| A new 3D acquisition regimen is presented which enables the acquisition of conventional, water-suppressed and fat-suppressed images with no increase in scan times compared to currently implemented 3D sequences. The method is based on conventional 3D steady state with interleaved selective excitation of the fat resonance resulting in acquisition of a fat based image during the TR period experienced by the water spins. This new sequence is relatively tolerant to susceptibility artifacts and results in excellent fat- and water-based images. Since the idea we propose is independent of the type of steady state imaging protocol utilized, it can be easily applied with regimens which are specifically tailored to enhance contrast. Phantoms containing fat and water have been utilized to demonstrate the feasibility of this technique. Clinical applications normally requiring lipid suppression, such as orbit, head and breast studies, have also been conducted. Using this technique, the final fat and water images are devoid of chemical shift artifacts since separate fat and water images are created. In addition, the use of interleaved acquisition allows us to essentially eliminate subtraction artifacts caused by patient motion since correlated excitations are separated by a time interval of only 30 msec. Compared to other 3D suppression techniques, the sequence we proposed also has the advantages of short scan times while maintaining both water and fat signals and thus providing more diagnostic information. Slice selective capability is also presented in a modified 3D MELISSA sequence. In achieving slice selection, conventional and composite pulses were utilized. These preliminary results can be further enhanced using adiabatic slice selection.;In the appendix of this dissertation, the design of an actively shielded gradient coil for a high field magnet is described. This actively shielded gradient coil was utilized in conducting a water suppression experiment. Excellent water suppression results were obtained using gradient selected multiple quantum coherence methods with this coil assembly. |
