High field MR imaging: Importance of magnetic field homogeneity, motion, and sample dimension

Most clinical magnetic resonance imaging is performed at a field strength of 1.5 Tesla or lower. Whole-body magnetic resonance imaging at 4 Tesla offers several potential advantages, including higher signal-to-noise ratios, increased spectral resolution, enhanced deoxygenation contrast for susceptibility magnetic resonance imaging, and longer T$\sb1$ relaxation times, which increase the sensitivity of spin-tagging perfusion experiments. However, initial cardiovascular magnetic resonance studies at 4 Tesla have suffered from decreased signal-to-noise ratios and increased image artifacts compared to 1.5 Tesla images. This has prompted an investigation into mechanisms that may be degrading 4 Tesla cardiac images. From this high-field investigation, three general results are presented in this thesis. First, to determine if magnetic field variation was important in 4 T cardiovascular studies, the magnetic field distribution over the heart was spatially mapped at 4.0 T. A novel method of magnetic field correction over the in vivo human heart was developed, with demonstrated improvements in cardiac $\sp{31}$P magnetic resonance spectroscopy and cardiac echo-planar imaging. Next, flow and motion artifacts in cardiovascular imaging were investigated. Two motion correction methods were implemented to reduce artifact due to flow in the phase-encode direction in echo-planar imaging. New experimental results from phantoms and the cardiovascular system demonstrate that a centrically ordered k-space trajectory is a superior motion compensation method compared to gradient moment nulling. Third, electromagnetic losses of human-size samples at 4 Tesla were studied. From this investigation, we show that dielectric resonators can be used as radiofrequency coils for high-field magnetic resonance imaging and spectroscopy