The group Allison is referring to (Hyare et al., 2008) found that storing tissue in Fomblin for 2 days prior to MRI had no effect on scan or histology results when compared to matched samples stored in 10% formol-saline.
Synopsis: Formalin-fixed mouse brain tissue placed in Fomblin for 2 days prior to T1, T2, DWI, and MTR scans @ 9.4T produced the same results as formalin-fixed controls placed in 10% formol-saline solution 2 days prior to scanning. Both were scanned in Fomblin. They compared ROI-based quantitative MR measures, anatomical & cellular histology, and immunohistochemical staining.
Fluorinert has been used in a number of studies. A quick search came back with +5 exvivo studies since 2004 using Fluorinert as an in-scan solution - typically Flourinert FC-77, not FC-3283.
3M Fluorinert Electronic Liquids for Electronics: https://www.3m.com/3M/en_US/company-us/all-3m-products/~/All-3M-Products/Electronics/Chemicals/Chemicals/Fluorinert-Electronic-Liquids/?N=5002385+8709318+8710710+8711017+8736409+8745514+3294857497&rt=r3
Studies using Flourinert: Noristani et al., 2015, Colon-Perez et al., 2015, Kovačević et al., 2004, etc. (https://scholar.google.com/scholar?hl=en&as_sdt=0,22&q=ex+vivo+mri+fluorinert)