Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: As compared to the prior radiograph , the retrocardiac opacity has improved and may represent a focal area of atelectasis or aspiration . The lungs are otherwise clear , the cardiac , hilar , mediastinal contours are stable , and the calcified aortic knob is also stable . No pleural abnormality is seen .
    OUTPUT:
    
Interval improvement in retrocardiac opacity which may represent atelectasis or aspiration .