Characterisation of Acinar Airspace Involvement in Asthma using Inert Gas Washout

51 52 Background 53 The multiple breath washout (MBW) parameter Sacin is thought to be a marker of acinar 54 airway involvement, but has not been validated using quantitative imaging techniques in 55 asthma. 56 57 Objective 58 We aimed to utilise 3 He diffusion magnetic resonance ( 3 He-MR) at multiple diffusion 59 timescales and quantitative computed tomography (CT) densitometry to determine the nature 60 of acinar airway involvement in asthma. 61 62 Methods 63 Thirty-seven patients with asthma and seventeen age-matched healthy controls underwent 64 spirometry, body plethysmography, MBW (using the tracer gas sulphur hexafluoride) and 65 3 He-MR. A subset of patients with asthma (n = 27) underwent quantitative CT densitometry. 66 67 Results 68 Ninety-four percent (16/17) of patients with an elevated Sacin had GINA treatment steps 4/5 69 asthma and 13/17 had refractory disease. and Sacin was a significant predictor of refractory 70 asthma. The apparent diffusion coefficient (ADC) of 3 He at 1s was significantly higher in 71 patients with Sacin-high asthma compared to healthy controls (0.024 vs 0.017, p < 0.05). Sacin 72 correlated strongly with ADC at 1s (R = 0.65, p < 0.001), but weakly with ADC at 13ms (R = 73 0.38, p < 0.05). ADC at both 13ms and 1s correlated strongly with the mean lung density 74