Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The heart size is normal . The hilar and mediastinal contours are normal . There is a subtle opacity in the retrocardiac region seen only on the lateral view , which may be secondary to atelectasis . There is no pleural effusion or pneumothorax . The visualized osseous structures are unremarkable .
    OUTPUT:
    
Subtle opacity in the retrocardiac region may be secondary to atelectasis ; however , an acute infectious process can not be excluded .