Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: AP portable semi upright view of the chest . Lung volumes are low . Subtle retrocardiac opacity may represent mild atelectasis versus subtle aspiration . Right lung is clear . No definite signs of effusion or pneumothorax . Cardiomediastinal silhouette appears within normal limits . No acute bony abnormalities .
    OUTPUT:
    
Subtle retrocardiac opacity likely atelectasis though difficult to exclude mild aspiration .