Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: As before , there is a persistent retrocardiac opacity which may represent atelectasis or infection . The lungs are otherwise clear aside from moderate emphysesma . Cardiomediastinal contours are normal . No pleural effusion . Blunting of the costophrenic angles bilaterally may represent a small amount of pleural fluid .
    OUTPUT:
    
Retrocardiac opacity may represent atelectaisis or infection .