Spin-manipulation methods for efficient magnetic resonance imaging.

摘要

Magnetic resonance imaging (MRI) is a powerful, non-invasive medical imaging modality that provides excellent soft-tissue contrast compared with other methods. Although MRI is extensively used in clinical practice, reductions in imaging times and improvements in image contrast are still possible. In MRI, the “spins” of atomic nuclei are excited to a state where they produce an image signal. This thesis considers two excitation methods and a fast imaging method, all of which attempt to both improve image contrast and reduce scan times.; Driven equilibrium is a technique where spins are excited, but after signal readout, are driven back towards their equilibrium state. In knee imaging, driven equilibrium produces a bright cartilage appearance, but an even brighter synovial fluid signal. This combination allows excellent visualization of cartilage structure as well as a diagnostically useful contrast compared with previous knee imaging methods.; In “steady state” imaging sequences, spins are excited rapidly and never return to equilibrium, but rather reach a certain steady state. The transient time during which the steady state evolves can be long, and is typically not useful for imaging. A new analysis of the transient dynamics of steady-state sequences suggests methods of spin-manipulation to speed up the evolution of the steady state that are validated in simulations and experiments.; Spiral imaging is an efficient method for imaging in the presence of motion or flow. The design of spiral imaging sequences for new MR systems is presented. The methods are applied to three-dimensional imaging and angiography, as well as for contrast-enhanced perfusion imaging, with comparatively low scan times compared with other methods.