Several observational studies have shown that cystic fibrosis (CF) patients less than or equal to 6 years of age have clinically silent airway damage. There is growing interest in early initiation of therapies to prevent or delay the progression of this lung disease in CF. In SHIP-CT, the investigators will evaluate treatment effects of HS relative to IS on measures of structural lung disease obtained from chest CT using a novel scoring system sensitive to early lung changes, the Perth-Rotterdam Annotated Grid Morphometric Analysis method for CF (PRAGMA-CF), that quantifies the volume percentage of diseased airways (%Dis), bronchiectasis (%Bx), and trapped air (%TA). As a secondary evaluation of structural airway damage, the investigators will use an image analysis system to measure airway dimensions relative to adjacent arteries (AA-system). Longitudinal changes in CT measures will also be compared to changes in lung function measured by the lung clearance index (LCI) obtained by N2 Multiple Breath Washout (MBW) and to clinical outcomes.