Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The cardiac , mediastinal and hilar contours are unchanged , with the heart size within normal limits . Lung volumes are low which causes crowding of the bronchovascular structures . No overt pulmonary edema is demonstrated . Linear opacity in the left mid lung field likely reflects subsegmental atelectasis or scarring , unchanged . Patchy opacity in the retrocardiac region may reflect atelectasis . Infection , however , can not be completely excluded . No pleural effusion or pneumothorax is present . No acute osseous abnormalities visualized . There are multilevel degenerative changes in the thoracic spine .
    OUTPUT:
    
Patchy retrocardiac opacity may reflect atelectasis but infection can not be completely excluded