Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: A single portable erect chest radiograph was obtained . The lung volumes are low . There is an ill - defined retrocardiac opacity , which does not silhouette the left hemidiaphragm . Median sternotomy wires are intact . The aortic arch is calcified .
    OUTPUT:
    
Low lung volumes . Retrocardiac opacity likely represents atelectasis rather than focal consolidation .