Improving in-vivo myelin and iron mapping from relaxation rates maps by incorporating relaxation rate changes from in-vivo to ex-vivo conditions

Motivation: Estimating myelin and iron concentration maps from in-vivo MRI yields biased estimates because MR-to-microstructure linear mappings are derived from fixed-postmortem human brain tissue. Goal(s): We assessed whether taking into account the changes of relaxation rates from in-vivo to hydrated fixed ex-vivo specimens would allow the use of current MR-to-microstructure linear mappings for in-vivo MRI. Approach: We introduced a pipeline that accounts for the major relaxation-rate changes during fixation and hydration, and compared the estimated MRI-based myelin parameters to their counterparts from light microscopy in the human corpus callosum. Results: We found that including these changes significantly improved the accuracy of the myelin estimates. Impact: We proposed a new method that significantly improves the MRI-based myelin and iron maps estimation from in-vivo longitudinal and effective transverse relaxation rates.