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

Diffusion Magnetic Resonance Imaging (dMRI) has been used to infer the axon diameter of the tissues using biophysical models of diffusion. In recent years, a new method was proposed for estimating axon diameter directly from the ensemble average propagator (EAP), and in particular from one of its derived index, the return to the axis probability (RTAP). In this work we study the effect that different acquisition parameters have on the estimation of the axon diameter with three different EAP models: diffusion tensor imaging (DTI), 3D-SHORE, and MAPMRI. We quantify the effect that moving from the long diffusion time condition has on the diameter estimation by simulating the diffusion signal inside impermeable cylinders. Results on in-vivo data shows that EAP models estimation of axon diameter is heavily influenced by extra-axonal water and incoherence of fiber orientation, which hides the intra-axonal signal.