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

摘要

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.