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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, R2∗, R2, R2′, 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 R2∗ 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 R2∗, more paramagnetic QSM values, and a lower Medawar coefficient (0.9 mm/normalh; without PVP: 1.5). Regardless of fixative, switching to phosphate‐buffered saline as embedder caused a paramagnetic shift in QSM and decreased R2∗ 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 R2∗, and more paramagnetic QSM than without additives. The kinetics of R2∗ allowed estimation of fixative tissue penetration.

Magnetoreception in animals illustrates the interaction of biological systems with the geomagnetic field (geoMF). However, there are few studies that identified the impact of high magnetic field (MF) exposure from Magnetic Resonance Imaging (MRI) scanners (>100,000 times of geoMF) on specific biological targets. Here, we investigated the effects of a 14 Tesla MRI scanner on zebrafish larvae. All zebrafish larvae aligned parallel to the B0 field, i.e. the static MF, in the MRI scanner. The two otoliths (ear stones) in the otic vesicles of zebrafish larvae older than 24 hours post fertilization (hpf) fused together after the high MF exposure as short as 2 hours, yielding a single-otolith phenotype with aberrant swimming behavior. The otolith fusion was blocked in zebrafish larvae under anesthesia or embedded in agarose. Hair cells may play an important role on the MF-induced otolith fusion. This work provided direct evidence to show that high MF interacts with the otic vesicle of zebrafish larvae and causes otolith fusion in an “all-or-none” manner. The MF-induced otolith fusion may facilitate the searching for MF sensors using genetically amenable vertebrate animal models, such as zebrafish.