Staining methods for magnetic resonance microscopy of the rat fetus

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Abstract
Purpose: To develop a magnetic resonance histology (MRH) staining and fixation method by immersion to enhance the signal‐to‐noise ratio (SNR) with a paramagnetic contrast agent permitting microscopic acquisition within a 3‐hour scan time.Materials and Methods: Methods were optimized for embryonic day 18.5 (E18.5) rat fetuses and imaging at 9.4T with an RF refocused spin‐echo pulse sequence (TR/TE = 75 msec/5.2 msec). Fixation/staining was performed by immersion in Bouin's fixative containing varied concentrations of ProHance (from 10:1 to 500:1 Bouin's:ProHance) and for varied immersion durations (up to 24 hours).Results: The results showed a significant change in T1 and T2 relaxation times as a function of concentration of contrast agent and immersion duration. As the contrast agent penetrated the tissues, T1 was reduced as desired (typically by 10×), but at the same time T2 was profoundly reduced (typically by 3×) due to both protein cross‐linking from the fixative and the high concentration of contrast agent. A systematic assessment of this staining protocol showed an increased SNR (by 5×) over that in unstained specimens.Conclusion: This staining protocol reduced scan time for very‐high‐resolution images (19.5 μm) to only 3 hours, making MRH a routine tool for evaluating fetal development. J. Magn. Reson. Imaging 2007;25:1192–1198.