Modeling diffusion of intracellular metabolites in the mouse brain up to very high diffusion‐weighting: Diffusion in long fibers (almost) accounts for non‐monoexponential attenuation

Purpose To investigate how intracellular metabolites diffusion measured in vivo up to very high q/b in the mouse brain can be explained in terms of simple geometries. Methods 10 mice were scanned using our new STE‐LASER sequence, at 11.7 Tesla (T), up to q max  = 1 μm −1 at diffusion time t d  = 63.2 ms, corresponding to b max  = 60 ms/µm². We model cell fibers as randomly oriented cylinders, with radius a and intracellular diffusivityD i n t r ac y l, and fit experimental data as a function of q to estimateD i n t r ac y land a . Results Randomly oriented cylinders account well for measured attenuation, giving fiber radii andD i n t r ac y lin the expected ranges (0.5–1.5 µm and 0.30–0.45 µm 2 /ms, respectively). The only exception is N‐acetyl‐aspartate (NAA) (extracted a ∼0), which we show to be compatible with a small fraction of the NAA pool being confined in highly restricted compartments (with short T 2 ). Conclusion The non‐monoexponential signal attenuation of intracellular metabolites in the mouse brain can be described by diffusion in long and thin cylinders, yielding realistic D intra and fiber diameters. However, this simple model may require small “corrections” for NAA, in the form of a small fraction of the NAA signal originating from a highly restricted compartment. Magn Reson Med 77:343–350, 2017. © 2016 Wiley Periodicals, Inc.