Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The lungs are well - expanded . New retrocardiac opacity with silhouetting of the lateral border of the descending aorta , most likely secondary to atelectasis . Otherwise , no pleural effusion , pulmonary edema , or pneumothorax . The heart is top - normal in size . Mediastinal contours and hila are unchanged .
    OUTPUT:
    
New retrocardiac opacity , most suggestive of atelectasis .