Tesi sul tema "Magnetic resonance imaging"
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1
Lee, Kuan Jin. "Fast magnetic resonance imaging". Thesis, University of Sheffield, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.397487.
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2
O'Neil, Shannon M. "Magnetic resonance imaging centers /". Online version of thesis, 1994. http://hdl.handle.net/1850/11916.
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Lu, Wenmiao. "Off-resonance correction in magnetic resonance imaging /". May be available electronically:, 2008. http://proquest.umi.com/login?COPT=REJTPTU1MTUmSU5UPTAmVkVSPTI=&clientId=12498.
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4
Manners, David Neil. "Magnetic resonance imaging and magnetic resonance spectroscopy of skeletal muscle". Thesis, University of Oxford, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.269250.
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5
Petropoulos, Labros Spiridon. "Magnetic field issues in magnetic resonance imaging". Case Western Reserve University School of Graduate Studies / OhioLINK, 1993. http://rave.ohiolink.edu/etdc/view?acc_num=case1060710667.
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6
Campbell, Jennifer 1975. "Magnetic resonance diffusion tensor imaging". Thesis, McGill University, 2000. http://digitool.Library.McGill.CA:80/R/?func=dbin-jump-full&object_id=30809.
Testo completoAbstract (sommario):
Magnetic resonance imaging (MRI) can be used to image diffusion in liquids, such as water in brain structures. Molecular diffusion can be isotropic or anisotropic, depending on the fluid's environment, and can therefore be characterized by a scalar, D, or by a tensor, D, in the respective cases. For anisotropic environments, the eigenvector of D corresponding to the largest eigenvalue indicates the preferred direction of diffusion.This thesis describes the design and implementation of diffusion tensor imaging on a clinical MRI system. An acquisition sequence was designed and post-processing software developed to create diffusion trace images, scalar anisotropy maps, and anisotropy vector maps. A number of practical imaging problems were addressed and solved, including optimization of sequence parameters, accounting for flow effects, and dealing with eddy currents, patient motion, and ghosting. Experimental validation of the sequence was performed by calculating the trace of the diffusion tensor measured in various isotropic liquids. The results agreed very well with the quantitative values found in the literature, and the scalar anisotropy index was also found to be correct in isotropic phantoms. Anisotropy maps, showing the preferred direction of diffusion, were generated in human brain in vivo. These showed the expected white matter tracts in the corpus callosum.
7
Lindsay, Alistair. "Magnetic resonance imaging of atherosclerosis". Thesis, University of Oxford, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.526491.
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Glover, Paul Martin. "High field magnetic resonance imaging". Thesis, University of Nottingham, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.335575.
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9
Yoo, Seung-Schik 1970. "Adaptive functional magnetic resonance imaging". Thesis, Massachusetts Institute of Technology, 2000. http://hdl.handle.net/1721.1/70893.
Testo completoAbstract (sommario):
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 2000.Some research performed with the Harvard-M.I.T. Division of Health Sciences and Technology.
Includes bibliographical references (leaves 132-140).
Functional MRI (fMRI) detects the signal associated with neuronal activation, and has been widely used to map brain functions. Locations of neuronal activation are localized and distributed throughout the brain, however, conventional encoding methods based on k-space acquisition have limited spatial selectivity. To improve it, we propose an adaptive fMRI method using non-Fourier, spatially selective RF encoding. This method follows a strategy of zooming into the locations of activation by progressively eliminating the regions that do not show any apparent activation. In this thesis, the conceptual design and implementation of adaptive fMRI are pursued under the hypothesis that the method may provide a more efficient means to localize functional activities with increased spatial or temporal resolution. The difference between functional detection and mapping is defined, and the multi- resolution approach for functional detection is examined using theoretical models simulating variations in both in-plane and through-plane resolution. We justify the multi-resolution approach experimentally using BOLD CNR as a quantitative measure and compare results to those obtained using theoretical models. We conclude that there is an optimal spatial resolution to obtain maximum detection; when the resolution matches the size of the functional activation. We demonstrated on a conventional 1.5-Tesla system that RF encoding provides a simple means for monitoring irregularly distributed slices throughout the brain without encoding the whole volume. We also show the potential for increased signal-to-noise ratio with Hadamard encoding as well as reduction of the in-flow effect with unique design of excitation pulses.
(cont.) RF encoding was further applied in the implementation of real-time adaptive fMRI method, where we can zoom into the user-defined regions interactively. In order to do so, real-time pulse prescription and data processing capabilities were combined with RF encoding. Our specific implementation consisted of five scan stages tailored to identify the volume of interest, and to increase temporal resolution (from 7.2 to 3.2 seconds) and spatial resolution (from 10 mm to 2.5-mm slice thickness). We successfully demonstrated the principle of the multi- resolution adaptive fMRI method in volunteers performing simple sensorimotor paradigms for simultaneous activation of primary motor as well as cerebellar areas.
by Seung-Schik Yoo.
Ph.D.
10
Eichner, Cornelius. "Slice-Accelerated Magnetic Resonance Imaging". Doctoral thesis, Universitätsbibliothek Leipzig, 2015. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-184944.
Testo completoAbstract (sommario):
This dissertation describes the development and implementation of advanced slice-accelerated (SMS) MRI methods for imaging blood perfusion and water diffusion in the human brain. Since its introduction in 1977, Echo-Planar Imaging (EPI) paved the way toward a detailed assessment of the structural and functional properties of the human brain. Currently, EPI is one of the most important MRI techniques for neuroscientific studies and clinical applications. Despite its high prevalence in modern medical imaging, EPI still suffers from sub-optimal time efficiency - especially when high isotropic resolutions are required to adequately resolve sophisticated structures as the human brain. The utilization of novel slice-acceleration methods can help to overcome issues related to low temporal efficiency of EPI acquisitions.
The aim of the four studies outlining this thesis is to overcome current limitations of EPI by developing methods for slice-accelerated MRI. The first experimental work of this thesis describes the development of a slice-accelerated MRI sequence for dynamic susceptibility contrast imaging. This method for assessing blood perfusion is commonly employed for brain tumor classifications in clinical practice. Following up, the second project of this thesis aims to extend SMS imaging to diffusion MRI at 7 Tesla. Here, a specialized acquisition method was developed employing various methods to overcome problems related to increased energy deposition and strong image distortion. The increased energy depositions for slice-accelerated diffusion MRI are due to specific radiofrequency (RF) excitation pulses. High energy depositions can limit the acquisition speed of SMS imaging, if high slice-acceleration factors are employed. Therefore, the third project of this thesis aimed at developing a specialized RF pulse to reduce the amount of energy deposition. The increased temporal efficiency of SMS imaging can be employed to acquire higher amounts of imaging data for signal averaging and more stable model fits. This is especially true for diffusion MRI measurements, which suffer from intrinsically low signal-to-noise ratios. However, the typically acquired magnitude MRI data introduce a noise bias in diffusion images with low signal-to-noise ratio. Therefore, the last project of this thesis aimed to resolve the pressing issue of noise bias in diffusion MRI. This was achieved by transforming the diffusion magnitude data into a real-valued data representation without noise bias. In combination, the developed methods enable rapid MRI measurements with high temporal efficiency. The diminished noise bias widens the scope of applications of slice- accelerated MRI with high temporal efficiency by enabling true signal averaging and unbiased model fits. Slice-accelerated imaging for the assessment of water diffusion and blood perfusion represents a major step in the field of neuroimaging. It demonstrates that cur- rent limitations regarding temporal efficiency of EPI can be overcome by utilizing modern data acquisition and reconstruction strategies.
11
Harvey, Ian. "Magnetic resonance imaging in schizophrenia". Thesis, University of Edinburgh, 1991. http://hdl.handle.net/1842/19829.
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Sharkey-Toppen, Travis P. "Imaging Iron and Atherosclerosis by Magnetic Resonance Imaging". The Ohio State University, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=osu1429796182.
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Yoshimaru, Eriko Suzanne. "Magnetic Resonance Imaging Techniques for Rodent Pulmonary Imaging". Diss., The University of Arizona, 2013. http://hdl.handle.net/10150/293388.
Testo completoAbstract (sommario):
Magnetic Resonance Imaging (MRI) is a safe and widely used diagnostic imaging method that allows in vivo observation of anatomy and characterization of tissues. MRI provides a method to monitor patients without invasive measures, making it suitable for both diagnostics and longitudinal monitoring of various pathologies. A notable example of this is the work carried out by the Alzheimer's Disease Neuroimaging Initiative (ADNI), which utilizes imaging, including multiple MRI techniques, to monitor disease progression in AD patients and evaluates treatment responses and prevention strategies. Similarly, MRI has been extensively used in evaluating diseases in a variety of animal models. In order to detect subtle anatomical changes over time, small differences in MR images must be accurately extracted. Furthermore, to ensure that the extracted differences are due to anatomical changes rather than equipment variance, it becomes essential to monitor and to assess the MRI system stability. In the first chapter of the dissertation, a method for monitoring pre-clinical MRI system performance is discussed. The technique developed during the study provides a fast and simple method to monitor pre-clinical MRI systems but also has applications for all areas of MRI. The second chapter describes the development of a 3D UTE MRI method for pulmonary imaging in freely breathing mice. The development of the 3D UTE sequence for pulmonary MRI has demonstrated its ability to collect images without noticeable motion artifacts and with appreciable signal from the lung parenchyma. Furthermore, images at two distinct respiratory phases were reconstructed from a single data set, providing functional information of the rodents' lungs. Finally, in the third chapter, 3D ¹⁹F UTE MRI is evaluated for imaging in vivo distributions of perfluorocarbon (PFC) nanoemulsions for measuring pulmonary inflammation. Building upon the development of pulmonary imaging, fluorinated contrast agents made from PFCs were used to target immune cells in response to pulmonary pathology. Both 3D ¹H and ¹⁹F UTE MRI were used to acquire pulmonary images of mouse models documented to have pulmonary pathology. Even though the mice had confirmed elevation in alveolar macrophage counts, no visible ¹⁹F signal accumulation within the pulmonary tissue was observed with MRI.
14
MA, DAN. "Magnetic Resonance Fingerprinting". Case Western Reserve University School of Graduate Studies / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=case1426170542.
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15
Lei, Hao. "Magnetic resonance perfusion imaging and double quantum coherence transfer magnetic resonance spectroscopy". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1999. http://www.collectionscanada.ca/obj/s4/f2/dsk1/tape7/PQDD_0021/NQ45007.pdf.
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16
McDougall, Mary Preston. "Single echo acquisition magnetic resonance imaging". Texas A&M University, 2004. http://hdl.handle.net/1969.1/3324.
Testo completoAbstract (sommario):
The dramatic improvement in magnetic resonance imaging (MRI) scan time over the past fifteen years through gradient-based methods that sample k-space more efficiently and quickly cannot be sustained, as thresholds regarding hardware and safety limitations are already being approached. Parallel imaging methods (using multiple receiver coils to partially encode k-space) have offered some relief in the efforts and are rapidly becoming the focus of current endeavors to decrease scan time. Ideally, for some applications, phase encoding would be eliminated completely, replaced with array coil encoding instead, and the entire image formed in a single echo.
The primary objective of this work was to explore that acceleration limit  to implement and investigate the methodology of single echo acquisition magnetic resonance imaging (SEA MRI). The initial evaluation of promising array coil designs is described, based on parameters determined by the ability to enable the imaging method. The analyses of field patterns, decoupling, and signal-to-noise ratio (SNR) that led to the final 64-channel array coil design are presented, and the fabrication and testing of coils designed for 4.7T and 1.5T are described. A detailed description of the obtainment of the first SEA images  64xNreadout images, acquired in a single echo  is provided with an evaluation of those images and highly accelerated images (through parallel imaging techniques) based on SNR and artifact power. Finally, the development of methodologies for various MR applications is described: applications that would particularly benefit from the speed of the imaging method, or those to which the method or the tool (array coil) lends itself. These applications include, but are not limited to, 3D imaging (phase encode in the slice select direction), resolution-enhanced imaging, large-scale (field-of-view) microscopy, and conformal surface imaging. Finally, using the primary enablement of the method  the ability to obtain complete MR images at speeds limited only by the time it takes to acquire a single echo  is presented with a discussion of extremely high frame rate imaging.
The contribution to the field of medical imaging is the first implementation, characterization, and demonstration of applications for the acquisition of MR images in a single echo.
17
Tymofiyeva, Olga. "Magnetic resonance imaging in dental medicine". Göttingen Sierke, 2010. http://d-nb.info/1002094976/04.
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Brown, David Gerald. "Instrumentation for parallel magnetic resonance imaging". Texas A&M University, 2005. http://hdl.handle.net/1969.1/4784.
Testo completoAbstract (sommario):
Parallel magnetic resonance (MR) imaging may be used to increase either the
throughput or the speed of the MR imaging experiment. As such, parallel imaging may
be accomplished either through a "parallelization" of the MR experiment, or by the use
of arrays of sensors. In parallelization, multiple MR scanners (or multiple sensors) are
used to collect images from different samples simultaneously. This allows for an
increase in the throughput, not the inherent speed, of the MR experiment. Parallel
imaging with arrays of sensor coils, on the other hand, makes use of the spatial
localization properties of the sensors in an imaging array to allow a reduction in the
number of phase encodes required in acquiring an image. This reduced phase-encoding
requirement permits an increase in the overall imaging speed by a factor up to the
number of sensors in the imaging array. The focus of this dissertation has been the
development of cost-effective instrumentation that would enable advances in the state of
the art of parallel MR imaging.
First, a low-cost desktop MR scanner was developed (< $13,000) for imaging small
samples (2.54 cm fields-of view) at low magnetic field strengths (< 0.25 T). The
performance of the prototype was verified through bench-top measurements and
phantom imaging. The prototype transceiver has demonstrated an SNR (signal-to-noise ratio) comparable to that of a commercial MR system. This scanner could make
parallelization of the MR experiment a practical reality, at least in the areas of small
animal research and education.
A 64-channel receiver for parallel MR imaging with arrays of sensors was also
developed. The receiver prototype was characterized through both bench-top tests and
phantom imaging. The parallel receiver is capable of simultaneous reception of up to
sixty-four, 1 MHz bandwidth MR signals, at imaging frequencies from 63 to 200 MHz,
with an SNR performance (on each channel) comparable to that of a single-channel
commercial MR receiver. The prototype should enable investigation into the speed
increases obtainable from imaging with large arrays of sensors and has already been
used to develop a new parallel imaging technique known as single echo acquisition
(SEA) imaging.
19
Hill, Richard J. "Developments in quantitative magnetic resonance imaging". Thesis, University of Surrey, 1999. http://epubs.surrey.ac.uk/843527/.
Testo completoAbstract (sommario):
Two magnetic resonance imaging studies based on relaxometry are presented. Firstly, various methods of measuring T1, T2, and flip angle are reviewed, along with various applications of relaxometry. After a study of the relevant background and theory, a method of measuring T1, T2, and the flip angle simultaneously using echo planar imaging is described, followed by a study of diffusion in a biological system employing T2 measurements. A series of echo planar images acquired with a repetition time that is short compared with the relaxation times T1 and T2 shows fluctuations in image intensity, which are dependent on these relaxation times and the flip angle. These fluctuations are best modelled using the Kaiser theory of isochromats. The Levenberg-Marquardt non-linear least squares algorithm can then be used to estimate the parameters from the data. This has been shown to work consistently in zero and one dimensions, but inconsistently in two dimensions when high gradient amplitudes affect coherence. Bacterial polysaccharides are known to exhibit a property known as anion exclusion, where the diffusion of cations is permitted, but the diffusion of anions is prevented. According to the theory of permselectivity, negatively-charged functional groups on the surfaces of pores not only block anions, but assist the diffusion of cations. The relationship between T2 and the concentration of paramagnetic species is used to follow the diffusion of Mn2+ ions through several polysaccharides. It is shown that the diffusion coefficients of Mn2+ ions are higher in neutral than in positively-charged polysaccharides, and greater still in negatively charged polysaccharides.
20
Kristoffersen, Wiberg Maria. "Magnetic resonance imaging in breast diagnosis /". Stockholm : Karolinska Univ. Press, 2002. http://diss.kib.ki.se/2002/91-7349-343-0.
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Blomqvist, Lennart. "Magnetic resonance imaging of rectal tumours /". Stockholm, 1997. http://diss.kib.ki.se/1997/91-628-2797-9.
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22
Munasinghe, B. D. Jeeva P. "Nuclear magnetic resonance imaging of mice". Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.337912.
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Potters, Kimberlee. "Magnetic Resonance Imaging of columnar reactors". Thesis, University of Cambridge, 1994. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.336775.
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Placidi, Elisa. "Magnetic resonance imaging of colonic function". Thesis, University of Nottingham, 2011. http://eprints.nottingham.ac.uk/13886/.
Testo completoAbstract (sommario):
The overall aim of this work was to develop MRI methods and techniques to study the physiology and the pathology of the gastrointestinal tract, with particular attention to the colon. Besides, the development of new methods was aimed in order to perform quantitative analysis using proton and fluorine MRI. In particular the first experimental chapter describes the development and the optimisation of imaging protocols for studying colonic function in undisturbed physiologically relevant conditions. In addition a texture analysis method based on Gabor filters is developed and used for the objective assessment of colonic content characteristics. The mechanisms of action of common anti-diarrhoeal and anti-constipation agents are also investigated. The last experimental chapter describes the development of methods for using markers to measure GI transit. Transit time, i.e. the time it takes for a marker to pass through the entire gut, is often affected by functional gastrointestinal disorders, therefore it is of primary importance to develop a non-invasive and effective technique for the diagnosis of such gastrointestinal diseases. The use of fluorinated agents and its many advantages compared to other techniques is outlined and the first in vivo studies at high field are presented. The use of gadolinium based compounds as an additional marker is also discussed.
25
Noterdaeme, Olivier. "Magnetic Resonance Imaging of the Liver". Thesis, University of Oxford, 2008. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.490296.
Testo completoAbstract (sommario):
In this thesis, we develop image analysis techniques, applied and tested in a clinical environment, to support the management of patients with (metastatic) liver cancer The incidence of this cancer is rising and represents approximately 10% of cancer cases in men and women. Image analysis of the liver is difficult, in part because it is the only organ mixing arterial and (portal) venous blood, and in part because of the large excursion it is undergoing during respiration.
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Herreros, Quentin. "Very low field magnetic resonance imaging". Phd thesis, Université René Descartes - Paris V, 2013. http://tel.archives-ouvertes.fr/tel-01064053.
Testo completoAbstract (sommario):
The aim of this thesis is to perform Magnetic Resonance Imaging at very low field (from 1 mT to 10 mT). A new kind of sensor called "mixed sensor" has been used to achieve a good detectivity at low frequencies. Combining a superconducting loop and a giant magnetoresistance, those detectors have a competitive equivalent field noise compared to existing devices (Tuned coils, SQUIDs and Atomic Magnetometers). They have been combined with flux transformers to increase the coupling between the sample and the sensor. A complete study has been performed to adapt it to mixed sensors and then maximize the gain. This set has been incorporated in an existing small MRI device to test its robustness in real conditions. In parallel, several MRI sequences (GE, SE, FLASH, EPI, ...) have been integrated and adapted to very low field requirements. They have been used to perform in-vivo three dimensional imaging and relaxometry studies on well known products to test their reliability. Finally, a larger setup adapted for full-head imaging has been designed and built to perform images on a larger working volume.
27
Häberle, Thomas [Verfasser]. "Nanoscale Magnetic Resonance Imaging / Thomas Häberle". München : Verlag Dr. Hut, 2017. http://d-nb.info/1128467259/34.
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Magill, Arthur W. "Ultra-high frequency magnetic resonance imaging". Thesis, University of Nottingham, 2007. http://eprints.nottingham.ac.uk/10740/.
Testo completoAbstract (sommario):
This thesis addresses the problem of radiofrequency probe design for Ultra High Frequency Magnetic Resonance Imaging (7T). The signal-to-noise ratio available in Magnetic Resonance Imaging (MRI) is determined by the static magnetic field strength, causing a continued drive toward higher fields to enable faster image acquisition at finer spatial resolution. The resonant frequency increases linearly with static field strength. At 7T the proton resonant frequency is 300MHz, with a wavelength of approximately 13cm in tissue. As this is smaller than the dimensions of the human head, the phase of the radiofrequency (RF) signal varies considerably across the sample, producing field cancellation due to interference. A full wave electromagnetic simulator, using the Transmission Line Matrix (TLM) method, was developed to investigate RF probes at high frequency. A Birdcage probe operating at 64, 128 and 300MHz (corresponding to 1.5, 3 and 7T) was simulated, loaded with an anatomically detailed human head model. A half-wave microstrip was investigated for use as a high frequency probe element. Magnetic and electric fields produced by a single microstrip were simulated, and the strip dimensions varied to investigate the effect on field penetration into the head and Specific Absorption Rate (SAR). A transmit-receive array probe using four microstrip elements was then developed. Bloch simulations were run, using TLM generated magnetic fields, to investigate imaging at short wavelength. Parallel receive probes are demonstrated to offer considerable advantage over volume probes, as signals from receive elements can be combined without interference. There is no transmit equivalent to parallel reception; simultaneous excitation of independent probe elements causes interference in exactly the same manner as a volume probe. A new imaging sequence was developed using a Burst-like encoding to allow sequential excitation of probe elements, without interference, which can be recalled in a single readout. An improvement in image homogeneity was demonstrated, and SENSE acceleration of the new imaging sequence is shown. The sequence was implemented at 3T using a purpose built four element microstrip probe. An RF multiplexer was also built to enable transmit element switching during the imaging sequence. It was demonstrated that images due to different RF excitations, acquired in a single EPI readout, can be separated.
29
Gibson, Andrew Mark. "High speed functional magnetic resonance imaging". Thesis, University of Nottingham, 2002. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.252032.
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Rose, Heather E. L. "Magnetic resonance imaging of flow instabilities". Thesis, University of Birmingham, 2013. http://etheses.bham.ac.uk//id/eprint/4417/.
Testo completoAbstract (sommario):
This research project investigated the formation of flow instabilities and probed this formation using Magnetic resonance imaging. The coupling of hydrodynamic instabilities was investigated with regards to four different systems. The formation of 3-dimensional viscous fingers has been investigated in a packed bed using magnetic resonance imaging. Fingering patterns are produced as a result of two different chemically reactive interfaces and one non-reactive interface. By the formation of a highly viscous wormlike micelle solution formed at the interface between solutions of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal) viscous fingers were produced under flow. Both a non-reactive and reactive system with a pre-existing viscosity gradient, which remains unchanged throughout the experiment, is studied. The structure of the fingering patterns, which were found to be sensitive to flow rate, were analysed using histogram plots characterising the distribution of pixels containing High signal fluids. We also present an alternative method for plume visualisation and direct measurement of velocities within a solution of Chlamydomonas nivalis cells. We have carried out experiments to investigate the applicability of magnetic resonance imaging to investigate behaviour in this system.
31
Cooley, Clarissa Zimmerman. "Portable low-cost magnetic resonance imaging". Thesis, Massachusetts Institute of Technology, 2014. http://hdl.handle.net/1721.1/93060.
Testo completoAbstract (sommario):
Thesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2014.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 143-153).
Purpose: As the premiere modality for brain imaging, MRI could find wider applicability if lightweight, portable systems were available for siting in unconventional locations such as intensive care units (ICUs), physician offices, surgical suites, ambulances, emergency rooms, sports facilities, or rural healthcare sites. Methods: A truly portable (<100kg) proof-of-concept MRI scanner has been constructed and validated, which replaces conventional gradient encoding with a rotating lightweight, cryogen-free, low-field magnet. When rotated about the object, an inhomogeneous magnetic field pattern is used as a rotating Spatial Encoding Magnetic field (rSEM) to create generalized projections and encode the iteratively reconstructed 2D images. Multiple receive channels are used to disambiguate the non-bijective encoding field. Results: The system is validated with experimental images of 2D test phantoms. Similar to other non-linear field encoding schemes, the spatial resolution is position dependent with blurring in the center, but this will be improved with modifications to the magnet design. Conclusion: This novel MRI scanner demonstrates the potential for portability by simultaneously relaxing the magnet homogeneity criteria and eliminating gradient coils. This new architecture and encoding scheme shows convincing proof of concept images that are expected to be further improved with refinement of the calibration and methodology.
by Clarissa Zimmerman Cooley.
Ph. D.
32
Cheng, Joseph Yitan. "Gradient characterization in magnetic resonance imaging". Thesis, Massachusetts Institute of Technology, 2007. http://hdl.handle.net/1721.1/41257.
Testo completoAbstract (sommario):
Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2007.Includes bibliographical references (p. 53).
Special magnetic resonance (MR) scans, such as spiral imaging and echo-planar imaging, require speed and gradient accuracy while putting high demands on the MR gradient system that may cause gradient distortion. Additionally, high field MR scans are prone to inhomogeneities that disturb the gradient system. Regardless of the source, gradient characterization provides a simple tool for distortion correction. An improved method, named the self-encoded slice selection algorithm, of characterizing the gradient system of the magnetic resonance system is proposed. It improves and combines the self-encode method and the direct slice selection method. The new approach is simple and fast, and allows for the measurement of waveform gradients that reach the system's limits. The technique is used to model the gradient system as a linear time-invariant transfer function through frequency-domain analysis and time-domain analysis. A transfer function model of the gradient system on the 3T Siemens Tim Trio scanner is presented here along with the characterization and analysis of common waveform gradients. Possible distortion correction approaches are also suggested.
by Joseph Yitan Cheng.
M.Eng.
33
Robson, Philip Martyn. "Magnetic resonance imaging of filter function". Thesis, University of Cambridge, 2006. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.613956.
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Graves, Martin John. "Developments in fluoroscopic magnetic resonance imaging". Thesis, University of Cambridge, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.611775.
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Richards, Jennifer Margaret Jane. "Magnetic resonance imaging in cardiovascular disease". Thesis, University of Edinburgh, 2013. http://hdl.handle.net/1842/8079.
Testo completoAbstract (sommario):
Background Superparamagnetic particles of iron oxide (SPIO) are part of a novel and exciting class of ‘smart’ magnetic resonance imaging (MRI) contrast agents that are taken up by inflammatory cells. Ultrasmall SPIO (USPIO; ~30 nm diameter) can be used to assess cellular tissue inflammation and SPIO (80-150 nm) have the potential to be used to label cells ex vivo for in vivo cell tracking studies. Objectives The aims of the thesis were therefore (i) to develop and validate quantitative MRI methodology for assessing SPIO uptake within tissues, (ii) to demonstrate USPIO accumulation within the aortic wall and its implications in patients with abdominal aortic aneurysms (AAA), and (iii) to develop and apply a Good Manufacturing Practice (GMP) compliant method of SPIO cell labelling in healthy volunteers. Methods Patients with asymptomatic AAA >4.0 cm in diameter were recruited. Imaging sequences were optimised in eight patients using a 3 tesla MRI scanner. Data were analysed using the decay constant for multi echo T2* weighted (T2*W) sequences (T2*) or its inverse (R2*) and the repeatability of these measurements was established. A further twenty-nine patients underwent MRI scanning before and 24- 36 hours after administration of USPIO. T2 and multi echo T2*W sequences were performed and ultrasound-based growth rate data were collected. Operative aortic wall tissue samples were obtained from patients undergoing open surgical aneurysm repair. A GMP compliant protocol was developed for labelling cells with SPIO for clinical cell tracking studies. The effects of SPIO-labelling on cell viability and function were assessed in vitro. A phased-dosing protocol was used to establish the safety of intravenous administration of SPIO-labelled cells in healthy volunteers. The feasibility of imaging cells at a target site in vivo following local or systemic administration was assessed. Tracking of SPIO-labelled cells to a target site was investigated by inducing an iatrogenic inflammatory focus in the skin of the anterior thigh of healthy volunteers, following which autologous SPIO-labelled cells were administered and their accumulation was assessed using MRI scanning and histology of skin biopsies. Results Robust and semi-quantitative data acquisition and image analysis methodology was developed for the assessment of SPIO accumulation in tissues. In patients with AAA, histological analysis of aortic wall tissue samples confirmed USPIO accumulation in areas of cellular inflammation. USPIO-enhanced MRI detected aortic wall inflammation and mural USPIO uptake was associated with a 3-fold higher aneurysm expansion rate. Human mononuclear cells were labelled with SPIO under GMP compliant conditions without affecting cell viability or function. Both local and intravenous administration of SPIO-labelled cells was safe and cells were detectable in vitro and in vivo using a clinical MRI scanner. SPIO-labelled cells tracked to a focal iatrogenic inflammatory focus following intravenous administration in humans and were detectable on MRI scanning and histological examination of skin biopsies. Conclusions SPIO contrast agents have an extensive range of potential clinical applications. USPIO uptake in the wall of AAA appears to identify cellular inflammation and predict accelerated aneurysm expansion. This is therefore a promising investigative tool for stratifying the risk of disease progression in patients with AAA, and may also be considered as a biomarker for response to novel pharmacological agents. The ability to label cells for non-invasive cell tracking studies would facilitate the further development of novel cell-based therapies and would enable assessment of dynamic inflammatory processes through inflammatory cell tracking.
36
Bookwalter, Candice Anne. "CONTINUOUS SAMPLING IN MAGNETIC RESONANCE IMAGING". Case Western Reserve University School of Graduate Studies / OhioLINK, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=case1194049081.
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Williams, Catherine F. M. "Diffusion-weighted magnetic resonance imaging techniques". Thesis, University of Aberdeen, 1998. http://digitool.abdn.ac.uk/R?func=search-advanced-go&find_code1=WSN&request1=AAIU602003.
Testo completoAbstract (sommario):
The aim of this project was to compare and evaluate other, non-EPI, diffusion-weighted MRI (DWI) sequences, through imaging experiments, on a phantom and in vivo, (using a 0.95 T system) and computer simulations, and to develop improved DWI methodology which could be implemented on standard hardware. Pulsed gradient spin echo (PGSE) and diffusion-weighted STEAM are slow multiple shot sequences, with measurement times of several minutes. Both sequences are highly sensitive to patient motion, but motion artifact was virtually eliminated using navigator echo phase correction and EGG triggering when diffusion-sensitisation was in the phase-encoding direction. It was demonstrated that both sequences can provide high quality images and allow accurate and straightforward diffusion-coefficient measurement when an imaging time period in the region of 20-30 minutes is available and when diffusion-sensitisation is required in one or two directions. A third direction of diffusion-sensitisation may be feasible if more sophisticated immobilisation or phase correction techniques are employed. A choice between PGSE or STEAM for a given application should take account of the Ti and T2 values of the imaged tissues, since a higher SNR might be provided by STEAM when the T1T2 ratio is high. A diffusion-weighted CE-FAST sequence was implemented with the novel modification of acquisition of a navigator gradient-echo, which was shown to reduce motion artifact when diffusion-sensitisation was in the phase-encoding direction. However, it has been demonstrated by other workers that unknown signal losses due to motion-induced phase incoherence between signal components may remain. The SNR (normalised with respect to the square root of the imaging time) in the phantom and in white matter was similar to that obtained using PGSE, but an advantage of CE- FAST is that it can be performed in a fraction of the measurement time of PGSE. Diffusion-sensitivity was much higher than in other sequences and the diffusion- attenuation was found to agree with an analysis presented in the literature. However, a major disadvantage of the technique, which precludes its use for many DWI applications, is the requirement of accurate knowledge of Ti, T2 and flip angle in order to calculate the diffusion coefficient or tensor. Prior to a study of diffusion-weighted snapshot FLASH, the effects of magnetisation evolution during snapshot FLASH acquisition on image quality and parameter measurement accuracy were first investigated, through phantom experiments and computer simulations, in the context of a r2-weighted snapshot FLASH sequence. It was demonstrated that magnetisation evolution effects can lead to significant error in parameter measurement, but that this error can be eliminated by using crusher gradients to prevent evolved magnetisation from contributing to the acquired signal. However, qualitative effects are not entirely eliminated, since a significant degree of edge blurring may remain, and there is a 50% loss of SNR inherent to the crusher gradient technique. It was then shown, theoretically and experimentally, that in diffusion-weighted snapshot FLASH, the crusher gradient technique not only addresses the problem of magnetisation evolution, but also eliminates the effect of phase shifts arising during the diffusion-preparation sequence.
38
Lohamn, Martina. "Magnetic resonance imaging in orthopedic trauma". Helsinki : University of Helsinki, 2001. http://ethesis.helsinki.fi/julkaisut/laa/kliin/vk/lohman/.
Testo completo
39
Rej, Ewa. "Hyperpolarized Nanodiamond for Magnetic Resonance Imaging". Thesis, The University of Sydney, 2016. http://hdl.handle.net/2123/15915.
Testo completoAbstract (sommario):
This thesis describes a new MRI contrast agent based on hyperpolarized 13C in nanodiamond. Nanodiamonds are readily available, non-toxic, can be surface functionalized, and have long T1 relaxation times. Electronic defects in the nanodiamonds are used to hyperpolarize the 13C nuclear spins through dynamic nuclear polarization, building up large nuclear magnetic resonance signals that persist for a long time. Enhancements are comparable to those used in liquid state hyperpolarization experiments, and can be detected in an MRI scan with low background signal. This imaging modality encompasses no ionizing radiation, and long imaging windows stemming from the long T1 relaxation times of solids.
40
Li, Zhiqiang. "Novel Body Magnetic Resonance Imaging Techniques". Diss., The University of Arizona, 2006. http://hdl.handle.net/10150/193829.
Testo completoAbstract (sommario):
Body magnetic resonance imaging (MRI) has progressed rapidly over the last 12 years. The advances in hardware and software have allowed the implementation of faster and better pulse sequences for body imaging. Despite the improvements in MRI technology there are still problems associated with current body MRI techniques that limit their diagnostic capabilities. The main goal of this work is to develop novel body MRI methods to improve the diagnosis of cardiac and abdominal pathologies. One of the goals of this work is to develop a technique to improve the detection of lipid infiltration in the heart. For this purpose an interleaved double-inversion fast spin-echo technique was developed. The method yields co-registered lipid and water images of the heart from data acquired in a single breath hold, producing data with optimal contrast between lipid and myocardium as well as minimal artifacts caused by chemical shift and blood flow.A technique combining GRAdient and Spin-Echo (GRASE) data acquisition and an iterative algorithm for lipid-water separation (Iterative Decomposition of water and fat with Echo Asymmetry and Least-squares estimation, IDEAL) was also developed. By shifting the typical GRASE data acquisition and employing correction for phase errors due to eddy currents, the IDEAL-GRASE technique achieves more time-efficient and robust lipid-water decomposition in the presence of field inhomogeneities. The technique was developed to acquire data for Cartesian and radial k-space trajectories. The radial IDEAL-GRASE with auto-correction of phase errors was developed to accomplish insensitivity to motion artifacts as well as for the generation of high resolution parametric maps (T2 and T2) for tissue characterization.A radial "variable flip angle" Steady-State Free Precession (SSFP) technique with slice profile correction was also developed to obtain fast estimation of another parameter, i.e. the T1/T2 value. This method was developed as an alternative for fast parametric imaging.These body MRI techniques were evaluated in phantoms, healthy volunteers, and patients and demonstrated for a series of applications including pelvic, cardiac, abdominal, and musculoskeletal imaging.
41
Graff, Christian George. "Parameter Estimation in Magnetic Resonance Imaging". Diss., The University of Arizona, 2009. http://hdl.handle.net/10150/195912.
Testo completoAbstract (sommario):
This work concerns practical quantitative magnetic resonance (MR) imaging techniques and their implementation and use in clinical MR systems. First, background information on MR imaging is given, including the physics of the magnetic resonance, relaxation effects and how imaging is accomplished.Subsequently, the first part of this work describes the estimation of the T2 relaxation parameter from fast spin-echo (FSE) data. Various complications are considered, including partial volume and data from multiple receiver coils along with the effects of the timing parameters on the accuracy of T2 estimates. Next, the problem of classifying small (1 cm diameter) liver lesions using T2 estimates obtained from radially-acquired FSE data collected in a single breath-hold is considered. Several algorithms are proposed for obtaining lesion T2 estimates, and these algorithms are evaluated with a task-based metric, their ability to separate two classes of lesions, benign and malignant. A novel computer-generated phantom is developed for the generation of the data used in this evaluation.The second part of this work describes techniques that perform the separation of water and lipid signals while simultaneously estimating relaxation parameters that have clinical relevance. The acquisition sequences used here are Cartesian and radial versions of Gradient and Spin-Echo (GRASE). The radial GRASE technique is post-processed with a novel algorithm that estimates the T2 of the water signal independent of the lipid signal. The accuracy of this algorithm is evaluated in phantom and its potential use for detecting inflammation of the liver is evaluated using clinical data. Cartesian GRASE data is processed to obtain T2-dagger and lipid fraction estimates in bone which can be used to assess bone quality. The algorithm is tested in phantom and in vivo, and preliminary results are given.In the concluding chapter results are summarized and directions for future work are indicated.
42
Park, Daniel Joseph. "B1 Mapping for Magnetic Resonance Imaging". BYU ScholarsArchive, 2014. https://scholarsarchive.byu.edu/etd/5750.
Testo completoAbstract (sommario):
Magnetic Resonance Imaging (MRI) is a non-ionizing form of medical imaging which has practical uses in diagnosing, characterizing, and studying diseases in vivo. Current clinical practice utilizes a highly trained radiologist to view MR images and qualitatively diagnose, characterize, or study a disease. There is no easy way to compare qualitative data. That is why developing quantitative measures in MRI show promise. Quantitative measures of disease can be compared across a population, MRI sites, and over time. Osteoarthritis is one disease where those who have it may benefit from the development of quantitative MRI measures. Those benefits may include earlier diagnosis and treatment of the disease or treatment which may halt or even reverse the damage from the disease.The work presented in this dissertation focuses on analyzing and developing new methods of radiofrequency (B1) field mapping to improve quantitative MRI measures. The dissertation opens with an introduction and a brief primer on MRI physics, followed by an introduction to B1 and flip-angle mapping in MRI (Chapters 1-3). Chapter 4 presents a careful statistical analysis of a recent and popular B1 mapping method, the Bloch-Siegert shift (BSS) method, along with a comparison of the technique to other common B1 mapping methods. The statistical models developed in chapter 4 are verified using both Monte Carlo simulation and actual MRI experiments in phantoms. Chapter 5 analyzes and details the potential errors introduced in B1 mapping when a 3D slab-selective excitation is employed. A method for correcting errors introduced by 3D slab-selective B1 mapping is then introduced in chapter 6, along with metrics to quantify the error involved. The thesis closes with a summary of other scientific contributions made by the author in chapter 7. The chapters comprising the bulk of the presented research (4-7) are briefly summarized below. Chapter 4, the statistical analysis of B1 mapping methods, demonstrates the effectiveness of deriving the B1 estimate from the phase of the MR image. These techniques are shown to perform particularly well in low signal-to-noise ratio (SNR) applications. However, there are benefits and drawbacks of each B1 mapping technique. The BSS method deposits a significant amount of radiofrequency (RF) power into the patient, causing a concern that tissue heating may occur. The Phase-Sensitive (PS) method of B1 mapping outperforms the other techniques in many situations, but suffers from significant sensitivity to off-resonance. The Dual-Angle (DA) method is very simple to implement and the analysis is straightforward, but it can introduce significant mean bias in the estimate. No B1 mapping technique performs well for all situations. Therefore, the best B1 mapping method needs to be determined for each situation. The work in chapter 4 provides guidance for that choice. Many B1 mapping techniques rely on a linear relationship between flip angle and transmit voltage. That assumption breaks down when a 3D slab-selective excitation is used. 3D slab-selective excitation is a common technique used to reduce the field-of-view (FOV) in MRI, which can directly reduce scan time. The problem with slab-selective excitation in conjunction with B1 mapping has been documented, but the potential errors in B1 estimation have never been properly analyzed across different techniques. The analysis in chapter 5 demonstrates that the errors introduced in B1 mapping using a slab-selective excitation in conjunction with the ubiquitous DA B1 mapping method can be significant. It is then shown that another B1 mapping technique, the Actual Flip Angle Imaging (AFI) method, doesn't suffer from the same limitation. The analysis presented in Chapter 6 demonstrates that some errors introduced by 3D slab-selective B1 mapping may be modeled and corrected allowing the use of 3D slab-selective excitation to reduce field-of-view, and potentially reduce scan time. The errors are modeled and corrected with a general numerical method using Bloch simulations. The general method is applied to the DA method as an example, but is general and could easily be extended to other methods as well. Finally, a set of metrics are proposed and briefly explored that can be used to better understand the topology and severity of errors introduced into B1 mapping methods. With a better understanding of the errors introduced, the need for correction can be determined. Chapter 7 details other significant ancillary contributions made by the author including: (1) presentation of a new B1 mapping method, the decoupled RF-pulse phase-sensitive B1 mapping method, which has potential for parallel transmit MRI; (2) demonstration of an ultra-short TE method which has potential for imaging Alzheimers brain lesions in vivo; (3) introduction of a new steady-state diffusion tensor imaging technique; (4) phase-sensitive B1 mapping in sodium is demonstrated, a feat not previously demonstrated; (5) a comparison between a dual-tuned and single-tuned sodium coil; (6) introduction of a water- and fat-separation technique using multiple acquisition SSFP; (7) an inter-site and inter-vendor quantitative MRI study is introduced; (8) a relaxation and contrast optimization for laryngeal imaging at 3T is introduced; and (9) diffusion imaging with insert gradients is introduced.
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Boucneau, Tanguy. "Magnetic resonance imaging of respiratory mechanics". Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS165.
Testo completoAbstract (sommario):
La fonction respiratoire chez l'homme est indissociable du mouvement de déformation du poumon : les échanges gazeux entre l'organisme et son environnement sont rendus possibles, lors de l'inspiration, par le gonflement des alvéoles du parenchyme pulmonaire, et lors de l'expiration, par un retour passif à l'état d'équilibre statique du poumon. Les propriétés viscoélastiques des tissus pulmonaires jouent un rôle clé dans la fonction même de cet organe. Ces éléments de la mécanique respiratoire pourraient être des biomarqueurs très sensibles de l'état physiopathologique du poumon puisqu'ils dépendent de la structure des tissus et des conditions biologiques qui sont considérablement altérées par la plupart des maladies pulmonaires comme le cancer, l'emphysème, l'asthme ou la fibrose interstitielle. L'imagerie par résonance magnétique permet aujourd'hui, de manière non-invasive, l'obtention d'images anatomiques tridimensionnelles permettant, grâce aux résolutions spatiales et temporelles accessibles ainsi qu'aux contrastes riches observés au sein des tissus mous, la mesure de l'état de déformation d'un organe à un instant donné. Par ailleurs, par l'application de gradients d'encodage du mouvement, l'élastographie par résonance magnétique permet de suivre, sur la phase du signal de résonance magnétique, la réponse des organes à une contrainte mécanique externe afin de révéler leurs propriétés viscoélastiques, ce qui permet d'envisager l'exploration quantitative et spatialement résolue d'organes profonds que la main du médecin ne peut atteindre. Dans le poumon, l'IRM conventionnelle est cependant relativement inadaptée : la faible densité tissulaire, les grandes différences de susceptibilité magnétique à l'interface entre le gaz et le tissu et, corrélativement, les très faibles durées de vie du signal de résonance magnétique, conduisent à des rapports signal-à-bruit difficilement exploitables. De plus, les durées des acquisitions IRM tridimensionnelles sont généralement supérieures à la période du mouvement respiratoire, ce qui nécessite de prendre en considération ce mouvement au sein du processus d'imagerie. Ce projet de thèse, réalisé en collaboration avec GE Healthcare, vise à contourner les limitations citées précédemment en s'appuyant sur des techniques d'acquisition à temps d'écho sub-milliseconde de type UTE et ZTE, associées à des approches originales et innovantes de suivi intrinsèque des mouvements physiologiques ainsi qu'à des techniques de reconstruction d'images quadridimensionnelles tenant compte à la fois du mouvement respiratoire, de la redondance de l'information entre les différents canaux d'acquisition de données et de la parcimonie des images reconstruites à travers certaines représentations mathématiques. L'objectif ultime du projet est le développement et la validation de techniques d'exploration fonctionnelle respiratoire locales et quantitatives, mais aussi d'élastographie dynamique du poumon par résonance magnétique, afin d'extraire les paramètres ventilatoires et les modules viscoélastiques de cisaillement locaux du poumon au cours du cycle respiratoireThe respiratory function in human cannot be separated from the deformation motion of the lung: the gas exchanges between the organism and its environment are made possible, during the inspiration, by the swelling of the alveoli in the pulmonary parenchyma, and during the expiration, by a passive return to the static equilibrium state of the lung. The viscoelastic properties of lung tissue play a key role in the function of this organ. These elements of respiratory mechanics may prove to be very sensitive biomarkers of the pathophysiological state of the lung since they depend on the structure of tissues and biological conditions that are considerably altered by most pulmonary diseases such as cancer, emphysema, asthma or interstitial fibrosis. Magnetic resonance imaging enables non-invasive measurement of three-dimensional anatomical images that allow, thanks to the accessible spatial and temporal resolutions as well as the rich contrasts observed in the soft tissues, the measurement of the deformation state of an organ at a given moment. Moreover, by applying motion encoding gradients, magnetic resonance elastography gives the possibility to follow, onto to the magnetic resonance phase signal, the mechanical strain response of organs to an external mechanical stress in order to reveal their viscoelastic properties, which makes possible a quantitative and spatially-resolved exploration of deep organs that are nor reachable by the medical doctor's hand. In the lung, conventional MRI is, however, relatively difficult: the low tissue density, the large differences in magnetic susceptibility at the interface between gas and tissue and, correlatively, the very short lifetimes of the magnetic resonance signal, lead to signal-to-noise ratios that are difficult to exploit. In addition, the durations of three-dimensional MRI scans are generally longer than the period of the respiratory motion, which requires consideration of this motion within the imaging process. This PhD project, carried out in collaboration with GE Healthcare, aims at circumventing the limitations mentioned above by using UTE and ZTE sub-millisecond echo-time acquisition techniques, combined with original and innovative approaches of intrinsic physiological motions monitoring as well as four-dimensional image reconstruction techniques taking into account the respiratory motion, the redundancy of information between the different data acquisition channels and the sparsity of the reconstructed images through some mathematical representations. The ultimate goal of this project is the development and the validation of local and quantitative techniques to explore the respiratory function, as well as dynamic magnetic resonance lung elastography, in order to extract local ventilation parameters and viscoelastic shear moduli in the lung during the breathing cycle
44
Lee, Inhee. "Nanoscale Ferromagnetic Resonance Imaging using Magnetic Resonance Force Microscopy". The Ohio State University, 2010. http://rave.ohiolink.edu/etdc/view?acc_num=osu1281111992.
Testo completo
45
Tang, Mei-yee. "Medical imaging : applications of functional magnetic resonance imaging and the development of a magnetic resonance compatible ultrasound system /". View the Table of Contents & Abstract, 2006. http://sunzi.lib.hku.hk/hkuto/record/B36749710.
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Spence, Dan Kenrick. "Array combination for parallel imaging in Magnetic Resonance Imaging". Texas A&M University, 2003. http://hdl.handle.net/1969.1/5944.
Testo completoAbstract (sommario):
In Magnetic Resonance Imaging, the time required to generate an image is
proportional to the number of steps used to encode the spatial information. In rapid
imaging, an array of coil elements and receivers are used to reduce the number of
encoding steps required to generate an image. This is done using knowledge of the
spatial sensitivity of the array and receiver channels. Recently, these arrays have begun
to include a large number of coil elements. Ideally, each coil element would have its
own receiver channel to acquire the image data. In practice, this is not always possible
due to economic or other constraints. In this dissertation, methods are explored to
combine a large array to a limited number of receivers so as to optimize the performance
for parallel imaging; this dissertation focuses on SENSE in particular. Simple
combinations that represent larger coils that might be constructed are discussed. More
complex solutions form current sheets. One solution uses Roemer'ÃÂÃÂs method to optimize
image SNR at a set of points. In this dissertation, Roemer's solution is generalized to
give the weighting coefficients that optimize SNR over regions. Also, solutions fitted to
ideal profiles that minimize noise amplification are shown. These fitted profiles can
allow the SENSE algorithm to function at optimal reduction factors. Finally, a
description of how to build the combiner in hardware is discussed.
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Bao, Sumi. "Clinically relevant magnetic resonance imaging and spectroscopic imaging development". Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/9133.
Testo completoAbstract (sommario):
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1999.Includes bibliographical references (p. 129-137).
As one result of this thesis, a single slab 3D fast spin echo imaging (3DFSE) method has been implemented and optimized. This involved sequence design and implementation, SAR considerations, parameter adjustments and clinical testing. The method can deliver 3D Tl or T2 weighted brain image with isotropic Imm3 voxel resolution in approximately 10 minutes. The ability to obtain high spatial resolution in reasonable time periods has wide clinical applications such as improvement of treatment planning protocols for brain tumor patients, precise radiotherapy planning, and tissue segmentation for following the progression of diseases like multiple sclerosis. The other part of this thesis is devoted to developing and implementing spectroscopic imaging methods, which include 20 chemical shift imaging(2DCSI) methods, 20 line scan spectroscopic imaging(2D LSSI) methods, spin echo planar spectroscopic imaging(SEPSI) methods and ~ingle shot line scan spin echo planar spectroscopic imaging(SSLSEPSI) method. The former two methods are applied to oil phantoms and bone marrow studies. The SEPSI method can provide simultaneous spectroscopic measurements, R2 and R2' images and field distribution images. A time domain spectral analysis method, LP-HSVD was implemented and applied to spectroscopic imaging studies. The SEPSI method was applied to get lipid characterization of bone marrow as well as to get the R2 and R2' brain images. The SSLSEPSI method can provide instant line spectroscopic imaging which might be useful to image moving objects and can provide high temporal resolution for dynamic studies. With further development, both SEPSI and SSLSEPSI methods may prove useful for trabecular bone studies as well as functional magnetic resonance imaging( tMRI) studies.
by Sumi Bao.
Ph.D.
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Edalati, Ahmadsaraei Masoud. "Diffusion Tensor Imaging: Application to Cardiovascular Magnetic Resonance Imaging". University of Cincinnati / OhioLINK, 2016. http://rave.ohiolink.edu/etdc/view?acc_num=ucin1470754609.
Testo completo
49
Bianchi, Andrea. "Magnetic resonance imaging techniques for pre-clinical lung imaging". Thesis, Bordeaux, 2014. http://www.theses.fr/2014BORD0060/document.
Testo completoAbstract (sommario):
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 poumonsIn 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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Chen, Way Cherng. "Magnetic susceptibility-based white matter magnetic resonance imaging techniques". Thesis, University of Oxford, 2013. http://ora.ox.ac.uk/objects/uuid:7272b7e6-1fb9-4a1b-a71f-2ce5dfe93fde.
Testo completoAbstract (sommario):
Gradient echo (GRE) imaging, a magnetic resonance imaging (MRI) technique that is sensitive to changes in the magnetic susceptibility property of tissues, has recently revealed significant signal heterogeneity in white matter (WM) at high magnetic field B0 ≥ 3T. Various aspects of the underlying white matter microstructure have been linked to the observed contrast between white matter regions. This thesis investigates the origins of the observed differences in GRE signal behaviour. We proposed an explicit multi-compartmental model of WM that incorporates realistic representation of the geometry and magnetic susceptibility of the underlying microstructure that can be used to study the effects of WM microstructural changes on GRE signal characteristics. In particular, we looked at the apparent transverse relaxation rate (R2*) and the resonance frequency, as well as their respective deviations from mono-exponential decay and linear phase evolution. Next, we investigated the effect of WM fiber orientation on GRE signal using healthy human volunteers at 3T by correlating the GRE signal from different WM regions with WM fiber orientation information. Using literature-based parameters, we demonstrated that the geometric model predicted similar trends. Lastly, we studied the effect of myelin on GRE signal using a cuprizone mouse model at 7T . An ex vivo study was used to correlate GRE signal in fixed mouse brain with normalized myelin stain intensity. Simulated GRE signal from hypothetical scenarios of demyelination were then compared with the experimental results. R2* and resonance frequency were then used in an in vivo longitudinal study to track myelin changes during demyelination and subsequent remyelination.