The typical two-compartment model of diffusion in the white matter is associated with two plausible solutions, the choice between which relies on whether intra-axonal or extra-axonal axial diffusivity is faster. Here we use an intracerebroventricular perfusion of gadolinium in the rat brain to suppress the extra-cellular signal. Diffusion measurements before and after perfusion show a mild increase in axial diffusivity post-perfusion, which suggests intra-axonal diffusivity is higher than extra-axonal axial diffusivity. This can help solve the current indetermination in parameter estimation and allow diffusion models to regain their claimed specificity.
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