1,721,052 research outputs found
Investigation of maximum local specific absorption rate in 7T magnetic resonance with respect to load size by use of electromagnetic simulations
No full textLocal specific absorption rate (SAR) evaluation in ultra high field (UHF) magnetic resonance (MR) systems is a major concern. In fact, at UHF, radiofrequency (RF) field inhomogeneity generates hot-spots that could cause localized tissue heating. Unfortunately, local SAR measurements are not available in present MR systems; thus, electromagnetic simulations must be performed for RF fields and SAR analysis. In this study, we used three-dimensional full-wave numerical electromagnetic simulations to investigate the dependence of local SAR at 7.0T with respect to subject size in two different scenarios: surface coil loaded by adult and child calves and quadrature volume coil loaded by adult and child heads. In the surface coil scenario, maximum local SAR decreased with decreasing load size, provided that the RF magnetic fields for the different load sizes were scaled to achieve the same slice average value. On the contrary, in the volume coil scenario, maximum local SAR was up to 15% higher in children than in adults. Bioelectromagnetics. 36:358-366, 2015. (c) 2015 Wiley Periodicals, Inc
Quadrature birdcage coil with distributed capacitors for 7.0 T magnetic resonance data acquisition of small animals
No full textHigh static magnetic field magnetic resonance imaging (MRI) is commonly used for preclinical studies in rodents. In this context, minimization of coil losses is mandatory to scan samples that are small compared to the radiofrequency wavelength in the medium. In this study we construct a radiofrequency (RF) birdcage probe with distributed capacitors, operating in quadrature, tailored for 7.0T 1H MRI of small animals. The design eliminates the need for extra electrical components on the probe structure and affords a high SNR, a uniform B1+ field (homogeneity of 93% in the axial plain of the phantom) and a coil sensitivity of 9.8 μT/W. Feasibility experiments of mouse imaging are conducted and the competitive capability of a 7.0 T human system equipped with the proposed coil is demonstrated in both body and brain preclinical imaging
Characterization of high-resolution Gradient Echo and Spin Echo EPI for fMRI in the human visual cortex at 7 T
No full textThe increased signal-to-noise ratio (SNR) offered by functional Magnetic Resonance Imaging (fMRI) at 7T allows the acquisition of functional data at sub-millimetric spatial resolutions. However, simply reducing partial volume effects is not sufficient to precisely localize task-induced activation due to the indirect mechanisms that relate brain function and the changes in the measured signal. In this work T2* and T2 weighted Echo Planar Imaging (EPI) schemes based on Gradient Recalled Echo (GRE) and Spin Echo (SE) were evaluated in terms of temporal SNR, percent signal change, contrast to noise ratio (CNR), activation volume, and sensitivity and specificity to gray matter.Datasets were acquired during visual stimulation at in-plane resolutions ranging between 1.5 x 1.5 mm(2) and 0.75 x 0.75 mm2 targeting the early visual cortex. While similar activation foci were obtained in all acquisitions, at in-plane resolutions of 1.0 x 1.0 mm2 and larger voxel sizes the T2 weighted contrast of SE-EPI allowed the identification of the activation site with better spatial accuracy. However, at sub-millimetric resolutions the decrease in temporal SNR significantly hampered the sensitivity and the extent of the activation site. On the other hand, high resolution T2* weighted data collected with GRE-EPI provided higher CNR and sensitivity, benefiting from the decreased physiological and partial volume effects. However, spurious activations originating from regions of blood drainage were still present in GRE data, and simple thresholding techniques were found to be inadequate for the removal of such contributions. The combination of 2-class and 3-class automated segmentations, performed directly in EPI space, allowed the selection of active voxels in gray matter. This approach could enable GRE-EPI to accurately map functional activity with satisfactory CNR and specificity to the true site of activation. (C) 2017 Elsevier Inc. All rights reserved
Cortical neuronal loss and hippocampal sclerosis are not detected by voxel-based morphometry in individual epilepsy surgery patients
No full textVoxel-based morphometry (VBM) has detected differences between brains of groups of patients
with epilepsy and controls, but the sensitivity for detecting subtle pathological changes in single subjects
has not been established. The aim of the study was to test the sensitivity of VBM using statistical parametric
mapping (SPM5) to detect hippocampal sclerosis (HS) and cortical neuronal loss in individual patients.
T1-weighted volumetric 1.5 T MR images from 13 patients with HS and laminar cortical neuronal loss
were segmented, normalised and smoothed using SPM5. Both modulated and non-modulated analyses
were performed. Comparisons of one control subject against the rest (n ¼ 23) were first performed to ascertain
the smoothing level with the lowest number of SPM changes in controls. Each patient was then compared
against the whole control group. The lowest number of SPM changes in control subjects was found
at a smoothing level of 10 mm full width half maximum for modulated and non-modulated data. In the
patient group, no SPM abnormalities were found in the affected temporal lobe or hippocampus at this
smoothing level. At lower smoothing levels there were numerous SPM findings in controls and patients.
VBM did not detect any abnormalities associated with either laminar cortical neuronal loss or HS. This
may be due to normalisation and smoothing of images and low statistical power in areas with larger interindividual
differences. This suggests that the methodology may currently not be suitable to detect particular
occult abnormalities possibly associated with seizure onset zone in individual epilepsy patients with
unremarkable standard structural MRI
Translation of Novel Imaging Techniques into Clinical Use for Patients with Epilepsy
Get PDFTemporal lobe epilepsy is the most common focal epilepsy. Up to 40% of patients are refractory to medication. Anterior temporal lobe resection (ATLR) is an effective treatment but damage to the optic radiation can result in a visual field deficit (VFD) that precludes driving, a key goal of surgery. Diffusion tensor imaging tractography allows the in vivo delineation of white matter tracts such as the optic radiation. This thesis addresses the role of optic radiation tractography in planning and subsequently improving the safety of epilepsy surgery. I show how tractography assists risk stratification and surgical planning in patients with lesions near the optic radiation and assess the utility of different tractography methods for surgical planning. To derive the greatest benefit, tractography information should be available during surgery which requires correction for intraoperative brain shift and other sources of image distortion. I apply software developed at UCL in a clinical population underlying ATLR to show that postoperative imaging can predict the VFD and then use this software in real time during surgery in an intraoperative MRI suite. Updated anatomical scans can be acquired during surgery and tractography data accurately mapped on to these and displayed on the operating microscope display. I demonstrate that this image guidance allows the neurosurgeon to avoid significant VFD without affecting the seizure outcome. Diffusion imaging can also probe tissue microstructure. I explore how structural changes within the frontoparietal working memory network and temporal lobes are related to working memory impairment in TLE. I describe the structural changes that occur following ATLR showing both Wallerian degeneration and structural plasticity. Finally, I show how a novel diffusion model (NODDI) could aid the clinical assessment of patients with focal cortical dysplasia. The emphasis throughout this thesis is how diffusion imaging can be clinically useful and address clinically relevant outcomes
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
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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