Introduction and Objectives Apparent diffusion coefficient (ADC) and mean diffusive length scale (LmD) are diffusion-weighted (DW) MRI measurements of alveolar gas diffusion, providing novel lung microstructure information. Hyperpolarised 129-xenon (129Xe) MR spectroscopy is a quantitative marker of gas exchange, using the ratio of uptake of 129Xe in red blood cells to tissue/plasma (RBC:TP).
The objective was to evaluate hyperpolarised 129Xe MRI in differentiating between fibrotic and inflammatory ILD and assessing longitudinal change.
Methods A prospective, multicentre study of ILD patients including connective tissue disease ILD (CTD-ILD), drug induced ILD (DI-ILD), hypersensitivity pneumonitis (HP), idiopathic non-specific interstitial pneumonia (iNSIP) and idiopathic pulmonary fibrosis (IPF). Hyperpolarised 129Xe MRI was performed on a 1.5T scanner. Baseline HRCT scan was performed within a year prior to the MRI scan. Semi-quantitative visual CT analysis was performed by two consultant chest radiologists. In the non-IPF subtypes, a ground glass opacity score <2 and ≥2 was used to define fibrotic and inflammatory ILD respectively. All IPF subjects were classified as fibrotic.
Results To date, 34 patients (5 CTD-ILD, 9 DI-ILD, 7 HP, 2 iNSIP, 11 IPF) have complete MRI scan data for two separate visits (6 weeks apart for DI-ILD/HP/iNSIP and 6 months apart for CTD-ILD/IPF). There were 18 patients in the fibrotic group and 16 in the inflammatory group. At baseline visit there was no significant difference in mean RBC:TP between the fibrotic and inflammatory groups (0.17 vs 0.14; p=0.083), but a significant difference between the fibrotic and inflammatory groups in mean ADC (0.048 vs 0.043; p=0.030) (figure 1a) and mean LmD(261.3 vs 243.4; p=0.017) (figure 1b). In longitudinal change, there was a significant difference in mean RBC:TP between the fibrotic and inflammatory groups (-0.026 vs 0.0016; p=0.023), but no significant difference between the fibrotic and inflammatory groups in mean ADC (0.00089 vs -0.00025; p=0.25) and mean LmD (2.1 vs -0.19; p=0.39).
Conclusions 129Xe DW-MRI could have a role in differentiating changes in the airway microstructure between fibrotic and inflammatory ILD. 129Xe RBC:TP has sensitivity to longitudinal change with a decline in gas exchange observed in the fibrotic group but not in the inflammatory group.
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