title: The role of collateral paths in long-range diffusion of 3He in lungs.
abstract: OBJECTIVE
  The hyperpolarized (3)He long-range diffusion coefficient (LRDC) in lungs is sensitive to changes in lung structure due to emphysema, reflecting the increase in collateral paths resulting from tissue destruction. However, no clear understanding of LRDC in healthy lungs has emerged. Here we compare LRDC measured in healthy lungs with computer simulations of diffusion along the airway tree with no collateral connections.
  
  
  METHODS
  Computer simulations of diffusion of spatially modulated spin magnetization were performed in computer-generated, symmetric-branching models of lungs and compared with existing LRDC measurements in canine and human lungs.
  
  
  RESULTS
  The simulations predict LRDC values of order 0.001 cm(2)/sec, approximately 20 times smaller than the measured LRDC. We consider and rule out possible mechanisms for LRDC not included in the simulations: incomplete breath hold, cardiac motion, and passage of dissolved (3)He through airway walls. However, a very low density of small (micron) holes in the airways is shown to account for the observed LRDC.
  
  
  CONCLUSIONS
  It is proposed that LRDC in healthy lungs is determined by small collateral pathways.
authors.affiliation: Department of Physics, Washington University, Saint Louis, MO 63130, USA. msc@wuphys.wustl.edu
authors.forename: Mark S
authors.initials: MS
authors.lastname: Conradi
authors.forename: Dmitriy A
authors.initials: DA
authors.lastname: Yablonskiy
authors.forename: Jason C
authors.initials: JC
authors.lastname: Woods
authors.forename: David S
authors.initials: DS
authors.lastname: Gierada
authors.forename: Seth-Emil T
authors.initials: SE
authors.lastname: Bartel
authors.forename: Susan E
authors.initials: SE
authors.lastname: Haywood
authors.forename: Christopher
authors.initials: C
authors.lastname: Menard
mesh_terms: D000465:Algorithms; D000818:Animals; D003198:Computer Simulation; D004285:Dogs; D006371:Helium / Q000493:pharmacokinetics*; D006801:Humans; D007554:Isotopes; D008168:Lung / Q000502:physiology*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D011653:Pulmonary Diffusing Capacity / Q000502:physiology*; D011656:Pulmonary Emphysema / Q000175:diagnosis / Q000378:metabolism
chemical_list: D007554:Isotopes; D006371:Helium