Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Heart size is normal with mildly tortuous thoracic aorta . Hilar contours are unremarkable . There is a subtle increased density in the posterior lower lung fields on lateral view only without frontal correlate which could represent atelectasis or a subtle pneumonia . There is no pleural effusion or pneumothorax .
    OUTPUT:
    
Slightly increased density in the lower posterior lung field on lateral view only , which is likely atelectasis ; however , infection is not excluded given the correct clinical circumstance .