Background Multiple breath inert gas washout (MBW) is a technique for detecting abnormal ventilation distribution in patients with asthma and other airway diseases. S_cond and S_acin are measures of convective-dependent inhomogeneity (CDI) and diffusion-convection-dependent inhomogeneity (DCDI) respectively. Hyperpolarised ³He diffusion magnetic resonance imaging (³He-MRI) may be used to probe lung microstructure at a variety of length scales, with short timescale (14 ms) apparent diffusion coefficient (ADC) corresponding to diffusion within an alveolus or a single acinar airway, and long timescale (1.5s–6s) ADC corresponding to diffusion path lengths of up to 8mm. We aimed to determine the microstructural correlates of S_cond and S_acin in patients with asthma, using ³He-MRI.

Methods Twenty-nine patients with asthma underwent MBW using sulphur hexafluoride as the inert tracer gas, and the parameters S_cond and S_acin were calculated. ³He-MRI was performed and the ADC was calculated at both short (14ms) and long (1.5s, 3s and 6s) timescales. ³He-MRI data was also fitted to a previously reported geometrical model of the acinus (Yablonskiy DA et al , J Appl Physiol. 2009;107(4):1258–65), and estimates of the alveolar duct outer radius (R) and alveolar sleeve width (h) were derived.

Results Correlations between MBW and ³He-MRI parameters are shown in Table 1. The approximate length scales probed by short and long timescale ADC are also indicated for reference. Significant positive correlations were observed between S_acin and ADC at 14ms, 1.5s and 3s, but not 6s. In a stepwise linear regression model, ADC at 1.5s was the only significant determinant of S_acin, with a model R² of 0.334. S_cond did not correlate significantly with any of the MRI parameters. Yablonskiy model estimates of alveolar sleeve width and alveolar duct outer radius did not correlate significantly with either S_cond or S_acin.

Conclusion S_acin in patients with asthma is associated with an elevated ADC at 1.5s, corresponding to length scales of the order of 4mm. This suggests that DCDI in asthma is associated with structural asymmetries at the level of the distal acinar airways and/or collateral ventilation between parallel intra-acinar airways.