I used 1% agar dissolved in phosphate buffered saline (PBS, AbD Serotec, MorphoSys, Munich, Germany)

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0022835

Thanks Lejla. I had hoped it was that simple, but thought worth asking rather than have it melt half way through the scan or something unexpectedly odd. Happy New Year!

It may be advantageous to us a material that is MRI invisible (Fomblin has been used and is a good susceptibility match to the tissue), you will still need to restrain the sample though, perhaps with a close fitting tube.