Diffusional effects in magnetic resonance imaging and microscopy

Improvements in the information content of nuclear magnetic resonance (NMR) images have recently been sought through the development of new contrast mechanisms for magnetic resonance imaging (MRI) and NMR microscopy. This thesis addresses the role of molecular diffusion in NMR and develops new methods to obtain image contrast and to infer diffusional action.NMR microscopy of liquid samples provides a means of imaging the spatial distribution of the magnetization arising from nuclei in the liquid, at a resolution of several microns. Usually, the effect of molecular diffusion in NMR microscopy is to degrade resolution and sensitivity due to destructive interference of signals from the moving spins.In this thesis it is demonstrated, both theoretically and experimentally, that diffusional barriers, such as exist in biological tissues, can be made to appear extraordinarily bright in NMR micrographs. These effects are described by the line shape function of the magnetization signal, and by numerical evaluation of the diffusive phase dispersion of the signal. This "edge enhancement" provides a means of visualizing structures as thin and permeable as cell membranes which would otherwise be invisible in NMR microscopy.In the realm of clinical MRI, a description of nuclear spin relaxation mediated by MRI contrast agents and transport processes in compartmentalized systems is given. The relaxation-enhancing action of contrast agents can be most generally explained by exchange and diffusional dephasing, and simple analytic expressions are derived for typical situations. More complex solutions are obtained by efficient numerical methods, including the so-called generalized moment expansion. For appropriate models, it is demonstrated that while the extended influence of contrast agents through diffusion of water can contribute to blurring, such a loss of contrast may be avoided if the multiexponential character of the signal decay is exploited.U of I OnlyETDs are only available to UIUC Users without author permissio