Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Heart size is normal . Mediastinal and hilar contours are unchanged . Pulmonary vasculature is normal . Patchy opacity within the retrocardiac region may reflect atelectasis but infection or aspiration can not be excluded . There is a probable trace left pleural effusion . Right lung is clear . No pneumothorax is present . No acute osseous abnormalities seen .
    OUTPUT:
    
Patchy retrocardiac opacity could reflect atelectasis but infection or aspiration is not excluded in the correct clinical setting . Probable trace left pleural effusion .