Long-range diffusion of hyperpolarized ³He in explanted normal and emphysematous human lungs via magnetization tagging

Long-range diffusivity of hyperpolarized ³He gas was measured from the decay rate of sinusoidally modulated longitudinal nuclear magnetization in three normal donor and nine severely emphysematous explanted human lungs. This (long-range) diffusivity, which we call D_sec, is measured over seconds and centimeters and is ∼10 times smaller in healthy lungs (0.022 cm²/s) than the more traditionally measured D_msec, which is measured over milliseconds and submillimeters. The increased restriction of D_sec reflects the complex, tortuous paths required to navigate long distances through the maze of branching peripheral airways. In emphysematous lungs, D_sec is substantially increased, with some regions showing nearly the unrestricted value of the self-diffusion coefficient (0.88 cm ²/s for dilute ³He in air, a 40-fold increase). This suggests the presence of large collateral pathways opened by alveolar destruction that bypass the airways proper. This destruction was confirmed by comparison with histology in seven lungs and by removal of trapped gas via holes in the pleural surface in five lungs.