{"record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"0df266833b99d01ecf2978185ec290895e657be11da2effa581ba0f6b282a0db","created":"2026-10-03T07:18:06Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"75c487a9ea75a5190b967a89a3b790db2b6fbf010285e65fe52f59f1fa32a3aa","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"value","assisted_by":[],"external_id":"med.diffusion-mri.standard-model-degeneracy","kind":"well-posed","literature_status":"partially-resolved","n":"1","parents":[],"plain":"Models of brain white matter describe the signal using a few water compartments, but different parameter sets fit the same data equally well. 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 data","topic_ref":"7d6543da8b95cbfe0f88251dd0987163345fc9bcbe4e99e44fdf7b56a0f8bcd0"},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"e570bad89c59f66f0378ed1fe982e108dba2a773fad29ccc944700e13f137bab","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"aaf0c1c7d591fe640290f9e403c19cdf7b65b5db776544f2744fa1cedca39c7d","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"wRZ5RyN71DeC0Az_Uf3X4X5CMAZm1_DL1pcyA7j0cxZ4X9y-vhT1j6ygxmzUHkvaW3R_Td7sxARnBUtx1d-fAw"},"schema":"pubphys.envelope/1"},"record_hash":"e570bad89c59f66f0378ed1fe982e108dba2a773fad29ccc944700e13f137bab","leaf_index":1671}