Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The lungs are well expanded . A retrocardiac opacity is seen which is likely due to atelectasis although infection is hard to exclude . Given the linear shape of the opacity , atelectasis is perhaps more likely . The heart is top - normal in size . The cardiomediastinal silhouette is otherwise unremarkable . There is no pneumothorax or pleural effusion . Visualized osseous structures are unremarkable .
    OUTPUT:
    
Retrocardiac opacity , likely due to atelectasis but possibly due to pneumonia in the appropriate setting .