Simulation and optimization of RF-resonators for magnetic resonance imaging at high fields PDF Download
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Author: Sven Junge
Publisher:
ISBN:
Category :
Languages : de
Pages : 266
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Author: Sven Junge
Publisher:
ISBN:
Category :
Languages : de
Pages : 266
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Author: Gangchea Lee
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Magnetic resonance imaging (MRI) is one of the major medical imaging diagnostic tool in todays daily clinical routine. This technique produces either two-dimensional (2D) or three-dimensional (3D) images with high resolution and contrast predominantly in soft tissues non-invasively, therefore, providing physiological information that other imaging modality cannot offer. The quality of a MRI image often depends on the signal to noise ratio (SNR), and the high SNR can be sacrificed to achieve either higher resolutions or shorter acquisition times. Therefore, MR research is striving for the highest achievable SNR in the images. There are two major ways to achieve higher SNR: 1) the use of higher static magnetic fields (B0), or 2) the use of optimized radiofrequency (RF) resonator for the image acquisition. However, the optimization of RF resonators at higher B0 becomes harder as the working frequency of the RF resonator increases proportional to B0 (radiation losses, lumped element losses, coil to cable interactions, low quality factors, and dielectric effect becomes problematic at high frequencies).This dissertation describes conventional RF resonator designs and new concepts of RF resonators at a very high magnetic field of 14.1 T. Proton MRI will be conducted on selected biological applications requiring the RF resonator to operate at 600 MHz. The fabrication of conventional coils a surface coil, a birdcage coil, and scroll coils is described in the earlier chapters (chapter 2, 3, 4). These coils were fabricated to image the mouse eye in vivo, the mouse brain in vivo, and fixed 3D printed cell strands respectively. Simultaneous imaging concept is also elaborated in chapter 4. In the later chapters (chapter 5, 6), two unconventional design RF resonators a dielectric resonator, and a patch antenna were fabricated, tested, and compared with state of the art RF resonators to investigate the possibility of substituting the conventional RF resonators. These two designs provided higher homogeneity but less SNRs. Therefore, improvements to be made to increase SNR are discussed in each chapter.
Author: Xiaoliang Zhang
Publisher:
ISBN:
Category : Magnetic resonance imaging
Languages : en
Pages : 560
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Author: Andrew G Webb
Publisher: Royal Society of Chemistry
ISBN: 1782623876
Category : Science
Languages : en
Pages : 402
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Magnetic resonance systems are used in almost every academic and industrial chemistry, physics and biochemistry department, as well as being one of the most important imaging modalities in clinical radiology. The design of such systems has become increasingly sophisticated over the years. Static magnetic fields increase continuously, large-scale arrays of receive elements are now ubiquitous in clinical MRI, cryogenic technology has become commonplace in high resolution NMR and is expanding rapidly in preclinical MRI, specialized high strength magnetic field gradients have been designed for studying the human connectome, and the commercial advent of ultra-high field human imaging has required new types of RF coils and static shim coils together with extensive electromagnetic simulations to ensure patient safety. This book covers the hardware and engineering that constitutes a magnetic resonance system, whether that be a high-resolution liquid or solid state system for NMR spectroscopy, a preclinical system for imaging animals or a clinical system used for human imaging. Written by a team of experts in the field, this book provides a comprehensive and instructional look at all aspects of current magnetic resonance technology, as well as outlooks for future developments.
Author: Roberta Kriegl
Publisher:
ISBN:
Category :
Languages : en
Pages : 0
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Magnetic resonance imaging (MRI), among other imaging techniques, has become a major backbone of modern medical diagnostics. MRI enables the non-invasive combined, identification of anatomical structures, functional and chemical properties, especially in soft tissues. Nonetheless, applications requiring very high spatial and/or temporal resolution are often limited by the available signal-to-noise ratio (SNR) in MR experiments. Since first clinical applications, image quality in MRI has been constantly improved by applying one or several of the following strategies: increasing the static magnetic field strength, improvement of the radiofrequency (RF) detection system, development of specialized acquisition sequences and optimization of image reconstruction techniques. This work is concerned with the development of highly sensitive RF detection systems for biomedical ultra-high field MRI. In particular, auto-resonant RF coils based on transmission line technology are investigated. These resonators may be fabricated on flexible substrate which enables form-fitting of the RF detector to the target anatomy, leading to a significant SNR gain. The main objective of this work is the development of a flexible RF coil array for high-resolution MRI on a human whole-body 7 T MR scanner. With coil arrays, the intrinsically high SNR of small surface coils may be exploited for an extended field of view. Further, parallel imaging techniques are accessible with RF array technology, allowing acceleration of the image acquisition. Secondly, in this PhD project a novel design for transmission line resonators is developed, that brings an additional degree of freedom in geometric design and enables the fabrication of large multi-turn resonators for high field MR applications. This thesis describes the development, successful implementation and evaluation of novel, mechanically flexible RF devices by analytical and 3D electromagnetic simulations, in bench measurements and in MRI experiments.
Author: Julianna Denise Ianni
Publisher:
ISBN:
Category : Electronic dissertations
Languages : en
Pages :
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Author: Xin Chen
Publisher:
ISBN:
Category :
Languages : en
Pages : 153
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The focus of this dissertation is on the finite RF (radio frequency) wavelength effect in high field MRI (magnetic resonance imaging). As the RF frequency increases proportionally with the main magnetic field strength, and due to the high dielectric permittivity of human tissue, the wavelength of the electromagnetic (EM) field produced by the MRI RF coil is substantially shortened in that tissue. The shortened RF wavelength is comparable with or even shorter than the size of the imaging subject (for example, the human head or torso.) The oscillation of the RF field in an imaging subject with such a short wavelength is the underlying physical cause of RF field and image inhomogeneities. Additionally, due to the short wavelength, the EM field can no longer be considered static or quasi-static, and full wave calculations must be applied to determine the field. In this work, we develop analytical calculations to simulate the RF field for several different types of RF coil models. Since the oscillation of the RF field is derived from Maxwell equations and cannot be eliminated, we first demonstrate that RF field with a high planar homogeneity in a specific plane can be achieved by completely restricting the oscillation of the field to the spatial axis perpendicular to that plane. Furthermore, RF coil models with a number of independent current sources are simulated. By virtue of efficient analytical calculations, optimizations are applied to search for optimal current sources that can produce desired RF field patterns with significantly improved homogeneity. Last but not least, the current distribution and the RF field patterns of a circular loop antenna operating at ultra-high RF frequency are studied. Calculations demonstrate that the shortened RF wavelength has an impact on not only the field pattern, but also the source current distribution on the RF coil. Through the comparison with FDTD (finite difference time domain) numerical simulations, we find that our analytical calculations based on simplified models can accurately simulate the field for a more realistic situation. The optimization significantly improves the RF field homogeneity. Additionally, the extremely high computation efficiency of analytical calculations is also shown.
Author: Anca Irina Stefan
Publisher:
ISBN:
Category : Cavity resonators
Languages : en
Pages : 120
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Abstract: The purpose of this work was to use computer-aided design methods to analyze resonant cavities for electron paramagnetic resonance (EPR) and magnetic resonance (MR) imaging. As a result of this analysis, a new design and modifications to existing ones were proposed. The behavior of a L-band transverse electric reentrant resonator (TERR) for in-vivo EPR spectroscopy and imaging was analyzed. The influence of various geometrical parameters on the B1 field distribution was investigated. The optimal size of a sample that can be imaged with this type of resonator was determined, as well as the dependence of the quality factor on the sample size and geometry. Some of the numerical results were compared with experimental data. Finally, a design for a 300 MHz TERR was proposed. In the second part of this work, numerical studies using a model of a 16-strut transverse electromagnetic (TEM) resonator for MR imaging were performed. Simulations were performed either with a saline-filled spherical phantom or with a 4-mm resolution head model. The purpose was to investigate ways to improve the B1 homogeneity, obtain localized imaging, and explore the potential use of the first mode of the TEM structure. Models of TEM structures with reduced number of struts to allow for better patient access were designed. The effect of partially filling the TEM resonator with high dielectric constant material on the field homogeneity was analyzed.
Author: Jason E. Moore
Publisher:
ISBN:
Category : Electronic dissertations
Languages : en
Pages : 292
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Author: Joseph Vincent Rispoli
Publisher:
ISBN:
Category :
Languages : en
Pages :
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The growing availability of high-field magnetic resonance (MR) scanners has reignited interest in the in vivo investigation of metabolics in the body. In particular, multinuclear MR spectroscopy (MRS) data reveal physiological details inaccessible to typical proton (1H) scans. Carbon-13 (13C) MRS studies draw considerable appeal owing to the enhanced chemical shift range of metabolites that may be interrogated to elucidate disease metabolism and progression. To achieve the theoretical signal-to-noise (SNR) gains at high B0 fields, however, J-coupling from 1H-13C chemical bonds must be mitigated by transmitting radiofrequency (RF) proton-decoupling pulses. This irradiated RF power is substantial and intensifies with increased decoupling bandwidth as well as B0 field strength. The preferred 13C MRS experiment, applying broadband proton decoupling, thus presents considerable challenges at 7 T. Localized tissue heating is a paramount concern for all high-field studies, with strict Specific Absorption Rate (SAR) limits in place to ensure patient safety. Transmit coils must operate within these power guidelines without sacrificing image and spectral quality. Consequently, RF coils transmitting proton-decoupling pulses must be expressly designed for power efficiency as well as B1 field homogeneity. This dissertation presents innovations in high-field RF coil development that collectively improved the homogeneity, efficiency, and safety of high field 13C MRS. A review of electromagnetic (EM) theory guided a full-wave modeling study of coplanar shielding geometries to delineate design parameters for coil transmit efficiency. Next, a novel RF coil technique for achieving B1 homogeneity, dubbed forced current excitation (FCE), was examined and a coplanar-shielded FCE coil was implemented for proton decoupling of the breast at 7 T. To perform a series of simulation studies gauging SAR in the prone breast, software was developed to fuse a suite of anatomically-derived heterogeneous breast phantoms, spanning the standard four tissue density classifications, with existing whole-body voxel models. The effects of tissue density on SAR were presented and guidance for simulating the worst-case scenario was outlined. Finally, for improving capabilities of multinuclear coils during proton coil transmit, a high-power trap circuit was designed and tested, ultimately enabling isolation of 13C coil elements during broadband proton decoupling pulses. Together, this work advanced the hardware capabilities of high-field multinuclear spectroscopy with immediate applicability for performing broadband proton-decoupled 13C MRS in the breast at 7 T. The electronic version of this dissertation is accessible from http://hdl.handle.net/1969.1/155400
