Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The lungs are well expanded . There is a somewhat linear opacity in the retrocardiac region , along the left margin of the heart , which appears to correlate with an opacity in the retrocardiac region in the lateral view . Cardiomediastinal and hilar contours are unremarkable . There is no pleural effusion or pneumothorax .
    OUTPUT:
    
Linear retrocardiac opacity , potentially atelectasis although infection can not be entirely excluded .