Magnetic Resonance Imaging of 3He Apparent Diffusion Coefficient Anisotropy in Emphysema

Perhaps one of the most unique implementations of hyperpolarized 3He is diffusion- weighted MR imaging, taking advantage of the much larger apparent diffusion coefficient (ADC) of 3He ( 0.2 cm2∕s), compared to 1H (1.1 × 105 cm2∕s). 3He diffusion in the lungs is restricted by airway and alveoli walls and therefore is highly dependent on lung microstructure. 3He ADC has been shown to be sensitive to changes in terminai airway anatomy, specifically alveolar damage due to emphysema. At the terminai airway, 3He diffusion has been demonstrated to be anisotropic and described by two components: (i) a longitudinal diffusion coefficient (Dl)i reflective of diffusion along the length of the duct, and (ii) a transverse diffusion coefficient (Dτ), reflective of diffusion perpendicular to the duct. The purpose of this thesis was to first determine the sensitivity of Dτ and Dl to emphysema through a finite difference simulation in a budded cylinder model at submillisecond diffusion times. In-vivo experiments were performed in elastase-instilled rats to compare Dτ and Dl to those of sham-instilled rats. MR imaging was performed at 3T using a custom-built high performance insert gradient. D1 and DT were then mapped pixel-by-pixel by fitting a multi-component function to the data. Following imaging, the rats were euthanized and lungs were extracted and fixed under inflation at the end of each experiment for histological measurement of mean linear intercept. We hypothesized that Dτ measured at sub-millisecond diffusion times would be more sensitive than Dl for discriminated damage. Results from both simulations and in-vivo experiments supported this hypothesis. Overall, anisotropy measurement of the diffusion coefficient in Hyperpolarized Noble Gas (HNG) MR imaging could be a way to detect emphysema in its early stages.