The impact of axon orientation dispersion and 3D diameter variations on the transverse apparent diffusion coefficient

We extract the orientation dispersion (OD) and 3D axon diameter variations of long axons (>100 µm) from an ultra-high resolution, synchrotron X-ray nano-holotomography (XNH) scan of the monkey splenium. From this, we discover a relationship between mean axon diameter and along-axon diameter variations. Monte Carlo simulations are then performed on the intra-axonal spaces (IAS) of different substrates which inherit their morphological features from the segmented axons. These simulations show that the OD significantly affects the transverse apparent diffusion coefficient (ADC⟂) of the axon substrate at diffusion times up to 50 ms, while diameter variations do not.