Longitudinal quantitative MRI of whole human brains during formaldehyde immersion, PBS wash-out, and tissue volume change

Postmortem MRI links in vivo MRI contrast to microstructure measured with ex vivo histology, yet fixation systematically alters relevant physical MRI parameters. We quantified these effects by longitudinally characterizing multi-parametric mapping (MPM) measures - longitudinal (R1) and effective transverse (R2*) relaxation rates, normalized proton density proxy (NA), and magnetization transfer saturation (MTsat) - and tissue-volume change in five whole human brains scanned unfixed ex vivo in situ, during formaldehyde immersion fixation, and, when available, after transfer to phosphate buffered saline solution. We included an independent in vivo cohort of 25 younger healthy participants to provide a qualitative in vivo reference. The largest changes were found for R1 during fixation relative to in situ values (up to 351% in dGM), followed by R2* (up to 60% in WM). NA was largely unchanged during fixation. Although MTsat decreased by up to 25% in gray matter from in vivo to in situ, it largely preserved tissue contrast during fixation and PBS immersion, making it an ideal candidate for longitudinal registration across tissue conditions. Phenomenological models characterized fixation-induced parameter changes and relative tissue-volume change. R1 and fixation-associated volume loss evolved under broadly similar timescales, consistent with a contribution from fixation-related water loss or redistribution. This densely sampled dataset and analyses provide a resource for linking in vivo MRI to postmortem fixed-tissue MRI and ex vivo histology under a standardized immersion-fixation protocol.