Purpose: MR imaging of post mortem brains has the potential to yield volumetric information and define the extent of structural changes prior to pathological examination. Although standard T2-weighted clinical imaging sequences have been used for the examination of formalin-fixed brains, these protocols may not yield optimum contrast. We examined the effect of varying durations of formalin fixation on the transverse relaxation time (T2) and the tissue spin density.
Material and methods: Three post mortem brains were examined weekly during formalin fixation from the unfixed state to 35 days fixation. Standard MR spin-echo imaging was used at 5 echo times (20-100 ms) to calculate transverse relaxation time (T2) and spin density.
Results: T2 decreased significantly (ANOVA, p<0.001) in both grey and white matter by 7 days fixation and there was a further (but non-significant) trend towards lower values between 7 and 35 days. Grey and white matter T2 times converged with fixation. Conversely, the grey to white matter spin density ratio increased from 1.19+/-0.01 to 1.54+/-0.06 over five weeks of fixation.
Conclusion: Our results suggest that spin density-weighted imaging sequences would provide improved grey to white matter contrast over T2-weighted sequences.