Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Portable upright frontal view of the chest . The lung volumes are low ; however , they have improved since ___ . Linear bibasilar opacities most likely represent atelectasis . No pulmonary edema , pleural effusion or pneumothrax is seen . The aortic knob is calcified . The heart size is normal . There is no free air beneath the hemidiaphragms . No acute osseous abnormality is seen .
    OUTPUT:
    
Bibasilar opacities most likely represent atelectasis .