Anisotropic Nature of 3He Gas Diffusion in Mice Lungs

Introduction: Lung morphometry technique that utilizes 3He MRI [1, 2] demonstrated in vivo quantification of lung geometrical parameters in humans, comparable to those obtained through histology. As part of the broader effort, this technique, optimized for mouse lungs, allowed us to calculate mouse lung geometry comparable to histological findings. In our approach, lung airways are considered as cylinders covered by alveolar sleeve; diffusion of 3He atoms in airways is described by two distinct diffusion coefficients DL (along airway) and DT (perpendicular to airway axis). Diffusion-attenuated MR signal in such a microscopically anisotropic but macroscopically isotropic system (the uniform orientation distribution of airways is assumed) is non-mono-exponential in b-value in human [1, 2] and mice. However, such a non-mono-exponential dependence can, in principle, be caused by other factors, e.g., due to the presence of multiple spherical compartments (that mimic alveoli) with a variety of sizes. We used a triple-gradient pulse sequence consisting of 3 consecutive bipolar gradient pairs with orthogonal gradient orientations (e.g., along the Cartesian axes X, Y, Z) to prove that diffusion of 3He gas in the mice lung acinar airways is really microscopically anisotropic.