Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Single portable view of the chest compared to previous exam from ___ . Lower lung volumes seen on the current exam . Given differences in positioning and technique , the lungs are clear , noting that the retrocardiac region is relatively dense which could be technical and positional . Cardiomediastinal silhouette is unchanged . Osseous and soft tissue structures are unremarkable .
    OUTPUT:
    
Retrocardiac opacity , potentially technical in nature ; however , if concern , repeat exam with frontal and lateral views may help further characterize .