13. Participating in any other research study involving exposure to ionizing radiation in the past year will be excluded if the total cumulative exposure from the past research studies and the current research study would exceed the limits set by the FDA in 21 CFR 361.1. Specifically, the total cumulated dose to the whole body, active blood-forming organs, lens of the eye, and gonads must remain below 5 rems, and the cumulated dose to all other organs must remain below 15 rems. Potential participants who have had exposure to ionizing radiation in the past year will not be allowed to participate if we are unable to obtain proper documentation quantifying the amount of past exposure.