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

Magnetic structure in manganites as probed by magnetic force microscopy

Israel, Emil Casey 28 August 2008 (has links)
Not available / text

In vivo MRI investigation of liver in normal and fibrotic stage

Gao, Shan, 高珊 January 2011 (has links)
published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

Advanced magnetic resonance spectroscopy techniques and applications

Cao, Peng, 曹鹏 January 2013 (has links)
Magnetic resonance (MR) is a well-known non-invasive technique that provides spectra (by MR spectroscopy, MRS) and images (by magnetic resonance imaging, MRI) of the examined tissue with detailed metabolic, structural, and functional information. This doctoral work is focused on advanced methodologies and applications of MRS for probing cellular and molecular changes in vivo. A single-voxel diffusion-weighted (DW) MRS method was first developed for monitoring the size changes of intramyocellular lipid droplets in vivo. This DWMRS technique was then utilized for exploring the vascular origins of the functional blood-oxygen-level-dependent (BOLD) signal. Magnetic resonance spectroscopic imaging (MRSI) enables simultaneous MRS acquisition in multiple voxels. However, MRSI is conventionally time-consuming. Therefore, a compressed sensing (CS) method was proposed in this thesis to accelerate the acquisition speed of the in vivo MRSI. It holds the potential for promoting the realization of multiple-voxel DW-MRS experiments, though the latter is still constrained by hardware in the present. The single-voxel DW-MRS method for probing lipid diffusion was first developed and evaluated in oil and muscle phantoms. The experimental sequence was demonstrated to be sensitive to diffusion restriction and free of significant artifacts. Experiments were then performed in rat hindlimb muscles in vivo. The restricted lipid diffusion behavior was characterized by apparent diffusion coefficient (ADC) changes and utilized for quantifying the sizes of intramyocellular lipid (IMCL) droplets in normal, fasting, diabetic and obese rats. The sizes of IMCL droplets reflect their vital roles in muscle energy metabolism. The IMCL droplet size estimated by ADC here was closely correlated with that measured by transmission electron microscopy. IMCL ADC was sensitive to metabolic alterations, decreasing in the fasting and diabetic groups while increasing in the obese group. These results clearly demonstrate DW MRS as a new means to examine the dynamics of IMCL metabolism in vivo. The DW-MRS technique was then utilized to characterize water ADC during neuronal activation to explore the vascular origins of the BOLD signal in rat brains. MRS experiments with acoustic stimulation were performed with a dynamic point-resolved spectroscopy (PRESS) acquisition on conditions with or without the diffusion gradient for blood suppression in the same voxel and same experimental session, which enabled the simultaneous T2/T2*/diffusion measurements. The T2*% changes with and without diffusion gradient showed no significant difference, while the spin echo (SE)-BOLD% (T2%) change significantly decreased after applying the diffusion gradient, suggesting an intravascular component in the SE-BOLD signal. This intravascular component was not venous blood, as the T2* of this component was comparable with the T2* of the brain tissue. These results provide new insights into the vascular origins of BOLD signals. A CS approach was developed to accelerate in vivo magnetic resonance spectroscopic imaging (MRSI) which enables multi-voxel MRS measurements. The CS undersampling was performed by acquiring a pseudo-random and density-varying subset of phase encodings. The proposed CS approach preserved the spectral and spatial resolution, while substantially reduced the number of phase encodings with accelerations up to seven fold for phantom and up to six fold for in vivo rat brains. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Multi-compartment model estimation and analysis in high angular resolution diffusion imaging

Zhu, Xinghua, 朱星华 January 2014 (has links)
Diffusion weighted magnetic resonance images offer unique insights into the neural networks of in vivo human brain. In this study, we investigate estimation and statistical analysis of multi-compartment models in high angular resolution diffusion imaging (HARDI) involving the Rician noise model. In particular, we address four important issues in multi-compartment diffusion model estimation, namely, the modelling of Rician noise in diffusion weighted (DW) images, the automatic determination of the number of compartments in the diffusion signal, the application of spatial prior on multi-compartment models, and the evaluation of parameter indeterminacy in diffusion models. We propose an expectation maximization (EM) algorithm to estimate the parameters of a multi-compartment model by maximizing the Rician likelihood of the diffusion signal. We introduce a novel scheme for automatically selecting the number of compartments, via a sparsity-inducing prior on the compartment weights. A non-local weighted maximum likelihood estimator is proposed to improve estimation accuracy utilizing repetitive patterns in the image. Experimental results show that the proposed algorithm improves estimation accuracy in low signal-to-noise-ratio scenarios, and it provides better model selection than several alternative strategies. In addition, we derive the Cram´er-Rao Lower Bound (CRLB) of the maximum Rician likelihood estimator for the balland-stick model and general differentiable diffusion models. The CRLB provides a general theoretical tool for comparing diffusion models and examining parameter indeterminacy in the maximum likelihood estimation problem. / published_or_final_version / Computer Science / Doctoral / Doctor of Philosophy

Magnetic resonance imaging investigation of brain networks

Cheng, Shi, 程实 January 2015 (has links)
Brain operates on a network level. Magnetic resonance imaging (MRI) provides structural and functional images noninvasively with large field of view and at high spatial resolution and thus assumes an extremely valuable role in studying brain networks. The objectives of this doctoral work were to develop and apply novel MRI methods on human and rodent brains, for in vivo and global assessments of functional brain networks at resting and task-evoked states. Firstly, the feasibility of passband balanced steady-state free precession (bSSFP) imaging for distortion-free and high-resolution resting-state fMRI (rsfMRI) was investigated. Resting-state networks (RSNs) derived from bSSFP images were shown spatially and spectrally comparable to those derived from conventional gradient-echo echo-planar imaging (GE-EPI) with considerable intra- and inter-subject reproducibility. High-resolution bSSFP corresponded well to the anatomical images, with RSNs exquisitely co-localized to gray matter. Furthermore, RSNs at areas of severe susceptibility were proved accessible including human anterior prefrontal cortex and rat piriform cortex. These findings demonstrated for the first time that passband bSSFP approach can be a promising alternative to GE-EPI for rsfMRI. It offers distortion-free and high-resolution RSNs and is potentially suited for high field studies. Secondly, to examine the macrovascular contributions to the spatial and spectral prosperities of resting-state networks, spin-echo echo-planar imaging (SE-EPI) with moderate diffusion weighting (DW) was proposed for rsfMRI. SE and DW suppressed the extravascular and intravascular contributions from macrovessels respectively. Significantly lower functional connectivity strength was observed in the posterior cingulate cortex of the default mode network derived from DW SE-EPI data comparing to that derived from SE-EPI, suggesting a confounding role played by the intravascular component from large veins, whereas no significant spectral difference was detected. Therefore, the DW SEEPI approach for rsfMRI may assist in better identifying and interpreting largescale brain networks with future improvement in temporal resolution by acceleration techniques and in sensitivity at higher field. Thirdly, rsfMRI was performed to evaluate the intrinsic functional networks in the corresponding anatomical visual brain connections traced by Mn-enhanced MRI (MEMRI). Strengths of resting-state functional connectivity appeared to couple with structural connectivity in MEMRI, demonstrating the sensitivity of these structural and functional connectivity MRI techniques for assessing the neuroarchitecture, neurophysiology and structural-functional relationships in the visual brain in vivo. Fourthly, the hypothesis that a regional activation identified via general linear model analysis of fMRI data reflects the summation of multiple distinct networks that carry different functional purposes was tested. Overlapping frontoparietal networks engaged in a simple single-digit multiplication task were found and their functional roles were evaluated through independent components analysis and contributive source analysis. Future studies incorporating different arithmetic tasks and resting state will shed more light upon how brain accomplishes arithmetic and more complex tasks in general. Lastly, benefiting from higher SNR, better spatial and temporal resolution at higher field, exploratory fMRI studies were conducted on rats at 7 T for in vivo assessments of 1) the effect of dark-rearing on postnatal visual development, 2) sound amplitude modulations and 3) sound frequency modulation sweep direction selectivity in auditory system. ( / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

Use of contrast agents with fast field-cycling magnetic resonance imaging

Ó hÓgáin, Dara January 2011 (has links)
Fast Field-Cycling (FFC) MRI allows the magnetic field to be switched during an imaging scan. FFC-MRI can be used to exploit a characteristic of contrast agents, i.e. the variation of its spin-lattice relaxation time (T1) or rate (R1= 1/T1) with magnetic field in order to increase contrast. Contrast agents play an essential role in MRI, allowing improved diagnosis and delineation of diseased tissue. However, the R1, and hence the effectiveness of contrast agents, varies significantly with magnetic field. Thus, Fast Field-Cycling (FFC) MRI can be used to take advantage of this variation to improve image contrast, allowing more sensitive detection of the agent. In this project new contrast agents, developed by a collaborating group (Invento S.r.l., Italy) were investigated for use with FFC-MRI. R1 dispersion curves of samples containing a range of contrast agents were first obtained using both a commercial relaxometer and a home-built whole-body FFC-MRI system, and the accuracy of the home-built FFC-MRI system was verified. The magnetisation behaviour of these samples during field-cycling pulse sequences was modelled in order to predict the pulse sequence parameters necessary for maximum T1 contrast. Images were obtained, using a number of novel imaging techniques developed on the home-built whole-body FFC-MRI system, and also, using standard T1 weighted imaging on a 3 T Philips clinical MRI scanner. A new FFC-MRI imaging method, ΔR1 mapping was employed to show an increase in contrast using a novel Mn2+ based liposomal contrast agent compared with T1 weighted images at 5 mT, 59 mT and 3 T. The low concentrations of Mn2+ based liposomal contrast agents used with ΔR1 mapping indicate suitability for molecular imaging

Fast field-cycling magnetism transfer contrast magnetic resonance imaging (FFC MTC MRI)

Choi, Chang-Hoon January 2010 (has links)
Magnetisation Transfer Contrast (MTC) is a well-established magnetic resonance imaging (MRI) contrast-generating mechanism, and is widely used for clarifying MR-invisible macromolecular information indirectly via MR-detectable free protons using an offresonance pre-saturation radiofrequency (RF) pulse (or MT pulse). As a result of MT pulse irradiation, magnetisation between both proton pools is exchanged and the signal intensity of mobile protons is decreased in relation to the amount of macromolecules. MTC MRI is normally implemented at a fixed magnetic field; however, it may be useful to evaluate changes of the MT effect as a function of magnetic field (B0). In order to explore fielddependent MTC experiments using a single MR instrument, two techniques are required, which enable simultaneously shifting both B0 and the resonance frequency of an RF coil (f0) during MT pulse irradiation and returning them to the original condition during MR data acquisition. Switching of B0 is achieved by fast field-cycling (FFC). FFC is a novel technique allowing B0 to shift between levels rapidly during the pulse sequence. This makes it possible to perform a number of beneficial field-dependent studies and/or to provide new MR contrast mechanisms. Switching of f0 requires an actively frequencyswitchable RF coil. This coil was designed and constructed for frequencies at and below 2.5 MHz proton Larmor frequency. The design employed PIN diodes, and enabled switching f0 between five different values. Using these techniques and tools, fielddependent MTC experiments were carried out with a control sample and samples with different concentrations of agarose gel. Due to the absence of macromolecules in the control, the MT effect was almost zero, whereas the MT effect observed in agarose samples increased with increasing concentration of macromolecules. Furthermore, MT effects ((for a given set of MT pulse conditions) were larger at higher B0.

Radio frequency coils for ultra-high field MRI

Jones, Alexa January 2008 (has links)
The push toward higher magnetic fields in MRI has consistently thrown up new challenges in hardware development. The recent development of a new generation of ultra-high field scanners for human imaging is no exception. The earch presented in this thesis aims to provide solutions to new technical challenges in radio-frequency probe design. All probe designs were developed for use at 7T on a Philips Acheiva full body scanner.

Mobile magnetic resonance imaging system and its application

揚思敏, Yeung, Sze-man. January 2001 (has links)
published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

Microstrip radio-frequency coil and array design for magnetic resonance imaging

Wu, Bing, 吳冰 January 2006 (has links)
published_or_final_version / abstract / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

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