Improved functional magnetic resonance imaging at 4.0 T
Brewer, Kimberly Dawn
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Functional magnetic resonance imaging (fMRI) is an important tool for the study of the human brain. When performed at higher field strengths, fMRI offers increased signal strength and sensitivity to activation. Unfortunately, traditional fMRI using echo-planar imaging (EPI), or spiral-out also suffers from increased image artifact at higher fields. The most significant artifact is signal dropout occurring near areas where the magnetic susceptibility of a material changes rapidly (for example, at air-tissue interfaces such as sinuses). As a result, signal in several important cognitive areas is often lost. In response to this problem, several new methods for dealing with these susceptibility artifacts have been developed. Modified pulse sequences such as spiral-in are the most common and tend to be the most effective at recovering signal. In order to fully understand these spiral techniques the differing artifact patterns and signal displacement behaviour of spiral-out and spiral-in due to susceptibility field gradients were first modeled and explored. Spiral-in was found to have superior signal recovery in SFG regions due to to increased intravoxel phase coherence. A new pulse sequence called asymmetric spin-echo (ASE) spiral, based on the spiral-in sequence, was designed to more efficiently recover signal and fMRI activation lost to susceptibility artifacts. ASE spiral has reduced geometric distortion, increased SNR over the whole brain and optimized BOLD weighting. The additional optimization of ASE spiral, via the addition of z-shim gradients, was also explored, but found to be unnecessary as z-shim did not offer significant benefits at the group level when added to spiral-in techniques. This work also examines the changes in R 2 and [Special characters omitted.] -weighting present in ASE spiral images and the subsequent effects on the specificity and sensitivity of the fMRI activation with later ASE images having increasing specificity compared to traditional gradient-echo sequences. ASE spiral can be used as both a SFG-insensitive technique, and as a technique more specific to extravascular (i.e. tissue) sources. This thesis will aid in the continuing development of fMRI by providing important information and tools that can be used by both clinicians and researchers for understanding brain function and treatment of disease.