The radioactive element used in some medical imaging may stop you from flying:

Certain medical scans can render people radioactive enough to trigger false security alarms at airports for up to a month, a Lancet article warns…

Thallium is apparently one of the worst culprits.
The article suggests that you get an information card from your physician if you are planning to travel subsquent to having one of these tests.
Of course, having one of these medical information cards in hand would give your average terrorist another tool to help avoid detection. So I expect the TSA and analogous agencies will be recommending, if not insisting, that if you are planning to that you do not undergo radioactive imaging x days before the flight (x being the number of days the element used in your test will cause security alarms).
Via MedGaget.