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This Article Full Text Full Text (PDF) All Versions of this Article: 104/5/1495    most recent 01005.2007v1 Submit a response Alert me when this article is cited Alert me when eLetters are posted Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Google Scholar Articles by Bartel, S.-E. T. Articles by Conradi, M. S. PubMed PubMed Citation Articles by Bartel, S.-E. T. Articles by Conradi, M. S.

Role of collateral paths in long-range diffusion in lungs

Departments of ¹Physics and ²Radiology, Washington University, St. Louis, Missouri

Submitted 21 September 2007 ; accepted in final form 13 February 2008

The long-range apparent diffusion coefficient (LRADC) of ³He gas in lungs, measured over times of several seconds and distances of 1–3 cm, probes the connections between the airways. Previous work has shown the LRADC to be small in health and substantially elevated in emphysema, reflecting tissue destruction, which is known to create collateral pathways. To better understand what controls LRADC, we report computer simulations and measurements of ³He gas diffusion in healthy lungs. The lung is generated with a random algorithm using well-defined rules, yielding a three-dimensional set of nodes or junctions, each connected by airways to one parent node and two daughters; airway dimensions are taken from published values. Spin magnetization in the simulated lung is modulated sinusoidally, and the diffusion equation is solved to 1,000 s. The modulated magnetization decays with a time constant corresponding to an LRADC of 0.001 cm²/s, which is smaller by a factor of 20 than the values in healthy lungs measured here and previously in vivo and in explanted lungs. It appears that collateral gas pathways, not present in the simulations, are functional in healthy lungs; they provide additional and more direct routes for long-range motion than the canonical airway tree. This is surprising, inasmuch as collateral ventilation is believed to be physiologically insignificant in healthy lungs. We discuss the effect on LRADC of small collateral connections through airway walls and rule out other possible mechanisms. The role of collateral paths is supported by measurements of smaller LRADC in pigs, where collateral ventilation is known to be smaller.

gas magnetic resonance imaging; hyperpolarized; collateral ventilation