Modelling Extra-Axonal Diffusion Spectra for Oscillating Gradient Measurements

Wilfred W. Lam¹, Saad Jbabdi¹, Karla L. Miller¹

Diffusion imaging has enormous potential for quantitative measurements of properties like axon diameter that are directly relevant to brain function and pathology. Most work has focused on intra-axonal diffusion, despite the fact that significant signal is expected to arise from the extra-axonal space. Current diffusion models do not account for the frequency dependence of extra-axonal signal in oscillating gradient experiments. Two analytic models are proposed for the diffusion attenuation spectra of spins diffusing in the space between regularly packed cylinders. We compare our model predictions to Monte Carlo simulations. These models capture salient properties of extra-axonal diffusion spectra.