It isn't altogether unlikely. Thallium is pretty nasty stuff and does accumulate in body tissue. Several interesting murder cases have incorporated it - unless you know to look for heavy metal contamination it's a pretty sneaky way to kill somebody. A guy named Graham Young got away with it in the 70s until he started to brag about it.
I had a stress test a few weeks ago and they used some chemical other than thallium. I have no idea what it was, but I could really see the blood flowing through my carotids on the instrument screen.