Background
The multiple breath washout (MBW) parameter Sacin is thought to be a marker of acinar
airway involvement, but has not been validated using quantitative imaging techniques in
asthma.
Objective
We aimed to utilise 3He diffusion magnetic resonance (
3 He-MR) at multiple diffusion
timescales and quantitative computed tomography (CT) densitometry to determine the nature
of acinar airway involvement in asthma.
Methods
Thirty-seven patients with asthma and seventeen age-matched healthy controls underwent
spirometry, body plethysmography, MBW (using the tracer gas sulphur hexafluoride) and
He-MR. A subset of patients with asthma (n = 27) underwent quantitative CT densitometry.
Results
Ninety-four percent (16/17) of patients with an elevated Sacin had GINA treatment steps 4/5
asthma and 13/17 had refractory disease. The apparent diffusion coefficient (ADC) of 3
He at 1s was significantly higher in
patients with Sacin-high asthma compared to healthy controls (0.024 vs 0.017, p < 0.05). Sacin
correlated strongly with ADC at 1s (R = 0.65, p < 0.001), but weakly with ADC at 13ms (R =
0.38, p < 0.05). ADC at both 13ms and 1s correlated strongly with the mean lung density
expiratory / inspiratory ratio, a CT marker of expiratory air trapping (R = 0.77, p < 0.0001 for
ADC at 13ms; R = 0.72, p < 0.001 for ADC at 1s).
Conclusion
Sacin is associated with alterations in long-range diffusion
within the acinar airways and gas trapping. The precise anatomical nature and mechanistic
role in severe asthma requires further evaluation