Hi Dawn,
I had an MRI scan 2 weeks ago on my brain and felt a bit spaced out afterwards. I felt pretty depressed and foggy headed during the following week but that could have been from a number of things. Who knows. I too was worried that it may stir up the mercury but I was literally only in the machine for 10 minutes.
The only unpleasantness was the thumping noises and the slight claustrophobia.
Bet they don't find anything though.