Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: There is an unchanged minor left retrocardiac opacity likely representing atelectasis . There is mild pleural thickening bilaterally . Mild cardiomegaly , but no pulmonary edema . Status post mitral valve replacement .
    OUTPUT:
    
Unchanged mild retrocardiac opacity likely representing atelectasis . No evidence of pneumonia .