It's was most probably in DICOM (http://en.wikipedia.org/wiki/Digital_Imaging_and_Communicati...) - pretty much a standard nowadays and you can find a lot of FOSS applications that can read it. Just watch out for the file sizes - those images are not compressed high-resolution 3d 16-or-more-bit grayscale, so you need massive amounts of ram/swap.

The format itself is cool though - it can support annotations, arrows and basically anything-you-want overlayed on certain parts of the image afterwards.

I didn't save my work unfortunately ;-)

I'll have a look when I get home tonight though - there's a decent chance everything is still in my bash history!