{"record":{"author":{"account_ref":null,"orcid":null},"builds_on":[],"content_schema":"pubphys.content.revision/1","content_sha256":"37b329216358c412b68cb68e97e62b32f9390762f245c8f41b0b0cab899747e8","created":"2026-10-03T07:18:06Z","files":[],"origin":{"assisted_by":[],"kind":"seed"},"parents":["5475cf5dc5b1a72283d26dc95dc6e00d7ea4e00096a7c6492e409a1816eeb011"],"salt":"d1861301df5007d7081ced8909124eb545a039053f5ba6f58e5897dcf045ed3c","schema":"pubphys.record/2","site":"pubphys.com","target":null,"type":"revision"},"content":{"answer_type":"value","assisted_by":[],"external_id":"med.diffusion-mri.axon-diameter-limit","kind":"well-posed","literature_status":"partially-resolved","n":"1","parents":[],"plain":"Most human axons are less than a thousandth of a millimetre across, near or below what the scanner gradient strength can resolve. Whether reliable axon-size maps are possible is unsettled.","posed_since":"","precise":"Determine the resolution limit on axon radius $r$ from the diffusion signal perpendicular to axons as a function of maximal gradient amplitude $G$ (30 to 50 G/cm in dedicated human scanners), diffusion time and signal-to-noise ratio, given that the signal attenuation scales as $r^4 G^2$ in the wide-pulse limit, and whether the estimated effective radius, weighted toward the largest axons, can be related to histological radius distributions. An answer is the limit in micron and a demonstrated agreement or disagreement with histology.","problem_ref":null,"references":"","settled_by":"Ex vivo and in vivo diffusion measurements on the same tissue followed by electron-microscopy histology of axon radii.","status_note":"Resolution-limit analyses place the threshold near the upper end of the human axon radius distribution, so only large-axon tails are accessible with current gradients.","title":"Smallest axon diameter measurable by diffusion MRI in humans","topic_ref":"7d6543da8b95cbfe0f88251dd0987163345fc9bcbe4e99e44fdf7b56a0f8bcd0"},"attested":{"attestation":{"batch":null,"client_id":null,"id_token_sha256":null,"kind":"platform"},"record_hash":"67479d9a5f498458b4da0838db686bfe0e6ddbc3083e0f4c036aeb2bc8e5e87d","schema":"pubphys.attested/1"},"envelope":{"attested_hash":"7b15f094a7aee510baa4af8f5c51b67586f6fb4aa6718446dbf8a3796bd61073","platform_signature":{"key_id":"c6afc19b31429869751f06879c75cd64ea92654423d15b44be775bf1310a60da","sig":"BSx4awdacGljbOEjiziK5nGqjDjN1VJPLwt7f7fd38WGPCC76UzFnFD-DH4hOFVJEYogEkn3Cjjd72q2hMoOBQ"},"schema":"pubphys.envelope/1"},"record_hash":"67479d9a5f498458b4da0838db686bfe0e6ddbc3083e0f4c036aeb2bc8e5e87d","leaf_index":1669}