Measuring and modelling lung microstructure with hyperpolarised gas MRI

This thesis is concerned with the development of new techniques for measuring and modelling lung microstructure with hyperpolarised gas magnetic resonance imaging (MRI). This aim was pursued in the following five chapters: Development of a framework for lobar comparison of lung microstructure measurements derived from computed tomography (CT) and 3He diffusion-weighted MRI evaluated in an asthmatic cohort. Statistically significant linear correlations were obtained between 3He diffusion-weighted MRI and CT lung microstructure metrics in all lobar regions. Implementation of compressed sensing (CS) to facilitate the acquisition of 3D multiple b-value 3He diffusion-weighted MRI in a single breath-hold for whole lung morphometry mapping. Good agreement between CS-derived and fully-sampled whole lung morphometry maps demonstrates that CS undersampled 3He diffusion-weighted MRI is suitable for clinical lung imaging studies. Acquisition of whole lung morphometry maps with 129Xe diffusion-weighted MRI and CS. An empirically-optimised 129Xe diffusion time (8.5 ms) was derived and 129Xe lung morphometry values demonstrated strong agreement with 3He equivalent measurements. This indicates that 129Xe diffusion-weighted MRI is a viable alternative to 3He for whole lung morphometry mapping. Implementation of an in vivo comparison of the stretched exponential and cylinder theoretical gas diffusion models with both 3He and 129Xe diffusion-weighted MRI. Stretched exponential model diffusive length scale was related to cylinder model mean chord length in a non-linear power relationship; while the cylinder model mean alveolar diameter demonstrated excellent agreement with diffusive length scale. Investigation of clinical and physiological changes in lung microstructure with 3He and 129Xe diffusion-weighted MRI. Longitudinal studies with 3He and 129Xe diffusion-weighted MRI were used investigate changes in lung microstructure in cystic fibrosis and idiopathic pulmonary fibrosis. Lung inflation mechanisms at the acinar level were also investigated with 3He and 129Xe diffusion-weighted MRI acquired at two different lung volumes.