Date of Award
Spring 5-2009
Document Type
Dissertation - Restricted
Degree Name
Doctor of Philosophy (PhD)
Department
Electrical and Computer Engineering
First Advisor
Richie, James E.
Second Advisor
Deibele, Steven
Third Advisor
Johnson, Michael
Abstract
A magnetic resonance (MR) bore's structure, loading parameters, and material composition influence the ability of a radio frequency signal to propagate. Many researchers have tried to create wireless telemetry networks within the MR bore. However, no research has generated a model for bore path loss. The goal of this work is to create a frequency-specific path loss model for the MR bore using electromagnetic theory and experimental results. This model is structured similar to the log-normal shadowing model, which has reference, environmental, and adjustment terms to describe path loss in differing environments.
Theory indicates that the MR bore should behave like a cylindrical waveguide or cavity if excited at ultra high frequencies (UHF). Therefore, the path loss behavior, or attenuation characteristic, of the bore should be governed by the mode structure created when the bore is excited by a single tone frequency. The bore should also have improved signal attenuation characteristics when compared to the same signal propagating in free space.
The relative path loss behavior of the MR bore is determined experimentally at UHF. An experiment is conducted to determine expected signal attenuation when the bore is empty (air-filled) or dielectrically loaded (patient-filled). This information is especially important to the wireless network designer who is implementing an in-bore physiological monitoring network.
A circuit model is proposed as a way to model each mode for the environmental portion of the overall path loss model. This model represents a transmitter exciting an empty bore at the transverse center and longitudinal edge of the bore. Experimental results are used to augment unknown parameters. The circuit model is extended to apply to different transmitter locations by adding reference and adjustment terms. It is also extended to address the addition of the dielectric body to the bore.
Results show that the MR bore exhibits impressive path loss performance at 434 MHz, with greatest signal strength arriving at the receiver when the bore is dielectrically loaded. The empty and dielectrically filled bore path loss models predict bore path loss within 1 dB of actual path loss in some bore locations.