Developing formalin-based fixative agents for post mortem brain MRI at 9.4 T

Abstract

Purpose: To develop fixative agents for high-field MRI with suitable dielectric properties and measure MR properties in immersion-fixed brain tissue.

Methods: Dielectric properties of formalin-based agents were assessed (100 MHz-4.5 GHz), and four candidate fixatives with/without polyvinylpyrrolidone (PVP) and different salt concentrations were formulated. B₁ field and MR properties (T₁ , $R_2^{\ast }$ , R₂ , $R_2^{\text{'}}$ , and magnetic susceptibility [QSM]) were observed in white and gray matter of pig brain samples during 0.5-35 days of immersion fixation. The kinetics were fitted using exponential functions. The immersion time required to reach maximum $R_2^{\ast }$ values at different tissue depths was used to estimate the Medawar coefficient for fixative penetration. The effect of replacing the fixatives with Fluoroinert and phosphate-buffered saline as embedding media was also evaluated.

Results: The dielectric properties of formalin were nonlinearly modified by increasing amounts of additives. With 5% PVP and 0.04% NaCl, the dielectric properties and B₁ field reflected in vivo conditions. The highest B₁ values were found in white matter with PVP and varied significantly with tissue depth and embedding media, but not with immersion time. The MR properties depended on PVP yielding lower T₁ , higher $R_2^{\ast }$ , more paramagnetic QSM values, and a lower Medawar coefficient (0.9 $\text{mm}/\sqrt{h}$ ; without PVP: 1.5). Regardless of fixative, switching to phosphate-buffered saline as embedder caused a paramagnetic shift in QSM and decreased $R_2^{\ast }$ that progressed during 1 month of storage, whereas no differences were found with Fluorinert.

Conclusion: In vivo-like B₁ fields can be achieved in formalin fixatives using PVP and a low salt concentration, yielding lower T₁ , higher $R_2^{\ast }$ , and more paramagnetic QSM than without additives. The kinetics of $R_2^{\ast }$ allowed estimation of fixative tissue penetration.

Keywords: B1 homogeneity; brain; dielectric properties; formalin; post mortem MRI.