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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
, R2 , R′2 , 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
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
, more paramagnetic QSM values, and a lower Medawar coefficient (0.9 mm/√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
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
, and more paramagnetic QSM than without additives. The kinetics of R∗2 allowed estimation of fixative tissue penetration.
