Measuring Collateral Pathways in Health via 3He MRI

Abstract

IntroductionThere has been recent interest in the use of 3He diffusion MRI because of the close correlations of the imaging measure with tissue morphometry. The long‐range diffusion coefficient (LRDC) has been shown to be a measure of collateral pathways in emphysema, but little is known of the role of collaterals in the LRDC in health. A recent publication attempted to explain the LRDC by simulations within single lung acini, without collateral pathways.MethodsWe measured the LRDC of 3He ex‐vivo in 4 human donor lungs that were rejected for transplantation and in 3 pig lungs, which are known to have greatly reduced collateral ventilation. We also simulated the interacinar diffusion of 3He (at 2‐cm diffusion distances) via the diffusion equation in a strictly branching model of human lung with no collateral pathways. (Airway radii, lengths, and branching angles were taken from detailed, morphometric literature.)ResultsLRDC in pig lungs (0.004 cm2/s) was 4x lower than in the human lungs (0.022 cm2/s), implying that collateral pathways can play a significant role in the measurements. The robustly tested simulations revealed a surprisingly low LRDC (24x lower than the measured value), implying that diffusion via the airway tree alone (no collaterals) cannot account for the measured value in healthy lung. A surprisingly small number of 5‐μ pores is shown to account for the discrepancy.