imaging process. Therefore, these antennas might show more efficiency imaging regions of interest where safety has to be ensured such as the heart, but for prostate imaging the fractionated dipole is preferable.Biomedical EngineeringBiomedical EngineeringMechanical, Maritime and Materials Engineerin
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
Modeling Electromagnetic Fields in Strongly Inhomogeneous Media: An Application in MRI
No full textModeling electromagnetic fields in MRI involves two main challenges: the solution of the scattering problem resulting from Maxwell's equations has to be accurate and it has to be obtained within short computation time. In this thesis a method that meets both requirements is searched for. The method of moments with rooftop basis functions is used to discretize different formulations of the volume integral equation corresponding to Maxwell's equations. A simple two-layer cylinder test case is used to compare each solution with a derived analytical solution for scattering on a two-layer cylinder. Two types of errors are analyzed. The good performance of a staggered grid with respect to a non-staggered grid shows that the way of treating the mixed derivative terms is of great importance. The performance of a higher order approximation scheme on a non-staggered grid is close to the performance of a second order approximation scheme on a staggered grid. A contrast study shows that these two methods are particularly beneficial for high contrasts and on low resolution. The performance of the iterative solvers IDR(s) and GMRES is tested for each discretization method. IDR(4) shows excellent performance in reducing the computation time that is obtained with GMRES. Finally, human body simulations confirm the findings from the two-layer cylinder test case.Applied MathematicsNumerical AnalysisElectrical Engineering, Mathematics and Computer Scienc
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