Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: The lung volumes are low . The cardiac , mediastinal and hilar contours appear stable allowing for differences in technique . There is fairly substantial retrocardiac opacification , although predominantly linear and streaky . Elsewhere , the lungs appear clear . There is no pleural effusion or pneumothorax .
    OUTPUT:
    
Retrocardiac opacity at the left lung base , somewhat striking although suggestive atelectasis ; an infectious process is difficult to exclude , however .