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  Simulation of Electromagnetic Fields for the Development of NMR Coils

Hoffmann, J. (2009). Simulation of Electromagnetic Fields for the Development of NMR Coils. Diploma Thesis, Eberhard Karls Universität Tübingen, Tübingen, Germany.

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Hoffmann, J1, 2, Author              
1Former Department MRZ, Max Planck Institute for Biological Cybernetics, Max Planck Society, Spemannstrasse 38, 72076 Tübingen, DE, ou_2528700              
2Max Planck Institute for Biological Cybernetics, Max Planck Society, Spemannstrasse 38, 72076 Tübingen, DE, ou_1497794              


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 Abstract: Magnetic resonance imaging is a powerful noninvasive method for imaging cross-sectional slices in the human body and brain, as well as metabolic processes therein. It capitalizes from the magnetizability of tissue in strong static magnetic fields and the possibility to make the local magnetization detectable with additional radiofrequency pulses, based on the physical principle of nuclear magnetic resonance. In 2008, magnets that produce a static field strength ( B 0 )of0 . 5or1 . 5 Tesla are the clinical standard for routine radiological exams on humans, but this standard is apparently moving slowly to the use of 3 Tesla magnets. However, there is a drive to even higher field strengths in order to explore its advantages for clinical and research imaging. In 1998, the first 8 Tesla system with an 80 cm bore was built and installed at Ohio State University, followed by the first 7T/90cm magnet at the Center for Magnetic Research at the University of Minnesota in 1999 [28]. Several other sites installed magnets with a field strength of 7 Tesla or higher since then, and in 2008, three operational 9 . 4 T systems for human research exist, one of them at the Magnetic Resonance Center of the Max Planck Institute for Biological Cybernetics in T ̈ ubingen, Germany. The main motivation for high-field magnets in MRI is the theoretical proportionality of the signal-to-noise ratio (SNR) to the B 0 field magnitude, because the increased signal from the sample allows for scanning with a higher spatial or temporal resolution. SNR-demanding techniques, as for example fMRI, parallel imaging methods or imaging of low γ nuclei strongly benefit from the increase in field strength. In addition, MR spectroscopy always aims at higher field strengths because the chemical shift dispersion increases with B 0 . However, some serious challenges exist with human high-field imaging. The most dominant of them are related to the increasing frequency of the magnetic field B 1 that must be generated by the RF coils since the resonance frequency grows proportional to the static magnetic field B 0 . The decreased wavelength and penetration depth of the radiofrequency field complicate the creation of a homogeneous circularly polarized magnetic RF field inside the sample, leading to inhomogeneous flip angle maps and therefore inhomogeneous images. In addition, the power deposition or specific absorption rate (SAR) due to the accompanying electric field E 1 increases at high frequencies, and this can possibly lead to a dangerous rise in temperature in the imaged subject. Unfortunately, no direct methods exist to map the local RF electric field in vivo ,and therefore a direct measurement of SAR is not possible. Numerical simulations have proved to be useful in addressing these issues. In the last couple of years, the finite-difference time-domain (FDTD) method was used by a few researchers to accurately calculate electromagnetic fields produced by RF coils in the presence of complex shaped biological tissue. Their work greatly improved the understanding of field behavior in high-field MRI and lots of universally valid results could be extracted from their efforts. This work mainly focuses on the precise simulation of electromagnetic fields produced by phased array coils for human brain imaging at 9 . 4 Tesla. Chapters 1, 2 and 3 deal with the basics of NMR, coil design and the numerical algorithm used, while the materials and methods are explained in chapter 4. The results of the simulations for RF coils at 9 . 4 Tesla are presented in chapters 5 and 6, the results of the simulations for coils at 16 . 4 Tesla can be found in chapter 7. Chapter 8 finally gives a brief discussion about the findings of this work.


 Dates: 2009-01
 Publication Status: Published in print
 Pages: 114
 Publishing info: Tübingen, Germany : Eberhard Karls Universität Tübingen
 Table of Contents: -
 Rev. Type: -
 Identifiers: BibTex Citekey: Hoffmann2009
 Degree: Diploma



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