Concomitant gradients in low-field NMR studies of restricted diffusion

Linear magnetic field gradients are used in conventional magnetic resonance (MR) imaging in order to spatially encode and/or resolve nuclear spin distributions. These gradients are normally thought of as being unidirectional, but are always accompanied by orthogonal field components and gradients. These concomitant gradients can largely be ignored as long as the curvature they induce in magnetic field lines is small compared to inverse dimensions of the field of view. Recent attempts to acquire MR images in milliTesla magnetic fields have sparked interest in trying to understand the potential influe nce of concomitant field gradients. In this thesis a series of NMR experiments intended to probe diffusion of 3He gas within a restricted geometry and in very-low magnetic fields are described. Data from these experiments show clear evidence for the perturbing influence of concomitant gradients, and can be qualitatively understood in terms of a simple model that ignores diffusion.