The investigators are developing a new nuclear medicine imaging technique for measuring liquid absorption in the airways that can be applied to screen new medications being developed to treat cystic fibrosis (CF). The investigators believe that the absorption of the small molecule radiopharmaceutical Indium 111 diethylene triamine pentaacetic acid (In-DTPA) will indicate changes in liquid absorption in the airways and demonstrate whether new CF medications will be effective. In this study the investigators will determine whether the imaging technique will demonstrate similar results when it is repeated on different days. They will also determine how their results change when subjects utilize several common CF medications.