Introduction: Impact of ultra-low-dose (ULD) computed tomography (CT) on quantification of structural disease and structure-function relationships in early Cystic Fibrosis (CF) lung disease have not been studied to date. Aim: To quantify structural lung disease and compare same session standard low radiation dose (LD) and ULD CT with pulmonary function measures in early CF lung disease. Methods: 46 CF subjects (age 11(6-17) yrs) with mild lung disease (FEV1≥70%pred) performed multiple breath washout (MBW), and spirometry directed paired inspiratory/expiratory CT with LD (CTDIvol: 1.23 mGy) and ULD (CTDIvol: 0.22 mGy). Quantification was performed automatically by validated institutional CT software. Results: Effective radiation dose was reduced by 78% in ULD vs. LD. Rates of abnormality were highest for CT and MBW and lowest for spirometry. Strong correlations were observed between LD and ULD readouts (gas trapping r=0.91, p
