Source: Annual Congress 2010 - State of the art imaging
Disease area: Airway diseases

Abstract

Introduction: A method for in vivo ³He MR lung morphometry (HeLM) has been developed (J. Magn. Reson. 2008, 190: 200-210) which estimates airway dimensions from ³He diffusion experiments using expressions obtained from computer models of alveolar ducts. This method has been proven to be sensitive to airway changes produced by emphysema. In this work, we investigate the the limitations imposed by the geometric assumptions (infinite non-connected ducts) of the model on the validity of this method.
Methods: Computer simulations were performed by solving the Bloch-Torrey equation using finite element method. The geometric model consisted of a central alveolar duct with branching nodes at both ends. The apparent diffusivities with gradients parallel (D_L) and perpendicular (D_T) to the central duct axis were computed for a range of diffusion times (τ =1.8- 6 ms).
Results: The results showed that the lack of dependence of D_L0 on diffusion time predicted by HeLM, is a consequence of the assumed simplified geometry. When branching is included, D_L0 shows a linear dependence on the diffusion time. For τ>2.5 ms, D_L increases with increasing b value, instead of decreasing as predicted by HeLM expressions. Branching effects also affects D_T, which decreases linearly with b value instead of increasing as predicted by HeLM.
Conclusions: Branching effects significantly affect ³He diffusivity, even at short diffusion times. The expressions of the HeLM method do not account for significant dependence on diffusion time and airway length as a consequence of the oversimplified geometric model. Further development and validation of this method is required.

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Citations should be made in the following way:
J. Parra-Robles, S. R. Parnell, J. M. Wild (Sheffield, United Kingdom). Finite element simulations of short-range ³He gas diffusion in a model of branching acinar airways: Implications for in vivo ³He MR lung morphometry. Eur Respir J 2010; 36: Suppl. 54, 5284

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