I'm not sure if this is entirely accurate, but I had also heard of "old" tattoo's being a potential problem . . .

" . . . about 20 years ago and further, tattoo ink was sometimes comprised of small fragments of metal as well as other ingredients. This was long before tattoos were ever regulated and before more serious thought was given as to the safety of tattoo ink ingredients. Some MRI patients who have had tattoos that dated back far enough to have received ink that contained metal bits have reported slight discomfort to severe pain during an MRI scan.

It is purported that the reason for this was that the magnetic force pulled on the metallic fragments so violently that it caused a burning sensation in the location of the tattoo. I have heard some theorize that this may have been caused by built up friction between the particles, and some say that the magnetic force was actually tearing at the skin as the fragments were pulled and attempted to actually break away from the skin.

I don’t know which, if either, is true; however, even if there is no pain at all, these fragments can cause artifacts, which is the technical term used for distortions in MRI scans. Artifacts can render a scanned image useless, requiring that the procedure be done again or even an alternate procedure be used to acquire accurate information. So, at the very least, you could be stuck with a very expensive bill for nothing -if, of course, your tattoo actually contains these metal particles."

http://tattoo.about.com/cs/tatfaq/a/mri_scan.htm

Cherie