Roflumilast (Daxas®, Takeda) is a selective phosphodiesterase type 4 (PDE4). Recently Roflumilast has been added as a therapeutic option for severe COPD patients. While the product seems to reduce the number of exacerbations in COPD patients, a number of questions remain the topic of ongoing research. In particular the interaction between the orally administered Roflumilast and concomitant medication such as the standard of care inhalation therapy needs further research. The inhalation therapy in COPD patients is often a combination of long acting beta 2 agonist (LABA), long acting anti-muscarinic agents (LAMA) and inhaled corticosteroids (ICS). One of the main challenges in developing novel anti-inflammatory compounds such as Roflumilast is that the current gold standard endpoints such as the forced expiratory volume in one second (FEV1) lack the sensitivity to detect subtle differences in the respiratory system. A novel image method called Functional Respiratory Imaging or FRI allows detecting changes in the lungs and airways with higher accuracy and hence could be used to describe the effect of anti-inflammatory interventions. The current study therefore aims to assess the mode of action of Roflumilast as ad-on to LABA/LAMA/ICS therapy in severe COPD patients. In addition the study aims to identify the characteristics of the responders.