Conventional $$$T_1$$$-mapping techniques are only sensitive to voxel-averaged $$$T_1$$$ values, which hinders the study of fiber-specific myelination changes in the developing, aging or diseased brain. While recent works have focused on combining diffusion- and $$$T_1$$$- weightings to access orientation-resolved $$$T_1$$$ values, they rely on assumptions regarding the voxel content. This work combines a prototype diffusion-relaxation MR acquisition and a Monte-Carlo inversion method to extract intra-voxel nonparametric 5D distributions of diffusion tensors and longitudinal relaxation rates $$$R_1=1/T_1$$$ without the use of limiting assumptions. Estimated $$$R_1$$$ values are then mapped onto nonparametric orientation distribution functions, thereby yielding fiber-specific longitudinal relaxation rates.
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