Response to Commentary on “The influence of lung airways branching structure and diffusion time on measurements and models of short-range 3He gas MR diffusion”

Our extensive investigation of the cylinder model theory through numerical modelling and purpose-designed experiments has demonstrated that it does produce inaccurate estimates of airway dimensions at all diffusion times currently used. This is due to a variety of effects: incomplete treatment of non-Gaussian effects, finite airway size, branching geometry, background susceptibility gradients and diffusion time dependence of the 3He MR diffusion behaviour in acinar airways. The cylinder model is a good starting point for the development of a lung morphometry technique from 3He diffusion MR but its limitations need to be understood and documented in the interest of reliable clinical interpretation.