Airway Microstructure in Idiopathic Pulmonary Fibrosis: Assessment at Hyperpolarized He Diffusion-weighted MRI.

Background MRI with inhaled hyperpolarized helium 3 (He) allows for functional and structural imaging of the lungs. Hyperpolarized gas diffusion-weighted (DW) MRI provides noninvasive and quantitative assessment of microstructural acinar changes in the lungs. Purpose To investigate whether microstructural imaging metrics from in-vivo hyperpolarized He DW MRI are sensitive to longitudinal changes in a cohort of participants with idiopathic pulmonary fibrosis (IPF) and to evaluate the reproducibility of these metrics and their correlation with existing clinical measures of IPF disease severity. Materials and Methods In this prospective study, 18 participants with IPF underwent He DW MRI at 1.5 T and 11 participants underwent an identical same-day examination for reproducibility assessment. Thirteen participants returned for 6- and 12-month follow-up examinations. Pulmonary function tests, including diffusing capacity of the lungs for carbon monoxide and forced vital capacity, were performed at each examination. The apparent diffusion coefficient (ADC) and stretched exponential model-derived mean diffusive length scale (Lm) from DW MRI was compared with baseline CT fibrosis scores and pulmonary function tests by using Spearman rank correlation coefficient. Longitudinal changes in DW MRI and pulmonary function test measurements were assessed with Friedman tests and post hoc Dunn test. Results He ADC and Lm were reproducible (mean Bland-Altman analysis bias, 0.002 cm · sec and -1.5 μm, respectively). Elevated ADC and Lm regions qualitatively corresponded to fibrotic regions at CT. ADC and Lm correlated with diffusing capacity of the lungs for carbon monoxide (respectively: r = -0.56, P = .017; and r = -0.54, P = .02) and CT fibrosis score (respectively: r = 0.71, P = .001; and r = 0.65, P = .003). Lm increased by 12 μm after 12 months (P = .001) whereas mean ADC (P = .17), forced vital capacity (P = .12), and diffusing capacity of the lungs for carbon monoxide (P > .99) were not statistically different between examinations. Conclusion Helium 3 diffusion-weighted MRI-derived mean diffusive length scale demonstrates longitudinal changes in lungs affected by idiopathic pulmonary fibrosis. © RSNA, 2019 Online supplemental material is available for this article. See also the editorial by Altes and Flors in this issue.