A mathematical description of the changes of quantitative MPM parameters in ex-vivo whole brain human brains during fixation and hydration

Motivation: Relaxation rates in the in-vivo human brain are strongly different to their counterparts in formalin-fixed postmortem tissue. Goal(s): To model the changes of the relaxation rate parameters for different tissue stages from in-vivo to ex-vivo: unfixed, during fixation and during hydration. Approach: The multi-parameter mapping (MPM) protocol was used to measure the changes of five whole-human brains across the aforementioned tissue stages, and different saturation models were tested to describe relaxation parameter changes during fixation. Results: The MPM parameters varied strongly per tissue stage, and a mathematical description of the change of the MPM during fixation was found. Impact: We characterised the MPM parameters during the fixation and hydration process across the entire brain and propose a mathematical model to describe the changes. This information could facilitate translating microstructure-mapping methods from fixed ex-vivo tissue samples to in-vivo application