In the time since magnetic resonance imaging (MRI) was introduced, scientific progress has allowed for a factor-of-ten increase in static magnetic (B 0) field strength, and has developed MR into a clinical workhorse. This increase in B0 field strength has the potential to provide significant gains to the inherent signal-to-noise ratio of resulting images. However, this progress has been limited by degradations in the spatial homogeneity of the radiofrequency magnetic fields used for nuclear excitation (B 1), which have wavelengths comparable to the dimensions of the human body in modern high-field MRI. Techniques to improve homogeneity, including B1-shimming and parallel transmission, require multi-element radiofrequency (RF) transmit arrays. Increasing B0 field strength is also associated with an increase in the deposition of RF energy into the subject, clinically measured and regulated as Specific energy Absorption Rate (SAR), deposited in tissue during image acquisition. High permittivity materials (HPMs) have the potential to augment RF coil performance outside of B1-shimming or parallel transmission methods. The use of HPM pads placed in existing RF coils has also been shown to provide a potential reduction of array SAR in nuclear excitation, as well as potential performance benefits in signal reception. However, the question of how best to strategically use these materials in the space between the coil and the sample in order to maximize benefit and alleviate any potential problems has not yet been thoroughly addressed.
The contributions presented in this dissertation demonstrate the potential utility of the integration of HPMs into transmit-receive RF coils, as an integral component of the hardware design. A framework to quickly choose the relative permittivities of integrated materials, optimized relative to an absolute standard (rather than relative to a different design) is introduced, and used to demonstrate that readily available material properties can provide significant improvements in multi-element transmit performance. A subsequent analysis of practical effects and limitations of these materials on the RF coil resonance properties is performed, including the description of a unique adverse resonance splitting phenomenon and how to avoid it. A transmit/receive RF coil design is built and evaluated, first on its own experimentally, and then in simulation with a helmet-shaped high permittivity material former to examine the benefits and challenges associated with HPM integration into RF coils.
|Advisor:||Sodickson, Daniel K.|
|Commitee:||Collins, Christopher M., Lattanzi, Riccardo, Turnbull, Daniel H., Webb, Andrew, Wiggins, Graham C.|
|School:||New York University|
|Department:||Basic Medical Science|
|School Location:||United States -- New York|
|Source:||DAI-B 80/03(E), Dissertation Abstracts International|
|Subjects:||Biomedical engineering, Electrical engineering, Medical imaging|
|Keywords:||Electromagnetic modeling, High magnetic fields, High permittivity materials, Magnetic resonance imaging, Radio frequency coils, Specific absorption rate|
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