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Tesi sul tema "Magnetic resonance"
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1
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.
Testo completo
2
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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3
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
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.
Testo completo
5
Norwood, Timothy John. "Nuclear magnetic resonance in inhomogeneous magnetic fields". Thesis, University of British Columbia, 1985. http://hdl.handle.net/2429/24875.
Testo completoAbstract (sommario):
The work described in this thesis was initiated in an attempt to overcome the limitations imposed upon NMR spectroscopy by magnetic field inhomogeneity in two specific areas: high resolution spectroscopy in isotropic liquids, and chemical shift resolved NMR imaging in isotropic liquids.
In both cases magnetic field inhomogeneity may degrade the resolution of spectra to such an extent that no useful information can be obtained from them. In high resolution NMR spectroscopy it is necessary to be able to extract accurately the parameters present within the spectrum such as chemical shifts, coupling constants and peak areas. In chemical shift resolved imaging experiments the requirements are less stringent; and it is only necessary that the resonances of different chemical species be resolved. However, even the less stringent requirements of NMR imaging are often difficult to meet as the sample volumes required are often several orders of magnitude larger than those required in conventional high resolution NMR spectroscopy.
The use of zero-quantum coherence has been investigated as a potential solution to the magnetic field inhomogeneity problem in both of these areas. Zero-quantum coherences are independent of magnetic field inhomogeneity and contain the parameters desired in both cases, though they are displayed in a way which differs from conventional NMR spectra.
In this thesis, existing zero-quantum coherence experiments have been evaluated for use with inhomogeneous magnetic fields, and, where necessary, adapted for this purpose. Several completely new experiments have been developed for producing broad-band decoupled zero-quantum coherence spectra and also for presenting coupling constants and chemical shifts in a manner which is as close to conventional NMR spectra as possible, hence facilitating ease of use. Zero-quantum coherence has been evaluated as a tool for identifying unknown compounds and also for identifying the components of complex mixtures by "signature" recognition.
Both decoupled and non-decoupled zero-quantum coherence experiments are adapted to provide imaging experiments which allow the separation of the images of different chemical species in inhomogeneous magnetic fields. The two-dimensional J-resolved experiment is also adapted for this purpose.Science, Faculty of
Chemistry, Department of
Graduate
6
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.
Testo completo
7
Cook, M. I. "Magnetic resonance in solids". Thesis, University of Oxford, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.253004.
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8
O'Connell, Andrew John. "Magnetic resonance in solids". Thesis, University of Oxford, 1992. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.305439.
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9
Lee, Kuan Jin. "Fast magnetic resonance imaging". Thesis, University of Sheffield, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.397487.
Testo completo
10
Sklar, Howard Fred. "Nuclear magnetic resonance logging". Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/10503.
Testo completoAbstract (sommario):
Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Earth, Atmospheric, and Planetary Sciences, 1997.Includes bibliographical references (leaves 119-121).
by Howard Fred Sklar.
M.S.
11
Tang, Xiao-wu 1972. "Nuclear magnetic resonance microscopy". Thesis, Massachusetts Institute of Technology, 1999. http://hdl.handle.net/1721.1/9542.
Testo completoAbstract (sommario):
Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1999.Includes bibliographical references (leaves 96-100).
This thesis describes the design and applications of an improved Nuclear Magnetic Resonance (NMR) microscope, which permits MRI to study small sample sizes ( < 2mm) at high resolution (up to 2[mu]m). The effects of molecular diffusion and local variations in the magnetic susceptibility in NMR microscopy are described, which, along with the intrinsic low sensitivity of NMR, are the fundamental limitations to resolution. Molecular diffusion in the presence of a magnetization grating not only broadens the point spread function but also reduces the signal intensity. The significance of these effects depends strongly on the magnetic field gradient strengths and imaging protocols. A NMR microscope for a standard bore 14.lT magnet was developed, it is equipped with a highly efficient. solenoidal RF coil and three orthogonal gradients with strengths of 1260G / cm for Gz , 760G/cm for Gy , and 410G/cm for Gx at 15A. A modified CTI sequence is presented which incorporates strong pulsed gradients, Ernst angle excitation, CP coherent detection and reduced k-space sampling. It is the optimal pulse sequence for acquiring high-resolution ( < 5[mu]m) NMR images (best signal-to-noise ratio per unit time) when the effect of molecular diffusion is significant. It is demonstrated that this new sequence makes it possible to acquire images with a high resolution of 2[mu]m x 2[mu]m x 8[mu]m within a few hours. A wide variety of images have been acquired using the new microscope, and representative images are presented to demonstrate the potential of NMR microscopy as a new tool in developmental biology research. In particular, used in combination with other biological techniques, NMR microscopy can provide a robust, non-invasive, 3D imaging approach to quantifying changes in structure due for instance to radiative exposure, therapy, and natural growth or genetic modifications.
by Xiao-wu Tang.
Ph.D.
12
O'Neil, Shannon M. "Magnetic resonance imaging centers /". Online version of thesis, 1994. http://hdl.handle.net/1850/11916.
Testo completo
13
Meier, Benno. "Nuclear Magnetic Resonance in pulsed high magnetic fields". Doctoral thesis, Universitätsbibliothek Leipzig, 2012. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-101205.
Testo completoAbstract (sommario):
Höchste Magnetfelder haben sich zu einem unverzichtbaren Werkzeug der Festkörperphysik entwickelt. Sie werden insbesondere verwendet, um die elektronischen Eigenschaften von modernen Materialien zu erforschen. Da Magnetfelder oberhalb von 45 Tesla nicht mehr mit statischen (z.B. supraleitenden) Feldern zu erreichen sind, haben sich weltweit verschiedene Labore auf die Erzeugung gepulster Magnetfelder mit angestrebten Maximalwerten von 100 Tesla spezialisiert.
In der vorliegenden Arbeit werden Anwendungsmöglichkeiten der kernmagnetischen Resonanz (NMR) in gepulsten Magnetfeldern aufgezeigt. Es ist gelungen, die starke Zeitabhängigkeit der gepulsten Magnetfelder mittels NMR präzise zu vermessen. Die genaue Kenntnis des Magnetfelds nach dem Puls ermöglicht, die Zeitabhängigkeit aus den Daten zu entfernen, sodass auch eine kohärente Signal-Mittelung möglich ist. Davon ausgehend werden erstmalig Messungen der chemischen Verschiebung, der Knight Shift, der Spin-Gitter-Relaxationsrate 1/T1 und der Spin-Spin-Relaxationsrate 1/T2 diskutiert.
Schließlich werden die im Zusammenhang mit gepulsten Magnetfeldern erarbeiteten Gleichungen in vereinfachter Form zur genauen Messung und Analyse des freien Induktions-Zerfalls von 19F Kernspins in Calciumfluorid verwendet. Durch Messung des Zerfalls über sechs Größenordnungen wird eine genaue Analyse bezüglich einer neuartigen Theorie ermöglicht, welche den Zerfall basierend auf der Annahme mikroskopischen Chaos\' erklärt. Diese Theorie hat das Potenzial, zu einem tieferen Verständnis von Quantenchaos in makroskopischen Vielteilchensystemen zu führen.
14
Kim, Jongjoo. "Localized Ferromagnetic Resonance using Magnetic Resonance Force Microscopy". The Ohio State University, 2008. http://rave.ohiolink.edu/etdc/view?acc_num=osu1222191966.
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15
Briand, Jacques. "Spatially localized nuclear magnetic resonance". Thesis, University of British Columbia, 1989. http://hdl.handle.net/2429/29062.
Testo completoAbstract (sommario):
The work presented in this thesis has involved the development and experimental implementation of a new method incorporating Nuclear Magnetic Resonance (NMR) methodology, and which enables a volume to be accurately defined and non-invasively interrogated within a larger object, by a sequence of radiofrequency (RF) and linear magnetic field gradient pulses.
The most important feature of the VOISINER (volume of interest by selective inversion, excitation and refocusing) sequence is its flexibility with respect to the location and size of the region of interest. The spatial coordinates and the size of the volume of interest can be directly selected from conventional NMR images and then converted into the VOISINER sequence by an appropriate setting of the radiofrequency carrier frequencies of the frequency-selective RF pulses and an appropriate scaling of the field gradient strengths used during those RF pulses. As part of the experimental protocol, the VOISINER sequence was actually combined with conventional spin echo imaging in order to facilitate the selection of the region of interest and the optimization of the spatial sensitivity profile of the localization process.
The applicability of the VOISINER sequence was then examined under various experimental conditions in order to evaluate the factors that can deteriorate or improve the efficiency of its spatial selectivity and detection sensitivity.
Potential extensions of the VOISINER technique for extracting a variety of high-resolution NMR information have been explored and experimentally demonstrated by combining it with conventional NMR methodology. In particular, it was combined with the inversion recovery method to measure on a model system, spatially localized spin-lattice (T₁) relaxation times. With regard to imaging, studies of a model system have been used to evaluate the technical prospects for using the VOISINER sequence as the basis for high-resolution imaging of small regions within a large object. Finally, to demonstrate that the technique is applicable for studies of living systems, it was tested on a human forearm and spatially localized ¹H high-resolution spectra were successfully obtained from muscle tissue and bone marrow.Science, Faculty of
Chemistry, Department of
Graduate
16
Estilaei, Mohammadreza. "Proton magnetic resonance of lung". Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape17/PQDD_0029/NQ27138.pdf.
Testo completo
17
Bergman, Harris L. "Knowledge-based magnetic resonance angiography". Diss., Georgia Institute of Technology, 1999. http://hdl.handle.net/1853/18247.
Testo completo
18
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.
19
Dick, Elizabeth Ann. "Magnetic resonance guided tumour ablation". Thesis, Imperial College London, 2003. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.404556.
Testo completo
20
Cox, A. "Magnetic resonance measurements on diamond". Thesis, University of Oxford, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.358599.
Testo completo
21
Batchelor, Stephen Norman. "Reaction yield detected magnetic resonance". Thesis, University of Oxford, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.334884.
Testo completo
22
Newton, M. E. "Magnetic resonance measurements in diamonds". Thesis, University of Oxford, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.253424.
Testo completo
23
Tronconi, Alvaro Luiz. "Magnetic resonance in crystalline solids". Thesis, University of Oxford, 1989. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.329981.
Testo completo
24
Broadhurst, R. William. "Flash photolysis nuclear magnetic resonance". Thesis, University of Oxford, 1990. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.257654.
Testo completo
25
Jardine, V. L. "Minimal preparation magnetic resonance colonography". Thesis, University of Cambridge, 2005. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.605059.
Testo completoAbstract (sommario):
Magnetic resonance colonography has the potential to combine accurate colorectal tumour detection with a minimally invasive approach that avoids potentially harmful ionising radiation. This thesis details the development and initial feasibility demonstration of a strategy for minimal preparation MR colonography. An initial review of the ‘normal’ unprepared colon was undertaken to establish the typical MR appearances. This indicated firstly that colonic intra-luminal material frequently contains foci of similar signal intensity to tumour, and secondly that combined analysis of several image contrast techniques might improve discrimination. Subsequently, several potential oral preparation strategies and imaging techniques were investigated in a trial involving healthy volunteers. Oral contrast agents and dietary manipulation significantly influenced the intra-luminal signal of the colon, potentially allowing discrimination between faeces, lesions and the bowel wall. Preparation with oral ferric ammonium citrate and a restricted high fat, low fibre diet proved optimal for faecal signal manipulation and palatability. A second healthy volunteer study evaluated potential modifications to both the preparation strategy and sequence parameters. Sequence optimisation was refined using phantom models constructed to simulate the bowel, polypoid lesions and surrounding abdominal fat. The feasibility of a minimal preparation MR technique was demonstrated in patients at high risk of colorectal cancer by comparing it to both conventional preparation CT colonography and colonoscopy. This work demonstrates the feasibility of minimal preparation MR colonography for colonic evaluation without cathartic cleansing. This technique, with further development and evaluation, could form the basis of a clinical investigation for colorectal tumour detection.
26
Lindsay, Alistair. "Magnetic resonance imaging of atherosclerosis". Thesis, University of Oxford, 2010. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.526491.
Testo completo
27
McGarry, Matthew. "Rayleigh Damped Magnetic Resonance Elastograpy". Thesis, University of Canterbury. Mechanical Engineering, 2008. http://hdl.handle.net/10092/1918.
Testo completoAbstract (sommario):
A three-dimensional, incompressible, Rayleigh damped magnetic resonance elastography (MRE) material property reconstruction algorithm capable of reconstructing the spatial distribution of both the real and imaginary parts of the shear modulus, density and bulk modulus from full-field MR-detected harmonic motion data was developed. The algorithm uses a subzone-based implementation of motion error minimization techniques, using 27 hexahedral finite elements, and is written in FORTRAN to run on high performance distributed computing systems. The theory behind the methods used is presented in a form that is directly applicable to the code's structure, to serve as a reference for future research building on this algorithm. Globally defined Rayleigh damping parameter reconstructions using simulated data showed that it is possible to reconstruct the correct combination of Rayleigh parameters under noise levels comparable to MR measurements. The elastic wave equation is used to demonstrate that use of a one parameter damping model to fit a Rayleigh damped material can lead to artefacts in the reconstructed damping parameter images, a prediction that is verified using simulated reconstructions. Initial results using MR-detected motion data from both gelatine phantoms and in-vivo cases produced good reconstructions of real shear modulus, as well as showing promise for successful imaging of damping properties. An initial investigation into an alternative elemental basis function approach to supporting the material property distribution produced some promising results, as well as highlighting some significant issues with large variations across the elements.
28
Alwesh, Nawar S. "Superresolution of magnetic resonance images". Thesis, University of Canterbury. Electrical and Electronic Engineering, 2000. http://hdl.handle.net/10092/6882.
Testo completoAbstract (sommario):
Magnetic resonance imaging (MRI) is used to image parts of the body using only electromagnetic interaction with the body's own atomic nuclei (hydrogen protons, in particular). The complex amplitude is directly measured at arbitrary locations in 2-D spatial frequency space; usually the locations are arranged in a square grid to allow reconstruction using the fast Fourier transform. The resolution of the obtained image is proportional to the number of locations and hence to the time allowed for data acquisition; and the bandlimit imposed by restricting the acquisition time is more severe in one of the coordinate directions than the other. To increase the efficiency of using expensive MR scanners and to reduce the time during which a patient must remain still, computational methods are sought to superresolve MR images.
Superresolution is commonly defined as the recovery of spatial frequency information of the object beyond the bandlimit imposed by the transfer function of the imaging system [Hunt 1995]. The well-known and straightforward Gerchberg-Papoulis (G-P) algorithm [Gerchberg 1974] was used to superresolve two sets of data, a simulated MR brain image and a phantom head image. These images were blurred by bandlimiting the spectrum (i.e. removing some of the high frequencies). Images resulting from applying the G-P algorithm were then compared to the corresponding original images. The algorithm was modified by making use of available information regarding the image, e.g. positivity, upper and lower bounds, and energy constraints, etc. The G-P algorithm and its modifications are special cases of the projection onto convex sets algorithm (POCS). The basic idea of POCS is that any prior information is used as a constraint on the image to lie in a closed convex set [Sezan and Stark 1982]. A more general form of the restoration algorithm, known as the method of generalized projections, extends the POCS method to utilizing nonconvex constraints, such as single level and neighbourhood based quantization. The different constraints were studied, and in some cases considerable improvement in the performance (as indicated by the improvement in the signal-to-noise ratio (ISNR)) and visually sharper images were achieved.
Since the superresolution problem is ill-posed [Hunt 1995] (i.e. trivial perturbation in the recorded data may lead to nontrivial perturbations in the solution), regularization methods are required to transform an ill-posed problem to a well-posed one, whose solution is an approximation to that of the ill-posed problem. In the case where no information regarding the nature of the image was assumed, the well-known Tikhonov-Miller regularization method [Tikhonov and Arsenin 1977] was used to stabilize the problem by using a penalty function that represents a bound on the energy of the image. Other regularization methods such as constraining smoothness or limiting total variation (which either smooth out the solution or preserve edges) were studied. The choice of applying these methods depends on prior knowledge of the nature of the object.
The second part of the research concentrated on recovery of undersampled images, where the noisy spectra were undersampled by multiplying them with different undersampling patterns. The algorithm, when applied on the undersampled images, in some cases resulted in useful recovery of even quite severely aliased regions. Both the determinacy of the system and the shape of the undersampling pattern used affected the amount of recovery achievable.
A study was also made on the recovery of a region of interest, which may be applied in situations where a follow-up image or dynamic imaging is required. The iterative region of interest (iROI) algorithm was presented in two versions. In the first version, information from a high resolution reference image is used to superresolve a low resolution dynamic image. This required some prior approximate knowledge of the location and extent of the region of interest. The total variation method was also used to improve the edges of the region of interest whenever accurate information regarding the region of interest was unavailable. The second version of the iROI method superresolves a low resolution dynamic image using a low resolution reference image; the resultant image is not as good as for the other version but it can be achieved with less prior knowledge.
Two other superresolving methods that utilize prior knowledge were investigated. The boxcar estimation method models the image by a series of boxcar functions with varying widths, locations and amplitudes. It results in sharp images with reduced
Gibb's oscillation when applied to piecewise images. The nonconvex level penalty
function utilizes prior knowledge of the levels present in the solution to superresolve
an image. Both methods are shown to be useful when the image genuinely contains
piecewise homogeneous regions.
29
Glover, Paul Martin. "High field magnetic resonance imaging". Thesis, University of Nottingham, 1993. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.335575.
Testo completo
30
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.
31
Ling, Yibo. "Nuclear magnetic resonance readable sensors". Thesis, Massachusetts Institute of Technology, 2010. http://hdl.handle.net/1721.1/57705.
Testo completoAbstract (sommario):
Thesis (Ph. D.)--Harvard-MIT Division of Health Sciences and Technology, 2010.Page 104 blank. Cataloged from PDF version of thesis.
Includes bibliographical references.
The monitoring of physiological biomarkers is fundamental to the diagnosis and treatment of disease. We describe here the development of molecular sensors which can be read by magnetic resonance (MR) relaxometry. MR is an advantageous bio-sensor readout because it can be determined from opaque samples and through intervening layers of matter. Wash steps can therefore be avoided in in vitro MR assays and non-invasive interrogation achieved for in vivo MR sensing. Functionalized magnetic nanoparticles originally developed as in vivo contrast agents have recently been adapted for use in magnetic relaxometry assays. The first half of this thesis describes a simple particle functionalization strategy and its application to the detection of myocardial infarction ("heart attack") associated biomarkers. The particles were subcutaneously implanted in the form of small discrete sensors and shown to be efficacious in measuring the physiological release of three protein biomarkers. Alternative contrast mechanisms may also be employed by MR readable sensors. The second half of this thesis introduces the novel use of 'smart' polymers which produce analyte-responsive changes in MR relaxivity. We show that MR responsive calcium-crosslinked and pH-swelling hydrogels can be incorporated within discrete sensors.
by Yibo Ling.
Ph.D.
32
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.
33
Harvey, Peter. "Paramagnetic probes for magnetic resonance". Thesis, Durham University, 2013. http://etheses.dur.ac.uk/7011/.
Testo completoAbstract (sommario):
Novel paramagnetic lanthanide complexes have been synthesised for use as probes for magnetic resonance imaging and spectroscopy. Initially, complexes containing a trifluoromethyl group were examined for 19F MRI/S, due to the large chemical shift range, favourable NMR properties, and the absence of a background signal in vivo. The use of paramagnetic lanthanide ions increases relaxation rates, allowing faster acquisition times and increased signal intensity per unit time. In addition, chemical shift non-equivalence is enhanced, which is important for the development of responsive probes. Examples are presented that report selectively on changes to citrate concentration levels in the presence of other anions or signal enzyme hydrolysis. To improve signal intensity for in vivo applications, conjugate systems have been developed containing a number of fluorinated lanthanide complexes covalently bound to high molecular weight adducts (glycol chitosan, PAMAM dendrimer). These systems resulted in one major species being observed in solution, despite the broad molecular weight ranges exhibited by these polymeric entities. Preliminary in vivo MRI studies in animal models were undertaken with the glycol chitosan conjugate. In order to enhance the signal further, a set of fluorinated complexes was synthesised containing a pyridyl moiety designed to increase the rigidity of the overall system, thereby reducing dynamic exchange broadening. Additionally, the 19F NMR spectra of these complexes displayed much larger lanthanide induced shifts than observed for the previous systems. By substituting the CF3 group for a tert-butyl moiety, the corresponding 1H NMR signal was shifted well beyond the standard diamagnetic range, leading to the development of a new series of complexes for use as 1H MRI and MRS probes. These complexes have been applied to preliminary in vivo MRI studies. A further system has been developed that displays pH-dependent behaviour, observed by both 1H NMR spectroscopy and in phantom imaging studies.
34
Morton, Geraint. "Cardiovascular magnetic resonance guided revascularisation". Thesis, King's College London (University of London), 2012. https://kclpure.kcl.ac.uk/portal/en/theses/cardiovascular-magnetic-resonance-guided-revascularisation(3dd5c5fe-c3a9-4287-a2e5-d2c9da4e8e27).html.
Testo completoAbstract (sommario):
Introduction: Coronary revascularisation is a key component of the management of patients with coronary artery disease (CAD). The importance of combining functional with anatomic information to select appropriate patients is increasingly recognised. Established Cardiovascular Magnetic Resonance (CMR) techniques already provide much of the relevant functional information and the absence of ionising radiation makes CMR ideal for serial examinations. However, new CMR techniques continue to emerge, and require appropriate clinical evaluation. Methods: This thesis comprises a series of clinical studies developing and evaluating techniques for guiding revascularisation: i. Comparison of a high-resolution k-t accelerated perfusion sequence with a standard sequence. ii. Validation of CMR quantification of absolute myocardial perfusion using the high-resolution sequence against PET. iii. Investigation of the relationship between an angiographic score (BCIS-1 Jeopardy score) and CMR estimations of CAD burden. iv. Determination of the inter-study reproducibility of perfusion imaging and strain analysis with CMR feature tracking (CMR-FT). v. Evaluation of a novel scar imaging technique using a dual-inversion recovery (dual-IR) pre-pulse for the first-time in patients. vi. The feasibility of combined CMR and coronary intervention in a hybrid laboratory. Results: The main findings for each compnent were: i. Perfusion imaging with the k-t accelerated sequence resulted in significantly improved image quality, signal and contrast to noise ratios and a reduction in dark rim artefacts compared to the standard sequence. ii. There was good correlation between quantitative myocardial perfusion reserve (MPR) derived from CPR and PET. CMR and PET-derived MPR were both comparable and accurate for the detection of CAD. However, absolute perfusion values from both modalities were only weakly correlated. iii. The correlation between the BCIS-1 Jeopardy score and CMR ischaemic burden was good and the score predicted a prognostically important ischaemic threshold of 12% with high specificity. iv. The inter-study reproducibility of quantitative myocardial perfusion and CMR-FT was reasonable and better for global rather than regional measures. There was no detectable variation in perfusion or strain during the day. v. The dual-IR sequence improved scar imaging compared to the IR technique. vi. Combined CMR and interventional coronary procedures were successful and well tolerated. Conclusions: Quantitative analysis of perfusion is an exciting prospect with considerable potential and has demonstrated clinically utility, however, its application remains challenging. Novel scar and strain imaging techniques have also shown promising results. Further method refinement and appropriate clinical studies should allow the full potential of these tools for guiding revascularisation to be realised.
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Miranda, João Pedro Ribeiro. "Functional studies on magnetic resonance". Master's thesis, Faculdade de Ciências e Tecnologia, 2010. http://hdl.handle.net/10362/4941.
Testo completoAbstract (sommario):
Dissertation submitted in Faculdade de Ciências e Tecnologia of Universidade Nova de Lisboa for the degree of Master in Biomedical EngineeringBackground: Magnetic Resonance Imaging (MRI) is an imaging technique used primarily to produce high quality structural and functional images of the human body. Functional MRI techniques, among which are included the Arterial Spin Labeling (ASL) and the Blood Oxygenation Level-Dependent (BOLD), are used to measure brain activity. Several studies have shown that ASL holds several advantages when compared with BOLD, namely the fact of being more reproducible and perfusion quantitative. Purpose: The main aim of this work is to obtain perfusion quantification of the human brain within several of its territories and to compare the results obtained using two different ASL protocols. Secondarily this study aimed to validate an ASL protocol to be used in clinical exams – Protocol #2 by comparing the values obtained for all the regions considered with the ones present in literature. Methods: The methodology used in this study was applied to fifteen adult volunteers. Two ASL protocols were used in a single functional imaging session. Subjects were asked to perform a motor finger tapping task with their right hand while being scanned. Images were acquired on a 3 Tesla equipment – Magnetom Verio MRI System from SIEMENS in Hospital da Luz. For the definition of the regions to study the Talairach anatomical atlas was used and the brain was segmented considering five different segmentation levels. Results: Perfusion quantification studies have demonstrated that ASL allows a correct calculation of Cerebral Blood Flow (CBF), especially when compared to other studies which used other invasive perfusion measuring techniques. The perfusion values obtained for several regions considered are in agreement with the ones available in literature. Conclusions: ASL protocols are now becoming commercially available and have been demonstrating coherent results with other techniques already established. The current study presents one of the first detailed perfusion studies using this technique to evaluate several structures of the brain. The adequacy of Protocol 2 for functional studies was also proved considering the stimulus used.
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Clarke, William. "Human cardiac magnetic resonance spectroscopy". Thesis, University of Oxford, 2016. https://ora.ox.ac.uk/objects/uuid:b75351dc-e4eb-4856-b901-4ba486ffe175.
Testo completoAbstract (sommario):
The status of the myocardial 'high energy phosphate' metabolism is a sensitive marker of the occurrence and progression of heart failure. Magnetic resonance spectroscopy enables non-invasive, direct and potentially quantitative measurements of the phosphate containing metabolites present in the human myocardium. This thesis is primarily concerned with the creation of measurement techniques for cardiac phosphorus magnetic resonance spectroscopy (31P-MRS) at the 7 tesla field strength. Chapter 1 provides an overview of the physical basis of magnetic resonance spectroscopy, the myocardial high energy phosphate metabolism, and the clinical relevance of the technique. Chapter 2 describes the advantage of 7 tesla scanners over lower field strengths. The radio frequency coil hardware is characterised experimentally. The multivoxel spectroscopy methods used throughout the thesis are described. Chapter 3 details the implementation of an open source spectroscopy fitting program. It is validated against previous closed-source implementations. The program's use is demonstrated in several clinical studies of heart failure, and to improve a previously implemented 1H spectroscopy coil combination method. In Chapter 4 the measurement of inorganic phosphate in the presence of overlapping peaks is attempted. Suppression of overlapping peaks, originating from the blood, is tried using Bo gradients, then saturation transfer. The myocardial pH of hypertrophic cardiomyopathy patients is measured. Chapter 5 describes the effect of creatine kinase catalysed chemical exchange on the 31P-MRS spectrum. A survey of methods suitable for measuring creatine kinase kinetics at 7 tesla is made. Multi-parametric fitting of variable repetition time saturation transfer data is explored in simulation and experiment. Chapter 6 describes the re-implementation and extension, for dynamic measurements, of the triple repetition time saturation transfer method for two clinical studies at 3 tesla. The creatine kinase forward rate constant is measured in heart failure and healthy cohorts, at rest, and during cardiac stress. In Chapter 7 a Bloch-Siegert B1 mapping sequence is implemented for 31P-MRS. An optimal Bloch-Siegert method for X-nuclear spectroscopy is calculated. B1maps are validated in skeletal muscle and collected in 5 volunteer's hearts. Chapter 8 uses the Bloch-Siegert B1 mapping sequence and the four angle saturation transfer method to implement creatine kinase rate measurement at 7 tesla. The first 3D localised creatine kinase rate measurements in the human myocardium are achieved in 10 volunteers.
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Elster, Lars. "Magnetic resonance in superconducting junctions". Thesis, Université Grenoble Alpes (ComUE), 2016. http://www.theses.fr/2016GREAY042/document.
Testo completoAbstract (sommario):
Dans cette thèse, on analyse la possibilité de changer un courant de charge dans des jonctions supraconductrices par une manipulation des propriétés de spin en utilisant la résonance magnétique. On considère deux jonctions différentes: Premièrement, une jonction Josephson non-conventionnelle entre un supraconducteur conventionel de type s et un supraconducteur non-conventionel de type px. Deuxièmement, une jonction entre un demi-métal et un supraconducteur conventionel. La jonction spx contient deux états liés d'Andreev qui sont 2pi-periodiques. Ils donnent lieu à une aimentation spontanée à l'équilibre. Ceci ouvre la possibilité de manipuler l'occupation des niveaux d'Andreev en utilisant un champ magnétique dépendant du temps. On demontre que ce champ induit des oscillations de Rabi cohérentes entre différents états de spin de la jonction. Ces oscillations se manifestent comme des résonances dans la relation courant-phase de la jonction. Pour un champ polarisé circulairement, on trouve une règle de sélection de spin qui autorise des oscillations de Rabi seulement dans un certain interval de phases dans la relation courant-phase permettant une éventuelle détection du spin. De plus, le champ induit des transitions non-cohérentes qui nécessitent la présence d'une quasiparticule dans le continuum d'états. Ces transitions agissent comme processus de recharge et d'ionization pour les niveaux d'Andreev. Pour un champ polarisé circulairement, ces processus induits par le champ ne donnent pas lieu à un mécanisme de relaxation pour les oscillations de Rabi à cause des contraintes en spin et en énergie. Pour un champ polarisé linéairement, il n'y a pas de règle de selection de spin et la largeur des résonances de Rabi dans la relation courant-phase est déterminée par les processus d'ionization induits par le champs. Dans la jonction entre le demi-métal et le supraconducteur conventionel, il n'y a pas de courant pour une aimentation statique, puisque la polarization parfaite en spin du demi-métal interdit les processus de réflexion d'Andreev à l'interface. On demontre que pour une géométrie de point contact, un courant d'Andreev passe, si le demi-métal est soumis à la résonance ferromagnétique. La précession de la direction de l'aimentation dans le demi-métal donne lieu au mécanisme de spin-flip nécessaire. Le courant est forcé par la précession de la direction de l'aimentation qui crée une situation hors équilibre pour les porteurs de charge. De plus, dans un matériau ferromagnétique avec une densité de porteurs minoritaires non-nulle, le courant est réduit et disparaît si les densités majoritaires et minoritaires sont égales. On considère, par ailleurs, une géométrie d'interface étendue, plus réaliste. Pour une jonction ballistique, le courant est augmenté par rapport à la géometrie de point contact, en raison du nombre plus élevé de canaux. De plus, on demontre que le désordre est le plus important dans le matériau ferromagnétique. Le courant d'Andreev à travers la jonction désordonnée est beaucoup plus grand que le courant à travers la jonction ballistique dans la même géométrieIn this thesis we investigate the possibility to change the charge current in superconducting junctions by manipulating the spin properties using magnetic resonance. We consider two different junctions: First, an unconventional Josephson junction between a conventional s-wave superconductor and an unconventional px-wave superconductor and second a half-metal/conventional superconductor junction. The spx junctions hosts two spin-polarized Andreev bound states, which are 2pi-periodic, giving rise to a spontaneous magnetization in equilibrium. This opens the possibility to manipulate the occupations of the Andreev levels using a time-dependent magnetic field. We show that the field induces coherent Rabi oscillations between different spin states of the junction that appear as resonances in the current-phase relation. For a cicularly polarized magnetic field, we find a spin selection rule, giving Rabi oscillations only in a certain range of superconducting phase differences, which provides a spin detection scheme. In contrary, for a linear polarization, there is no spin constraint on the Rabi oscillations. The field also induces non-coherent transitions including continuum states that act as refill and ionization processes for the Andreev levels. For a circularly polarized field, these field-induced processes do not provide a decay mechanism for Rabi oscillations, due to spin and energy constraints. For a linear polarization, the width of the Rabi resonances in the current-phase relation is determined by the field-induced ionization processes. In the half-metal/conventional superconductor junction no Andreev current may flow for a static magnetization direction, since the perfect spin polarization of the half-metal forbids Andreev reflection processes at the interface. We show that an Andreev current flows, if the half-metal is subject to ferromagnetic resonance. The precessing magnetization direction in the half-metal provides the necessary spin-flip mechanism. The current is driven by the precession of the magnetization direction that creates a non-equilibrium situation for the charge carriers. We also show for a point contact geometry that in a ferromagnet with non-zero minority carrier concentration the current is reduced and vanishes at equal minority and majority carrier concentrations. Additionally, we consider a more realistic, extended interface geometry. For a ballistic junction, the current is enhanced compared to a point contact geometry due to the larger number of transport channels. Furthermore, we show that disorder is most important in the ferromagnet. The Andreev current through the disordered junction is much larger than the current through a ballistic junction in the same geometry
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Jekic, Mihaela. "Exercise Stress Cardiac Magnetic Resonance". The Ohio State University, 2009. http://rave.ohiolink.edu/etdc/view?acc_num=osu1259633017.
Testo completo
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Greer, Mason. "Portable and Autonomous Magnetic Resonance". Case Western Reserve University School of Graduate Studies / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=case1586359944642158.
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40
Harvey, Ian. "Magnetic resonance imaging in schizophrenia". Thesis, University of Edinburgh, 1991. http://hdl.handle.net/1842/19829.
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Smith, Gillian Clare. "Cardiovascular magnetic resonance in cardiomyopathies". Thesis, Imperial College London, 2012. http://hdl.handle.net/10044/1/10114.
Testo completoAbstract (sommario):
Background: Cardiomyopathy (CM) frequently leads to heart failure which is associated with a high degree of mortality, morbidity and financial burden on healthcare providers. Confidence in the recognition of risk factors or in early diagnosis allows timely intervention, before heart failure develops. The right ventricle (RV) has historically been overlooked when assessing cardiac function and response to therapy but its importance is increasingly recognised. Cardiovascular magnetic resonance (CMR) is established as the gold standard for assessing the functional consequences of cardiovascular disease, being non-invasive and with high accuracy and reproducibility. It can also provide in-vivo tissue information which may be diagnostic before functional changes are apparent. With this backdrop I tested the hypothesis that CMR can provide early disease markers in patients at risk of heart failure in 3 unrelated conditions associated with heart failure including: Emery-Dreifuss muscular dystrophy (EDMD), anthracycline induced cardiomyopathy and cardiac iron overload in transfusion dependent patients. Methods and Results: Patients with 3 different clinical substrates for the development of CM were studied. In group 1, patients with EDMD were studied by CMR and echo. There was a significant reduction in inferior wall contractility using CMR tagging (-0.062±0.02 versus -0.094±0.03 in the control group, p=0.048) and in echo derived early diastolic myocardial posterior wall velocity gradients (4±1.2 vs. 7.1±2.7 s-1, p=0.02). Bi-ventricular ejection fraction (EF) was normal and no late gadolinium enhancement (LGE) was detected. These findings demonstrated the relative insensitivity of EF in the detection of early disease and the need for careful follow up in these patients. In group 2, anthracycline mediated cardiotoxicity (AMC) was studied. The risk of heart failure rises with cumulative dose but not all individuals are susceptible. In a cohort of patients with early breast cancer an increase in early gadolinium relative enhancement (EGRE) from baseline to day 3 correlated with a reduction in LVEF after a year (R=0.34, p=0.01) suggesting a potential clinical role for EGRE in the prediction of late AMC. In group 3, patients with thalassaemia major (TM) were studied. The coefficient of variance (CV) for a new black blood T2* sequence was found to be significantly lower than the white blood sequence (1.47% vs 4.23%, p<0.001) and this was adopted for clinical use. The RV response to iron chelation therapy was examined in 3 clinical trials. Using deferiprone monotherapy RVEF increased from 69.6±5.2 to 72.2±5.3% (p=0.001) with a reduction in RV end-systolic volume (ESV) from 37.7±11.7 to 34.2±11.3 mL (p=0.009). With deferiprone/deferoxamine combination therapy the RVEF increase from 60.2±7.2 to 63.8±5.9% (p<0.001) and RVESV decreased from 60.8±24.2 to 50.6±17.3 mL (p<0.01). These improvements mirrored the LV response. However, the response to deferasirox monotherapy was different with no change in LVEF, but an increase in RVEF from 66.1±6.1 to 68.8±5.4 (p=0.001) driven by an increase in RV end diastolic volume (EDV) from 69.3±19.8 to 76.1±17.1 (p<0.001) with a reduction in RV mass from 32.8±7.8 to 24.7±5.6 (p<0.001) and LV mass (78.6±16.9 to 66.5±12.9, p<0.001). These results suggest a different pharmacological action to deferiprone and deferoxamine, and indicate a possible role of RV measurements in risk assessment. Conclusions: This thesis has demonstrated that CMR can be used to identify a variety of markers of early CM, namely that: Strain abnormalities in EDMD are not associated with identifiable fibrosis in EDMD, early inflammatory changes post anthracycline exposure can predict late functional changes and that CMR provides sensitive markers of therapeutic efficacy on RV function in iron overloaded patients. These data improve our understanding of the early effects of CM and demonstrate that novel CMR techniques may play a clinically useful role in earlier detection of ventricular abnormality, and assessment of differential treatment responses.
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Hirsch, Sebastian. "Compression-sensitive Magnetic Resonance Elastography". Doctoral thesis, Humboldt-Universität zu Berlin, Mathematisch-Naturwissenschaftliche Fakultät, 2015. http://dx.doi.org/10.18452/17205.
Testo completoAbstract (sommario):
Diese Arbeit stellt das Konzept der kompressionssensitiven Magnetresonanzelastographie vor. Kompressionssensitive MRE analysiert die Ausbreitung von Kompressionswellen und liefert dadurch Erkenntnisse über die Kompressionseigenschaften eines Mediums auf Grundlage eines poroelastischen Modells. Anomalien bei der Regulation des Gewebedrucks stehen in Zusammenhang mit verschiedenen Krankheitsbildern, wie Normaldruck-Hydrozephalus und Pfortader-Hypertonie. Statischer Druck spielt als Porendruck eine zentrale Rolle in den poroelastischen Wellengleichungen; die kompressionssensitive MRE könnte daher ein nichtinvasives Diagnoseinstrument darstellen, das die durch konventionelle Scherwellen-Elastographie gewonnenen Informationen um weitere Aspekte ergänzt. Diese Arbeit beschreibt die Entwicklung einer schnellen Singleshot-EPI-Bildgebungssequenz, mit deren Hilfe die durch propagierende Druckwellen hervorgerufene volumetrische Verzerrung quantifiziert werden kann. Die Validierung der kompressionssensitiven MRE erfolgte an verschiedenen Systemen: an porösen Gelphantomen, an der menschlichen Lunge in zwei Atemzuständen, in einer ex-vivo Schafsleber bei unterschiedlichen hydrostatischen Drücken und schließlich am menschlichen Gehirn. Die Ergebnisse belegen, dass die Stärke der induzierten volumetrischen Verzerrung sensitiv gegenüber Druckänderungen ist, wohingegen die Scherverzerrung keine derartige Abhängigkeit aufweist. In einer weiteren Studie wurde intrinsische Pulsation des menschlichen Hirns anstelle einer externen Vibrationsquelle ausgenutzt. Dabei erzeugte die arterielle Pulswelle eine kurze lokale Expansion des Hirnparenchyms; in der sich anschließenden diastolischen Phase erfolgte eine langsame Rückkehr zum Ausgangszustand. Aus den gemessenen volumetrischen Verzerrungen wurden durch Inversion der Druckwellengleichung numerische Werte für den Druckwellenmodul M berechnet; Rauschen wurde als primäre Ursache für die systematische Unterschätzung von M identifiziert.This thesis introduces the concept of compression-sensitive Magnetic Resonance Elastography. Compression-sensitive MRE detects the propagation of pressure waves, providing insight into the compressibility of a material based on a poroelastic tissue model. Poroelastic models incorporate compressibility through interaction of compartments, even as each individual compartment remains incompressible. Hydrostatic tissue pressure abnormalities are associated with a number of diseases, such as normal pressure hydrocephalus or hepatic portal hypertension. Since pore pressure plays a central role in the poroelastic wave equations, compression-sensitive MRE could potentially serve as a diagnostic tool, providing information complimentary to shear-wave MRE data. This thesis describes the development of a fast single-shot EPI MR sequence capable of quantifying volumetric strain induced by external vibrations. Compression-sensitive MRE was validated in porous gel phantoms, in the human lung at two different respiratory states, in an ex vivo sheep liver at varying levels of hydrostatic pressure, and finally in human liver and brain. Results illustrate that compression-sensitive MRE is capable of quantifying volumetric strain in phantoms and in human organs. It was found that volumetric strain was sensitive toward pressure changes associated with different physiological states, whereas shear strain remained constant. In an additional study, pulsation of the human brain, driven by the heart cycle, was used as the actuation source instead of the external vibration generator. Results indicate local expansion of brain parenchyma upon the arrival of the arterial pulse wave, followed by a slow return to the initial state during the diastolic phase. Numerical values for the pressure wave modulus M were calculated from measured volumetric strain through inversion of the pressure wave equation. Measurement noise was identified as the primary effect causing a severe underestimation of M.
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Bienfait, Audrey. "Magnetic resonance with quantum microwaves". Thesis, Université Paris-Saclay (ComUE), 2016. http://www.theses.fr/2016SACLS297/document.
Testo completoAbstract (sommario):
Dans une expérience classique de résonance paramagnétique électronique (RPE), le couplage entre les spins et leur environnement électromagnétique est faible, limitant considérablement la sensibilité de la mesure. Grâce à l’utilisation combinée d'un amplificateur paramétrique Josephson et de micro-résonateurs supraconductuers de hauts facteurs de qualité refroidis à quelques millikelvins, ce travail rapporte la conception et la mise en œuvre d’un spectromètre RPE dont la sensibilité de détection est limitée par les fluctuations quantiques du champ électromagnétique au lieu d’un bruit d’origine thermique ou technique. Des mesures de RPE pulsée sur un ensemble de doneurs Bismuth dans le silicium permettent de démontrer une sensibilité de 1700 spins détectés par écho de Hahn avec un signal-sur-bruit unitaire. La sensibilité est encore améliorée en générant un état de vide comprimé dans le guide d'onde de détection, ce qui réduit les fluctuations quantiques au-delà de la limite quantique. Les hauts facteurs de qualité et le petit volume de mode du résonateur supraconducteur développés pour une sensibilité accrue accroit également le couplage spin-résonateur jusqu'au point où les fluctuations quantiques ont un effet dramatique sur la dynamique des spins. En effet, l’émission spontanée de photons dans le résonateur micro-onde est considérablement renforcée par l'effet Purcell, ce qui en fait le mécanisme de relaxation de spin dominant. Le taux de relaxation est augmenté de trois ordres de grandeur lorsque les spins sont accordés à résonance, démontrant que la relaxation de spin peut-être contrôlée sur demande. Nos résultats fournissent une méthode nouvelle et universelle pour initialiser des systèmes de spin dans leur état fondamental, avec des applications en résonance magnétique et en information quantiqueIn usual electron-spin resonance (ESR) experiments, the coupling between spins and their electromagnetic environment is quite weak, severely limiting the sensitivity of the measurements. Using a Josephson parametric microwave amplifier combined with high-quality factor superconducting micro-resonators cooled at millikelvin temperatures, this work reports the design and implementation of an ESR setup where the detection sensitivity is limited by quantum fluctuations of the electromagnetic field instead of thermal or technical noise. Pulsed ESR measurements on an ensemble of Bismuth donors in Silicon spins demonstrate a sensitivity of 1700 spins within a single Hahn echo with unit signal-to-noise (SNR) ratio. The sensitivity of the setup is improved one step further by generating squeezed vacuum in the detection waveguide, reducing the amount of noise beyond the quantum limit. The high-quality factors and small mode volume superconducting microwave ESR resonator developed for enhanced sensitivity also enhances the spin-resonator coupling up to the point where quantum fluctuations have a dramatic effect on the spin dynamics. As a consequence, the spin spontaneous emission of microwave photons in the resonator is dramatically enhanced by the Purcell effect, making it the dominant spin relaxation mechanism. The relaxation rate is increased by three orders of magnitude when the spins are tuned to resonance, showing that spin relaxation can be engineered and controlled on-demand. Our results provide a novel and general way to initialize spin systems into their ground state, with applications in magnetic resonance and quantum information processing
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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.
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Norwood, Timothy John. "Nuclear magnetic resonance in homogeneous and inhomogeneous magnetic fields". Thesis, University of Cambridge, 1988. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.304456.
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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.
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Krueger, Dirk. "Positive contrast magnetic resonance imaging with multifunctionalised magnetic nanoparticles". Thesis, King's College London (University of London), 2018. https://kclpure.kcl.ac.uk/portal/en/theses/positive-contrast-magnetic-resonance-imaging-with-multifunctionalised-magnetic-nanoparticles(37055346-b146-4133-9ee7-b836785cdc29).html.
Testo completoAbstract (sommario):
Superparamagnetic iron oxide nanoparticles (SPIONs) are used in many biomedical applications such as drug delivery, as therapeutic agent for hyperthermia treatment and contrast agent for molecular magnetic resonance imaging (MRI). Apart from the synthesis of novel SPIONs there have been technical developments such as new MRI pulse sequences for positive contrast imaging. The most prominent are GRadient-echo Acquisition for Superparamagnetic particles with Positive contrast (GRASP), Inversion Recovery with ON-resonant water suppression (IRON) and Susceptibility Gradient Mapping (SGM). Four novel SPIONs of different sizes, coatings and surface charges were assessed in terms of their application as MR contrast agents. The r₂ and r*₂ relaxivities in water and breast cancer cells were investigated in vitro and the effects on the relaxivity when SPIONs move from a free to a bound state were described. The most promising SPION was also investigated in vivo with a multi echo gradient-echo sequence and a range of positive contrast imaging techniques. For the GRASP and IRON technique a dynamic preparation phase was devised to optimise the positive contrast imaging parameters. We developed a novel positive contrast imaging technique based on the Dixon multi echo sequence for improved SPION visualisation and quantification. The Dixon positive contrast technique was compared to already established techniques such as GRASP, IRON and SGM. Finally, it was determined that fat suppression with Dixon is superior to SPIR for positive contrast imaging with the IRON technique. Overall, this work makes a contribution to the development of novel SPIONs for nanotheranostic applications. In particular, new developments for positive contrast imaging with SPIONs are presented that could further improve the usability of these techniques in future diagnostic applications.
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Nylund, Andreas. "Off-resonance correction for magnetic resonance imaging with spiral trajectories". Thesis, KTH, Medicinsk teknik, 2014. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-147925.
Testo completoAbstract (sommario):
The procedure of cardiographic magnetic resonance imaging requires patients to hold their breath for up to twenty seconds, creating an uncomfortable situation for many patients. It is proposed that an acquisition scheme using spiral trajectories is preferable due to their much shorter total scan time; however, spiral trajectories suffer from a blurring effect caused by off-resonance frequencies in the image area. There are several methods for reconstructing images with reduced blur and Conjugate Phase Reconstruction has been chosen as a method for implementation into Matlab-script for evaluation in regards to image reconstruction quality and computation time. This method finds a conjugate to the off-resonance from a field map to demodulate the image and an algorithm for frequency‑segmented Conjugate Phase Reconstruction is implemented along with an improvement called Multi-frequency Interpolation. The implementation is tested through simulation of spiral magnetic resonance imaging using a Shepp‑Logan phantom. Different off-resonance frequencies and field maps are used to provide a broad view of the functionality of the code. The two algorithms are then compared to each other in terms of computation speed and image quality. It is concluded that this implementation might reconstruct images well but that further testing on actual scan sequences is required to determine the usefulness. The Multi-frequency Interpolation algorithm yields images that are not useful in a clinical context. Further study of other methods not requiring a field map is suggested for comparison.
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Tymofiyeva, Olga. "Magnetic resonance imaging in dental medicine". Göttingen Sierke, 2010. http://d-nb.info/1002094976/04.
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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.