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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Development and Applications of 3D Ultra-short Echo Time MRI with Rosette k-Space Pattern

Xin Shen (13105116) 15 July 2022 (has links)
<p><br></p> <p>Magnetic resonance imaging (MRI) plays an important role in providing structural information, aiding in disease diagnosis, probing neuron activities, and etc. Sampling k-space, which is the Fourier transform of the image, is a necessary step in MRI scans. The most widely used k-space sampling strategy is the Cartesian trajectories. However, novel non-Cartesian trajectories are flexible and efficient in k-space sampling, permit shorter echo time, and are insensitive to motion artifacts. The non-Cartesian k-space patterns include radial, spiral, concentric rings, rosette, and etc. Some protons restricted by the chemical environment, or other nuclei because of their nature, have short transverse relaxation times (T<sub>2</sub>). Ultra-short echo time (UTE) and zero echo time (ZTE) modalities are the promising techniques to capture the rapid decaying signals directly. The common k-space pattern for UTE and ZTE applications is the three-dimensional radial acquisition, which allows a center-out trajectory. Rosette k-space trajectory, which also allows center-out sampling, is a potential candidate for UTE purposes. In addition, it acquires more samples in the peripheral k-space for better spatial resolution, and is more incoherent to stand image quality upon undersampling than radial. However, the rosette trajectories have not yet been applied in UTE.</p> <p> </p> <p>In this study, a 3D rosette k-space trajectory designed for UTE acquisition is developed. In addition, a rosette-based magnetic resonance spectroscopic imaging (MRSI) is also developed to measure metabolites with short echo time. A comparison between 3D rosette and 3D radial UTE sequences, based on both phantom and <em>in vivo</em> scans, was performed to test the performance of the novel sequence. In addition, the 3D rosette UTE sequence was also applied in 1) myelin bilayer imaging, 2) brain iron content mapping, 3) cartilage image by sodium MRI, and 4) phosphorus MRSI. In summary, the 3D rosette k-space trajectory performs better than radial, in terms of point spread function (PSF), signal-to-noise ratio (SNR), and ability to provide structural details. Furthermore, the applications have demonstrated that 3D rosette UTE sequence is able to capture fast decaying signals.</p>
2

Ultra Short MR Relaxometry and Histological Image Processing for Validation of Diffusion MRI

Nazaran, Amin 01 May 2016 (has links)
Magnetic Resonance Imaging (MRI) is an imaging modality that acquires an image with little to no damage to the tissue. MRI does not introduce foreign particles or high energy radiation into the body, making it one of the least invasive medical imaging modalities. MRI can achieve excellent soft tissue contrast and is therefore useful for diagnosis of a wide variety of diseases. While there are a wide variety of available techniques for generating contrast in MRI, there are still many open areas for research. For example, many tissues in the human body exhibit such rapid signal decay that they are difficult to image with MRI: they are "MRI invisible". Furthermore, some of the newer MRI imaging techniques have not been fully validated to ensure that they are truly revealing accurate information about the underlying anatomical microstructure that they purport to image. This dissertation focuses on the development of new techniques in two distinct areas. First, a novel method for accurately assessing the MRI signal decay properties of tissues that are normally MRI invisible, such as tendons, ligaments, and certain pathological chemical deposits in the brain, is presented. This is termed "ultrashort MRI relaxometry". Second, two new image processing algorithms that operate on high resolution images of stained histological slices of the ex vivo brain are presented. The first of these image processing algorithms allows the semi-automated extraction of nerve fiber directionality from the histological slice images, a process that is normally done manually, is incredibly time consuming, and is prone to human error. This new technique represents one significant step in the complicated problem of attempting to validate a popular MRI technique, Diffusion Tensor Imaging (DTI), by ensuring that DTI results correlate with the true underlying physiology revealed by histological slicing and staining. The second of these image processing algorithms attempts to extract and segment regions of different "cytoarchitectonic characteristics" from stained histological slices of ex vivo brain. Again, traditional cytoarchitectonic segmentation relies on manual segmentation by an expert neuroanatomist, which is slow and sometimes inconsistent. The new technique is a first step towards automated this process, potentially providing greater accuracy and repeatability of the segmentations in a much shorter time. Together, these contributions represent a significant contribution to the body of MR imaging techniques, and associated image processing techniques for validation of newer MR neuroimaging techniques against the gold standard of stained histological slices of ex vivo brain.
3

Magnetic resonance imaging techniques for pre-clinical lung imaging / Techniques d’IRM pour l’imagerie préclinique du poumon

Bianchi, Andrea 28 March 2014 (has links)
Dans ce travail, les s´séquences Imagerie par Résonance Magnétique (IRM) radiales à temps d’écho ultra-court (UTE) sont analysées pour évaluer leur potentiel dans l’étude non-invasive de différents modèles expérimentaux de maladies pulmonaires chez la souris. Chez le petit animal, les séquences radiales UTE peuvent efficacement limiter l’impact négatif sur la qualité de l’image dû au déphasage rapide des spins causé par les nombreuses interfaces air/tissu. En plus, les séquences radiales UTE sont moins sensibles aux artefacts de mouvement par rapport aux séquences Cartésiennes classiques. En conséquence, chez le petit animal, les séquences radiales UTE peuvent permettre d’obtenir des images du poumon avec une résolution bien inférieure au millimètre avec des rapports signal/bruit importants dans le parenchyme pulmonaire, tout en travaillant en conditions physiologiques (animaux en respiration spontanée). Dans cette thèse, il sera démontré que les séquences d’IRM protonique UTE sont outils efficaces dans l’étude quantitative et non-invasive de différents marqueurs distinctifs de certaines pathologies pulmonaires d’intérêt général. Les protocoles développés serontsimples, rapides et non-invasifs, faciles à implémenter, avec une interférence minimale sur la pathologie pulmonaire étudiée et, en définitive, potentiellement applicables chez l’homme. Il sera ainsi démontré que l’emploi des agents de contraste, administrés via les voies aériennes, permet d’augmenter la sensibilité des protocoles développés. Parallèlement, dans cette thèse des protocoles suffisamment flexibles seront implémentés afin de permettre l’étude d’un agent de contraste paramagnétique générique pour des applications aux poumons. / In this work, ultra-short echo time (UTE) Magnetic Resonance Imaging (MRI) sequences are investigated as flexible tools for the noninvasive study of experimental models of lung diseases in mice. In small animals radial UTE sequences can indeed efficiently limit the negative impact on lung image quality due to the fast spin dephasing caused by the multiple air/tissue interfaces. In addition, radial UTE sequences are less sensitive to motion artifacts compared to standard Cartesian acquisitions. As a result, radial UTE acquisitions can provide lung images in small animals at sub-millimetric resolution with significant signal to noise ratio in the lung parenchyma, while working with physiological conditions (freely-breathing animals). In this thesis, UTE proton MRI sequences were shown to be efficient instruments to quantitatively investigate a number of hallmarks in longitudinal models of relevant lung diseases with minimal interference with the lung pathophysiology, employing easilyimplementable fast protocols. The synergic use of positive contrast agents, along with anadvantageous administration modality, was shown to be a valuable help in the increase of sensitivity of UTE MRI. At the same time, UTE MRI was shown to be an extremely useful and efficacious sequence for studying positive contrast agents in lungs

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