title: Acinar determinants of the apparent diffusion coefficient for helium-3.
abstract: The apparent diffusion coefficient (ADC) obtained by helium-3 magnetic resonance imaging over several seconds is thought to reflect diffusion impairment due to both intra- and interacinar structure. In this study, numerical simulations of intra-acinar gas mixing and effective diffusion were performed in a multiple-branch-point model of the human acinus. Using a previously described method, we computed the instantaneous effective diffusion resulting from the diffusive impairment imposed by intra-acinar branching for varying times up to 5 s. We also tested the influence on effective diffusion of intra-acinar collateral channels in the fully alveolated intra-acinar airways to mimic the effect of emphysema. Randomly connecting two or four pairs of airways per generation (in generations 19-25) led to a 40 and 142% increase, respectively, in effective diffusion coefficient cumulated over the time interval of 0.2-5 s. Finally, we also used a system of two coupled multiple branch-point models to simulate diffusive attenuation over a 50-s interval in cases of purely acinar tagging (i.e., the initial gas concentration = 1 in one acinus and 0 in the other) and of partial tagging astride on two acini. It is shown that, in the latter case, the decay rate cannot be approximated by a mono-exponential with a several-fold faster decay for times below 10 s due to intra-acinar diffusion. We conclude that both the characteristic biphasic time dependence of simulated effective diffusion and its sensitivity to intra-acinar structural change mimic experimental ADC behavior. Additional simulations of combined inter- and intra-acinar diffusion strongly suggest that neglecting intra-acinar branching would in fact lead to considerable error of simulated ADC.
authors.affiliation: Respiratory Division, University Hospital UZ Brussel, Vrije Universiteit Brussel, Brussels, Belgium. sylvia.verbanck@uzbrussel.be
authors.forename: Sylvia
authors.initials: S
authors.lastname: Verbanck
authors.forename: Manuel
authors.initials: M
authors.lastname: Paiva
mesh_terms: D003198:Computer Simulation; D004058:Diffusion; D006371:Helium / Q000737:chemistry*; D006801:Humans; D007554:Isotopes; D008279:Magnetic Resonance Imaging / Q000379:methods; D008954:Models, Biological; D011650:Pulmonary Alveoli / Q000033:anatomy & histology* / Q000502:physiology*; D013997:Time Factors
chemical_list: D007554:Isotopes; D006371:Helium