Ensemble average propagator estimation of axon diameter in diffusion MRI: implications and limitations

Diffusion Magnetic Resonance Imaging (dMRI) has beenused to infer the axon diameter of the tissues using biophysicalmodels of diffusion. In recent years, a new methodwas proposed for estimating axon diameter directly fromthe ensemble average propagator (EAP), and in particularfrom one of its derived index, the return to the axis probability(RTAP). In this work we study the effect that differentacquisition parameters have on the estimation of the axondiameter with three different EAP models: diffusion tensorimaging (DTI), 3D-SHORE, and MAPMRI. We quantify theeffect that moving from the long diffusion time conditionhas on the diameter estimation by simulating the diffusionsignal inside impermeable cylinders. Results on in-vivo datashows that EAP models estimation of axon diameter is heavilyinfluenced by extra-axonal water and incoherence of fiberorientation, which hides the intra-axonal signal.