Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The lung volumes are low . A small opacity in the left retrocardiac region could be due to atelectasis given the poor inspiratory effort , although in the proper clinical setting , could be due to pneumonia . The lungs are otherwise clear . There is no pulmonary edema , pleural effusion , or pneumothorax . The cardiomediastinal silhouette is normal .
    OUTPUT:
    
Retrocardiac opacity in the setting of low lung volumes could be due to atelectasis or pneumonia . If desired , a repeat lateral radiograph with better inspiratory effort could help further characterize the opacity . Alternatively , could repeat a chest radiograph after treatment to document resolution .