AbstractPurposeThis study aims to evaluate two distinct approaches for fiber radius estimation using diffusion‐relaxation MRI data acquired in biomimetic microfiber phantoms that mimic hollow axons. The methods considered are the spherical mean power‐law approach and a T₂‐based pore size estimation technique.Theory and MethodsA general diffusion‐relaxation theoretical model for the spherical mean signal from water molecules within a distribution of cylinders with varying radii was introduced, encompassing the evaluated models as particular cases. Additionally, a new numerical approach was presented for estimating effective radii (i.e., MRI‐visible mean radii) from the ground truth radii distributions, not reliant on previous theoretical approximations and adaptable to various acquisition sequences. The ground truth radii were obtained from scanning electron microscope images.ResultsBoth methods show a linear relationship between effective radii estimated from MRI data and ground‐truth radii distributions, although some discrepancies were observed. The spherical mean power‐law method overestimated fiber radii. Conversely, the T₂‐based method exhibited higher sensitivity to smaller fiber radii, but faced limitations in accurately estimating the radius in one particular phantom, possibly because of material‐specific relaxation changes.ConclusionThe study demonstrates the feasibility of both techniques to predict pore sizes of hollow microfibers. The T₂‐based technique, unlike the spherical mean power‐law method, does not demand ultra‐high diffusion gradients, but requires calibration with known radius distributions. This research contributes to the ongoing development and evaluation of neuroimaging techniques for fiber radius estimation, highlights the advantages and limitations of both methods, and provides datasets for reproducible research.