This study capitalizes on the emerging technology of 19F MRI, using conventional 'thermally' polarized perfluorinated gas (perfluoropropane, or PFP) mixed with oxygen and studied with magnetic resonance imaging (MRI) to visualize ventilation. This technique has not been studied in children. Children and adolescents (6-17 years old) with cystic fibrosis (CF) who have normal spirometry will undergo 19F MRI with the inhalation of an inert contrast gas to study ventilation. Comparisons will be made to a cohort of healthy children (6-17 years old) who will perform the same measures. The primary outcome measure is the feasibility of conducting these studies in the pediatric population. Parallel performance of multiple breath nitrogen washout (MBW) and spirometry will be used to compare the sensitivity of these outcomes to the presence of mild lung disease in these children. Finally, the investigators will compare data obtained during standard breath holds with a novel free-breathing technique that will eliminate the need for breath holds during MRI acquisition.