Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: PA and lateral views of the chest provided . Allowing for low lung volumes , there is no overt evidence of pneumonia or CHF . There is mild retrocardiac opacity which is most compatible with atelectasis , though a very early pneumonia is impossible to exclude . No large effusion or pneumothorax is seen . The cardiomediastinal silhouette is normal . Imaged osseous structures are intact . No free air below the right hemidiaphragm is seen .
    OUTPUT:
    
Subtle retrocardiac opacity likely represents atelectasis , difficult to exclude a very early pneumonia .