Abstract
MR diffusion experiments using hyperpolarized gas are sensitive to emphysema. However, interpretation of non-Gaussian diffusion is complicated by its dependence on a variety of factors (e.g. branching structure, susceptibility gradients, diffusion time, lung inflation and gas mixture composition) [Parra-Robles et al, JMR 204, 2010]. In this work, a theoretical model based in the application of fractional order dynamics to anomalous diffusion is presented. The model does not require assumptions about the geometry and distribution of apparent diffusion rates [Magin et al, JMR 190, 2008]. This technique can be used to quantify non-gaussian diffusion signals arising from a multiplicity of sources.
MR diffusion scans of eight healthy volunteers and 10 COPD patients were performed using hyperpolarised 3He. While the diffusivity (DDC) depended strongly on experimental conditions, the heterogeneity index α remained constant (0.87 ± 5%, in healthy volunteers), regardless of the diffusion time (in the range 1.6-2.5 ms), lung inflation, gas mixture and subject's size. Both, DDC and α were significantly different for the COPD patients. These results seems to indicate that α is insensitive to a range of uncertainties in experimental conditions but sensitive to changes in lung structure due to COPD.
Since the model is not constrained to a specific range of timing parameters, it may be able to reveal information about lung geometry from diffusion data acquired over different time scales. This information may be sensitive to lung diseases that affect airway morphology at different generations of the lung branching structure.
- © 2012 ERS
