Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Compared to prior , there is opacity a partially obscuring the left heart border , concerning for pneumonia or atelectasis . The right lung is clear . No pleural abnormality is seen . Mediastinal contour is consistent with patient 's known thoracic aortic dissection and descending aortic dilatation , unchanged from prior .
    OUTPUT:
    
Possible lingular pneumonia vs . atelectasis .