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

Methods: Three-dimensional multiple b-value ³ He and ¹²⁹ Xe 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 ³ He and ¹²⁹ Xe 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 ³ He and ¹²⁹ Xe 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 ³ He and ¹²⁹ Xe, the stretched exponential model-derived Lm_D is related nonlinearly to cylinder model-derived Lm, and demonstrates excellent agreement with the cylinder model-derived L_Alv .