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At the typical spatial resolution of MRI in the human brain, approximately 60–90% of voxels contain multiple fiber populations. Quantifying microstructural properties of distinct fiber populations within a voxel is therefore challenging but necessary. While progress has been made for diffusion and T₁-relaxation properties, how to resolve intra-voxel T₂ heterogeneity remains an open question. Here a novel framework, named COMMIT-T₂, is proposed that uses tractography-based spatial regularization with diffusion-relaxometry data to estimate multiple intra-axonal T₂ values within a voxel. Unlike previously-proposed voxel-based T₂ estimation methods, which (when applied in white matter) implicitly assume just one fiber bundle in the voxel or the same T₂ for all bundles in the voxel, COMMIT-T₂ can recover specific T₂ values for each unique fiber population passing through the voxel. In this approach, the number of …