Comparison of in vivo lung morphometry models from 3D multiple b-value 3He and 129Xe diffusion-weighted MRI

Purpose: To compare in vivo lung morphometry parameters derived from theoretical gas diffusion models, the cylinder model and stretched exponential model, in a range of acinar microstructural length scales encountered in healthy and diseased lungs with He-3 and Xe-129 diffusion-weighted MRI. Methods: Three-dimensional multiple b-value He-3 and Xe-129 diffusion-weighted MRI was acquired with compressed sensing at 1.5 T from 51 and 31 subjects, respectively, including healthy volunteers, ex-smokers, idiopathic pulmonary fibrosis, and chronic obstructive pulmonary disease patients. For each subject, the stretched exponential model-derived mean diffusive length scale (Lm(D)) was calculated from the diffusion signal decay, and was compared with the cylinder model-derived mean chord length (Lm) and mean alveolar diameter (L-Alv) in order to determine the relationships among the different lung morphometry parameters. Results: For both He-3 and Xe-129 diffusion-weighted MRI, the mean global Lm(D) value was significantly related (P < .001) to Lm in a nonlinear power relationship, whereas the L-Alv demonstrated excellent linear correlation (P < .001) with Lm(D). A mean bias of +1.0% and -2.6% toward Lm(D) was obtained for Bland-Altman analyses of He-3 and Xe-129 Lm(D) and L-Alv values, suggesting that the two morphometric parameters are equivalent measures of mean acinar dimensions. Conclusion: Within the experimental range of parameters considered here for both He-3 and Xe-129, the stretched exponential model-derived Lm(D) is related nonlinearly to cylinder model-derived Lm(D), and demonstrates excellent agreement with the cylinder model-derived L-Alv.