| The work described in the first part of the thesis was stimulated by the very poor quality of the images obtained with an animal-size MRI system at the University of Alberta's In vivo NMR Facility. This was mainly caused by eddy current fields induced in the 40 cm bore Bruker superconducting magnet. The temporal and spatial dependence of these fields were therefore measured using the offset frequency of the proton signal from a small spherical sample. Three eddy currents were found, with decay times ranging from 13 to 480 ms. These currents generated very large gradients and significant field shifts, which led us to construct, in house, a set of distributed current, shielded gradient coils of minimum inductance. During the installation of the gradient coils, it became apparent that very accurate registration of the primary and shielding coils was required to achieve proper shielding. The effect of misregistration of the coils was therefore studied by determining the artifacts that would be produced in typical imaging and spectroscopic protocols by a small relative axial displacement of the primary and shielding coils in a shielded z gradient set from their proper location. Numerical values of the tolerance required for minimal artifacts ranged from 0.1 mm for STEAM proton spectroscopy to 1 mm for SE, FLASH and MBEST imaging.;Another defect found in the animal-size MRI system was an axial (z) variation in the magnetic field, which was predominantly fourth order in z, and could only be one-third compensated by the ;Many MRI protocols require strong gradients which can be switched rapidly. This requires gradient coils of low inductance and low power dissipation in generating a given gradient. Since these are strongly dependent on the volume within the coil, and since the human body is more elliptical than circular in "cross section", the second part of the thesis studies the design of gradient coils which utilize distributed currents flowing on the surface of an elliptic cylinder. General expressions were derived for the magnetic field and stored energy associated with currents flowing azimuthally and axially on the surface of an elliptic cylinder. It was then shown how these expressions could be used to design elliptical z gradient coils of minimum inductance. Allowing the current to flow axially as well as azimuthally greatly reduced the field nonuniformity in transverse planes. For a coil design with an axial ratio of 0.75, the gradient nonlinearity along the z axis and field nonuniformity in the transverse directions were... |
