Experimental investigation and numerical simulation of 3He gas diffusion in simple geometries: Implications for analytical models of 3He MR lung morphometry

Models of lung acinar geometry have been proposed to analytically describe the diffusion of 3He in the lung (as measured with pulsed gradient spin echo (PGSE) methods) as a possible means of characterizing lung microstructure from measurement of the 3He ADC. In this work, major limitations in these analytical models are highlighted in simple diffusion weighted experiments with 3He in cylindrical models of known geometry. The findings are substantiated with numerical simulations based on the same geometry using finite difference representation of the Bloch-Torrey equation. The validity of the existing “cylinder model” is discussed in terms of the physical diffusion regimes experienced and the basic reliance of the cylinder model and other ADC-based approaches on a Gaussian diffusion behaviour is highlighted. The results presented here demonstrate that physical assumptions of the cylinder model are not valid for large diffusion gradient strengths (above ∼15 mT/m), which are commonly used for 3He ADC measurements in human lungs.