Quantitative spin-lock magnetic resonance imaging: Technical development and biomedical applications.

摘要

Spin-lock magnetic resonance imaging (MRI) utilizes low amplitude spin-lock radiofrequency (rf) pulses to generate novel image contrast. The T1ϱ parameter, describing the relaxation of magnetization under the influence of spin-locking, can be measured noninvasively using spin-lock MRI to yield quantitative information about low frequency physico-chemical interactions between bulk water and surrounding molecules in biological tissues. This molecular-level information provides a unique insight into biological systems that is otherwise unattainable using conventional MR methods. However, the current state of spin-lock MRI has limitations in terms of the acquisition of volumetric data, rf power deposition, and length of experimental time.; The primary objective of this thesis is the technical development of quantitative spin-lock MRI and its applications to study biological tissue. Strategies are created to overcome the current experimental hindrances to facilitate clinical implementation. Novel techniques are developed to increase the time-efficiency and reduce the deposition of rf power during a spin-lock image acquisition. A pulse sequence is designed to produce multi-slice spin-lock images for complete coverage of an entire volume along with an algorithm to correct the multi-slice data for rf saturation effects to accurately measure T1ϱ. While the technical developments are generally applicable, their utility is demonstrated in cartilage and brain tissue of animals and humans.; Although spin-lock MRI can be used to investigate different tissue types and pathologies, this thesis focuses on the application of spin-lock methodologies to probe the biochemical and biomechanical properties of articular cartilage. The sensitivity of T1ϱ to changes in macromolecular content makes T1ϱ a promising MR marker of molecular changes in cartilage, degeneration of which plays a crucial role in osteoarthritis. To understand the relationship between T1ϱ and the progression of osteoarthritis, spin-lock MRI methods are applied to models of cartilage degeneration: enzymatically-treated bovine explants, a cytokine-induced in vivo porcine model, and naturally degraded osteoarthritic human cartilage specimens. The T1ϱ-based measurements of cartilage degeneration are corroborated by noninvasive quantification of proteoglycan using a novel sodium MRI strategy. T1ϱ MRI is taken one step further to track progressive changes in both cartilage biochemical content and biomechanical properties in an in vitro cytokine model of osteoarthritis.