Effects of Postmortem Intervals on Quantitative MRI in Unfixed and Fixed Swine Brain: Implications for Ex Vivo MRI...

The postmortem interval (PMI) alters tissue properties that shape quantitative MRI (qMRI) signals. We systematically investigated the effects of PMI on relaxation times, depending on tissue characteristics, in both unfixed and fixed pig brains.

Twelve pig brains (n = 12) were scanned in both unfixed and fixed states at 3 PMI windows (≈12, 24, 48h). Quantitative T1, T2, and T2* maps were acquired with identical protocols at controlled room temperature (~24°C). We assessed the PMI effects on relaxation times in the gray matter (GM) and the white matter (WM) at both group and sample levels, while fixation-induced effects and inter-sample variability were evaluated using pairwise rank correlation and coefficient of variation (CV) visualization.

In unfixed tissues, T1 significantly differed among PMI groups in GM (P = 0.0141) and WM (P = 0.0315) in the early window (≤24h). In the same window, PMI-T1 correlations were observed in both GM (r = 0.921, P = 0.007) and WM (r = 0.876, P = 0.013). T2 showed no group differences but exhibited an inverse correlation with PMI in WM (r = -0.849, P = 0.015). No significant PMI-T2* relationships were detected (all P > 0.05). At later PMI (20-50h), PMI-qMRI correlations diminished. Fixation processes altered all qMRI parameters. Notably, the PMI effects on T1 in the unfixed brains were preserved even after fixation at an ordinal level, although fixation introduced substantial inter-sample variability.

PMI exerts the most robust effects on T1. Fixation has a significant impact on qMRI values, mitigating the apparent PMI effect and increasing the inter-sample variability. However, it is still possible to retrieve the PMI effects from fixed brain at the level of rank order, providing practical guidance for further ex vivo qMRI studies on PMI.