1,720,959 research outputs found
Quantitative Magnetic Resonance Imaging Methodology Development
Full text linkMagnetic Resonance Imaging (MRI) is a non-invasive medical imaging modality that provides excellent soft tissue contrast and resolution. Objects with high magnetic susceptibility distort the magnetic field, leading to severe artifacts in conventional MRI. It is very challenging to image around metal implants. Novel strategies may exploit the field distortion for spatial encoding. The magnetic field map is required in the development of these methods. A robust field map can also be employed to quantify high susceptibility particles that play a major role in cell tracking studies and hyperthermic treatment of cancers. Pure phase encoding (PPE) techniques with short encoding times are largely immune to magnetic field inhomogeneity artifacts. Artifact-free MR images around titanium were acquired with PPE techniques, from which the magnetic field distribution was derived. The approach was extended to quantify iron microparticles and was compared with conventional MRI to demonstrate its superiority
SPRITE MRI for Prosthetic Implant Imaging
No full textArthroplasty is the process by which a damaged joint in the human body is repaired or replaced by non-magnetic metallic implants. Areas of repair include the knees, the hips and the shoulders. It involves the insertion of a metal ball to replace the joint and a plastic socket to fit to the bone for growth and acclimatization. Despite recent advances in arthroplasty technology, implants fail over time leading to pain, discomfort and possible further injury in patients. Diagnosing areas of failure in an implant is required for identification and reparation of the damaged site. Magnetic Resonance Imaging (MRI) is an imaging modality that applies magnetic fields to non-invasively and safely image human anatomy. Metal implants distort the magnetic field, resulting in severe image artifacts. It is not possible to acquire diagnostic quality MRI images in the vicinity of metal implants with traditional methods. We propose to employ SPRITE MRI techniques, which are immune to the local magnetic field inhomogeneity, to acquire high quality distortion-free images. The method has been demonstrated in phantom measurements and compared to standard techniques. SPRITE MRI can be applied to other systems that are challenging for traditional MRI
Spatially Resolved Magnetic Resonance Spectroscopy
No full textIn nuclear magnetic resonance (NMR), the hydrogen 1H signal carries a certain frequency proportional to the static magnetic field strength. Each chemical bond induces a unique shift in the local magnetic field, which can be differentiated in the signal. This forms the basis behind NMR spectroscopy, which allows scientists to identify molecular compounds. A highly uniform static magnetic field is crucial so that the unique shifts can be purely attributed to the sample property. In a large and complicated biological system, such as a human brain, numerous chemical compounds exist. The various chemical shift peaks overlap in a bulk measurement, and individual components cannot be identified. It is also extremely challenging to achieve static magnetic field homogeneity for an extended sample size, which results in a broader spectrum, further confounding the results. To solve this, MR imaging techniques are used to acquire localized NMR spectra. Each individual pixel has less chemical complexity and the field inhomogeneity is less severe. Therefore, high quality spectra can be acquired. The broadened spectrum problem is tested and a solution in the form of MR spectroscopy is presented. Phantoms with multiple chemical shifts are also tested and compared using NMR and MR spectroscopy. Extending the capability of NMR spectroscopy to large biological systems will bring new insights in the study of biofunctions and diseases
Restricted Diffusion Measurements with Magnetic Resonance
No full textIn fluids, molecular movement is modelled as random translational motion, known as diffusion. The diffusion coefficient is a measure of the distance a particle travels within a certain amount of time and is related to the fluid property in an uninhibited environment. When the fluid is constrained in a space smaller than the distance it could travel, the inhibited diffusion coefficient, known also as the apparent diffusion coefficient (ADC), is lower. Therefore, the microscopic structure can be inferred by quantitatively analyzing the ADC of the fluid. Magnetic Resonance Imaging (MRI) acquires signal from water in the human body. MRI is well known for its rich soft tissue contrast. It is less well known that quantitative diffusion information could be acquired with magnetic resonance. MR is also capable of measuring the apparent diffusion coefficient in any spatial direction non-invasively, providing insights into the water's microscopic environment. The effect of the diffusion on the signal can be tuned to acquire a set of data points for different diffusion sensitivities. At higher ADC values, the signal amplitude is lower and vice versa. The ADC value can be calculated via data processing. The technique has been demonstrated with a phantom experiment, where restricted diffusion occurred. ADCs in different directions of the phantom were measured. With the knowledge of the ADC values in different directions, the dimensions of the phantom were estimated using Einstein's simple particle diffusion model. This technique can be extended to other systems to non-invasively determine their internal microstructures
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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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