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It is unclear which measurements make the answer unique.","posed_since":"","precise":"For the two- or three-compartment model of white matter (intra-axonal sticks with a fibre orientation distribution, an anisotropic extra-axonal compartment, free water), with parameters axonal water fraction f, intra-axonal diffusivity $D_a$, and extra-axonal diffusivities D_e_par and D_e_perp, determine the minimal set of acquisitions (diffusion weightings $b\\ \\text{up to about}\\ 10\\,\\mathrm{ms}/\\text{micron}^2$, b-tensor shapes, echo-time variation) that makes the parameters identifiable at clinical signal-to-noise ratio of 20 to 50, and state the residual estimation variance. An answer is the identifiability condition and the attainable precision.","problem_ref":null,"references":"","settled_by":"An analytic or Cramer-Rao analysis of the model with given protocols, confirmed by phantom or in vivo test-retest data with independent ground truth.","status_note":"Degeneracy of conventional multi-shell fits is established, and planar or spherical b-tensor encoding and echo-time variation reduce it, but a minimal clinical protocol is not agreed.","title":"Identifiability of white-matter compartment parameters from diffusion 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