From this post by P.Z. Myers about a recent paper:
[...]This paper tries something different: the investigators had people have sex in an MRI tube, and snapped a few pictures while they were at it.
This wasn't easy. If any of you have seen an MRI tube, they tend to be small and cramped, difficult for claustrophobics to handle, and a tight enough squeeze for one person, let alone two. The methods section is the most interesting I've ever read.
Just head over there and read the rest of it!