Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Linear retrocardiac opacity likely represents atelectasis . The lungs are otherwise clear and the cardiomediastinal contours are normal . Heart size is top normal . No pleural effusion or pneumothorax . No subdiaphragmatic free air is seen .
    OUTPUT:
    
Left lung base opacity likely represents atelectasis . No evidence of pneumonia . Heart size is top normal .