Finite element simulations of hyperpolarized gas DWI in micro-CT meshes of acinar airways: validating the cylinder and stretched exponential models of lung microstructural length scales.

Purpose: This work assesses the accuracy of the stretched exponential (SEM) and cylinder models of lung microstructural length scales that can be derived from hyperpolarized gas DWI. This was achieved by simulating He and Xe DWI signals within two micro-CT-derived realistic acinar airspace meshes that represent healthy and idiopathic pulmonary fibrosis lungs.

Methods: The healthy and idiopathic pulmonary fibrosis acinar airway meshes were derived from segmentations of 3D micro-CT images of excised human lungs and meshed for finite element simulations of the Bloch-Torrey equations. He and Xe multiple b value DWI experiments across a range of diffusion times ( He Δ = 1.6 ms; Xe Δ = 5 to 20 ms) were simulated in each mesh. Global SEM mean diffusive length scale and cylinder model mean chord length value was derived from each finite element simulation and compared against each mesh's mean linear intercept length, calculated from intercept length measurements within micro-CT segmentation masks.

Results: The SEM-derived mean diffusive length scale was within ±10% of the mean linear intercept length for simulations with both He (Δ = 1.6 ms) and Xe (Δ = 7 to 13 ms) in the healthy mesh, and with Xe (Δ = 13 to 20 ms) for the idiopathic pulmonary fibrosis mesh, whereas for the cylinder model-derived mean chord length the closest agreement with mean linear intercept length (11.7% and 22.6% difference) was at Xe Δ = 20 ms for both healthy and IPF meshes, respectively.

Conclusion: This work validates the use of the SEM for accurate estimation of acinar dimensions and indicates that the SEM is relatively robust across a range of experimental conditions and acinar length scales.