Global ETD Search
Search theses and dissertations gathered from participating repositories worldwide. Every result links back to the library that holds it. No account is needed.
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Showing 1 to 20 of 21 for “"field inhomogeneity"”.
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Bilateral Ortohogonality Generative Acquisitions Method for Transmit Field Inhomogeneity Correction in Magnetic Resonance Imaging
Ultra-high field (>7T) magnetic resonance imaging (MRI) systems result in higher intrinsic signal to noise ratio compared to the lower field systems and have been previously utilized for acquisition of high-resolution images and reducing scan time. However, transmit field inhomogeneity of …
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Distortion correction in magnetic resonance imaging using the simulated point spread function
… imaging (EPI), due to long readout times and field inhomogeneity. Previously, the measured point spread function (PSF) has been shown to be effective in correcting this distortion. In this work, we reconstruct an image quickly and address the distortion with a point spread function that was …
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[mu]SR measurement of the magnetic penetration depth and coherence length in the high-T superconductor YBa₂Cu₃O₆ 95
… as observed by OR measurements of the local field inhomogeneity produced by the vortex state. Measurements were taken on a mosaic of single crystals in applied fields in three field regimes: low (A < L), moderate (A > L) and high (A > L > e), where L is the intervortex spacing and e is the …
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Inherent insensitivity to RF inhomogeneity in FLASH imaging
… past three decades. In MRI, radiofrequency (RF) field inhomogeneity is an unavoidable problem in practice and becomes severe at high magnetic fields due to the dependence of B1 on the sample. It leads to nonuniformities in image intensity and contrast, causing difficulties in quantitative …
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INTERFACIAL THERMAL MANIPULATION FOR WATER-ENERGY HARVESTING
… co-generation. We then transform the field inhomogeneity into thermal/electrostatic field co-modulation for environmental energy harvesting with a two-fold output increase. Moreover, we delve into the interfacial heat localization-propagation decoupling in transverse pyroelectrics and …
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Local and global SAR constrained large tip angle 3D kT̳-points parallel transmit pulse design at 7 T
… there has been a push towards higher main field strengths in magnetic resonance imaging (MRI) to achieve better contrast and higher signal to noise ratio (SNR). A drawback to current imaging systems at high field is transmit magnetic field inhomogeneity, which degrades both SNR and …
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Examination of susceptibility effects on functional and dynamic magnetic resonance imaging
A homogeneous, static magnetic field is important in magnetic resonance imaging. When a homogeneous field is not present it can cause imaging artifacts and misestimation of tissue structures, flow, and brain function. However, magnetic field inhomogeneity also drives certain contrast mechanisms …
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An investigation into high temperature superconducting flux pump technology with the circular type magnetic flux pump devices and YBaCuO films
… under different conditions, such as zero-field cooling (ZFC), field cooling (FC), delta-shape trapped field, etc. A simulation based on the H-formulation using FEM software revealed that, after application of the travelling wave, the current density distribution inside the round shape YBCO …
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Knee Osteoarthritis: gagCEST MR Imaging of Articular Cartilage
… gagCEST MRI very sensitive to B<sub>0</sub> field inhomogeneity. Conventionally, an entire CEST spectrum with high spectral resolution is sampled to match the actual minimum signal with the water frequency. This will give rise to a long acquisition time and prevent it from reaching clinical …
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Double volumetric navigators for real-time simultaneous shim and motion measurement and correction in Glycogen Chemical Exchange Saturation Transfer (GlycoCEST) MRI
… based on chemical shift, it is very sensitive to field inhomogeneity which may arise from poor initial shimming, subject respiration, heating of shim iron, mechanical vibrations or subject motion. This is a particular problem for molecules that resonate close to water, such as - OH protons in …
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Joint estimation of water and fat images from magnetic resonance signals
… two key difficulties: (a) the presence of B0 field inhomogeneities, which makes the problem non-linear and ill-posed; and (b) the difficulty of accurately modeling the acquired signal, which can lead to bias in quantitative fat-water separation applications. The research in this thesis has …
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Improvements in magnetic resonance imaging excitation pulse design
… due to spin relaxation, tissue heating, and main field inhomogeneity limitations. When magnetic spins are tilted by only a small amount, pulse transmission may be interpreted as depositing energy in a continuous three-dimensional Fourier-like domain along a one-dimensional contour to form an …
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Machine Learning and its Application in Automatic Change Detection in Medical Images
… detection is a fundamental problem in various fields, such as image surveillance, remote sensing, medical imaging, etc. The challenge of change detection in medical images is to detect disease-related changes while rejecting changes caused by noise, patient position change, and imaging …
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Methods for chemical exchange saturation transfer magnetic resonance imaging
… CEST imaging is extremely sensitive to magnetic field inhomogeneity. Images suffer from large susceptibility artifacts unless specialized BO inhomogeneity correction methods are employed that tremendously increase scan time. Second, the CEST contrast cannot be separated from the intrinsic …
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Plasma processes in pulsar environments
… propagating electromagnetic waves by a magnetic field inhomogeneity, thus creating a source of radiation in the pulsar magnetosphere. The full nonlinear equations in cylindrical geometry for a streaming cold pair plasma are solved numerically, together with Maxwell's equations, using a …
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Development of Optimised Pulse Sequences for Magnetic Resonance Imaging of Tumour Metabolism
… be limited by fast relaxation rates, magnetic field inhomogeneity and non-uniform spin excitation. A simple method for pulse sequence optimisation is presented that uses a stochastic numerical solver which, in principle, is capable of finding a better local optimum. The method provides a simple …
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¹H-MRSI at 7T using spectral-selective binomial excitation and adiabatic refocusing
… inherently low sensitivity of MRSI. Ultra-high field systems such as 7T can provide higher SNR and better spectral separation of metabolite peaks, but they also introduce pronounced B₀ and B₁ field inhomogeneity, larger chemical shift displacement error (CSDE), stricter specific absorption rate …
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Slice-selective parallel transmit magnetic resonance imaging at 7T
Ultra-high field (UHF) Magnetic resonance imaging (MRI) at 7 Tesla and beyond provides gains in signal-to-noise ratio (SNR), enabling faster and higher-resolution imaging. However, UHF MRI comes with significant technical challenges, especially radiofrequency (RF, or B+1 ) field inhomogeneity which …
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Subspace estimation for subspace-based magnetic resonance spectroscopic imaging
This Thesis was approved for publication on 2016-04-15 at 14:08.
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Quantitative susceptibility mapping and susceptibility-based distortion correction of echo planar images
The field of medical image analysis continues to expand as magnetic resonance imaging (MRI) technology advances through increases in field strength and the development of new image acquisition and reconstruction methods. The advent of echo planar imaging (EPI) has allowed volumetric data sets to be …
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