Recruiting period: 1 year Study period: 2 years per patient, 3 years as a whole LCI will help to evaluate early ventilation inhomogeneity due to small bronchi abnormalities in young children with CF. Concordance between LCI and CT endpoints evaluation will help to define the frequency of lung CT administration in young CF children. Predictive value for later abnormal lung function development will be evaluated. This will help to detect the children at risk to develop abnormal lung function and to target those requiring active treatment. Finally this endpoint will be very valuable in the future therapeutic trials for CFTR correctors or potentiators.