Effects of Nycthemeral Variations on Computed Tomography (CT) Parameters Reflecting Airways Remodelling, and Pulmonary Emphysema Extent in Chronic Obstructive Pulmonary Disease (COPD)

Patients : COPD patients aged 40 years or more, with a smoking history of \> 10 pack-years (PY), a post-bronchodilator forced expiratory volume in one second to vital capacity ratio (FEV1/VC) \< 0.7 and an optimal treatment according to GOLD guidelines will be included. Exclusion criteria are: COPD exacerbation or respiratory infection in the 4 weeks before the begin of the study, concomitant pulmonary disease (tuberculosis, significant bronchiectasis, lung cancer), pulmonary resection, active malignancy or malignancy of any organ system within the past 5 years. Procedures: All measurements will be performed on the same day, after obtaining the written informed consent of the patient. Medical history, smoking status and patient's treatment will be collected. At 08:00 AM (T0): PFT will be performed, including vital capacity (VC), forced vital capacity (FVC), functional residual capacity (FRC), total lung capacity (TLC), residual volume (RV), forced expiratory volume in one second (FEV1), diffusion lung capacity for carbon monoxide (DLCO), and alveolar volume (VA) measurements (either in absolute values and percentage of predicted values). A first CT scan will be performed using the following technique: * Supine CT scan after full inspiration. * Acquisition parameters: (Topogram 35 mA 120 kV 512 mm length) 90 quality ref mAs with care-dose ON 120 kV Pitch 1.4 Rotation time 0.33 s Acquired images 64 x 0.6 mm * Reconstructions parameters: B60f 1mm-thick every 0.7 mm, B20f 1mm-thick every 10.0 mm, B35f 1mm-thick every 0.7 mm, B60f 5mm-thick every 5.0 mm and B35f 5mm-thick every 5.0 mm At 04:00 PM (T0+8h): PFT will be performed, including vital capacity (VC), forced vital capacity (FVC), functional residual capacity (FRC), total lung capacity (TLC), residual volume (RV), forced expiratory volume in one second (FEV1), diffusion lung capacity for carbon monoxide (DLCO), and alveolar volume (VA) measurements (either in absolute values and percentage of predicted values). A second CT scan will be performed using the following technique: * Supine CT scan after full inspiration. * Acquisition parameters: (Topogram 35 mA 120 kV 512 mm length) 90 quality ref mAs with care-dose ON 120 kV Pitch 1.4 Rotation time 0.33 s Acquired images 64 x 0.6 mm * Reconstructions parameters: B60f 1mm-thick every 0.7 mm, B20f 1mm-thick every 10.0 mm, B35f 1mm-thick every 0.7 mm, B60f 5mm-thick every 5.0 mm and B35f 5mm-thick every 5.0 mm Data analysis: * Emphysema index: from B20f reconstructions, calculation of RA960 using Pulmo CT software. * Airway index: from B60f reconstructions (1mm-thick every 0.7 mm), calculations of luminal area and wall area for several bronchi. Statistical analyses : Comparisons of CT parameters values or derived values (for example : wall area to airway area ratio) reflecting emphysema and airways remodelling measured on CT scans obtained à T0 and T0+8h. Correlations with PFT values and derived values.