RF Spatial Combining and Field Control in Dielectric Media

Bluem, Patrick

Graduate Thesis Or Dissertation

RF Spatial Combining and Field Control in Dielectric Media Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/4j03cz76p

Abstract

Recent developments in higher static magnetic field MRIs have allowed for increased sensitivity and improved imaging compared to current clinical MRIs due to the inherent low SNR of nuclear magnetic resonance. Ultra-high field MRI promises to improve medical imaging technology with higher SNR, increased parallel imaging performance, and higher spectral dispersion. With the decrease in wavelength inside of the imaging volume, standing waves develop and decrease the high homogeneity required for accurate diagnosis of medical problems. Additionally, at higher operating frequencies the bore becomes a loaded waveguide which supports propagating modes. This thesis focuses on the development of circularly polarized patch probes for traveling wave MRI at small bore 16.4 T, wide-bore 7T and 10.5T systems with boundary modification structures to improve field coupling and homogeneity. The attenuation of the wave in the unloaded portion of the waveguide becomes an issue with available RF systems as the excitation is placed away from the imaging volume. Additionally, boundary modifying structures are developed to improve coupling and SNR inside of the imaging volume. These boundary modifying structures placed around the imaging volume can be relatively simple and provide an increase in field homogeneity. To improve the relatively poor received spatial coverage of the single patch probe, an additional array is developed with interdigitated capacitor probes at 10.5 T. Multiple excitations can be combined with various relative magnitude and phase excitations, known as B1 shimming. These two techniques can be combined to drastically increase the field coverage inside of an imaging volume. The techniques proposed for cavity excitations in MRIs can be translated to microwave heating systems that convert waste to carbon-rich fuel. Recent military interest in the development of waste-to-fuel techniques have used pyrolysis and incineration, however, these approaches are relatively large due to the necessary supporting infrastructure to operate them.

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