Not an urban legend - bmj.com (I think) is the site, and the past issues are available there. The Xmas issues (nearest issues to 25th Dec.) are always on the humorous side, though still medical. The main subject of the MRI was a trained acrobat (and well-known street performer), but they did get several others, and orgasms were involved.
The other one - unrestrained metal objects (including one that actually killed the poor patient) hurtling into the magnet is also not an urban legend. There is/was a website (sorry, I don't have the URL) with ten or a dozen photos of such objects - mainly chairs, but also an oxygen tank.