The purpose of this study is to conduct a comparative study for the study of bone mineralization evaluated with Quantitative computed tomodensitometry (QCT) compared to the reference technique, Dual-emission X-ray absorptiometry (DXA).

Patients with cystic fibrosis can have a deficit in bone mineralization. This is particularly well described in adults. However, in the pediatric population the results are more heterogeneous. The evaluation is hindered by difficulties in standardization of DXA interpretation.

This measurement depends from the mass and size of the bone as well as the mass of soft tissue covering the bone area. These two characteristics, intrinsic to the measurement, pose a significant problem of interpretations in children because a change in bone density may reflect both a change in bone mineral content or changes related to growth, for example, the increase in bone size or volume of soft tissue covering the bone of interest.

Quantitative computed tomography (QCT) provides a direct measure of bone mineralization densitometry volume. It quantifies bone mineral content in relation to the volume of the bone, in reference to an external phantom. This method therefore overcomes the size size.

This technique can be considered without an additional radiation exposure to patients during a lung CT because it is usual that lumbar vertebrae are included in the measurement window because of pulmonary hyperinflation. This exam would be ideal for patients with Cystic fibrosis.

All patients have these two evaluations during their routine management. This study compare study on the bone mineralization evaluated by (QCT) compared to the reference technique by (DXA) for the patient affected by cystic fibrosis for each patient.

Inclusion Criteria:

• Patients affected by cystic fibrosis
• Aged over 5 years
• Patients whose clinical condition warrants an indication of chest CT in relation to the criteria of the french consensus conference on Cystic fibrosis in May 2002 (Palace of the Luxembourg).
• Patients whose clinical condition warrants an indication of DXA examination in accordance with French recommendations (Consensus of the Working Group "bone mineralization and cystic fibrosis" In children, the examination is recommended for ages 8 every 2 years if the Z-score is greater than - 1, every year if the Z-score is less than - 1.. In adults, the exam is recommended every 5 years if the T-score is greater than> - 1, every 2 years if it is between -1 and - 2; annually if less than - 2.)
• Patient does not exhibit a phase of bronchial exacerbation
• Collection of non-opposition of the patient
• Patient affiliated to social security

Exclusion Criteria:

• Patient transplanted
• Patient with an infective exacerbation phase