DJ coffins are probably a good place to start, but they're not going to protect the media from fire or water damage. Of course, you're using a proven unreliable media, so maybe that doesn't matter. ;-) Really, you should be using online storage (read: servers with magnetic disks) or a combination of online and nearline storage (tape on demand or something similar) with offsite tape archiving in a controlled environment.
If your MRI system isn't networked, then optical media is fine for temporary storage as you shuttle data from the MRI to a server.
* I don't work in the medical field, and I have no idea how the UK deals with patient privacy issues. In the US, locking up a bunch of discs in a file cabinet probably wouldn't be acceptable under current regulations.
