Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: Given the patient 's rotation , the mediastinum and cardiac silhouette is within normal limits . There is a left - sided pacemaker battery pack with leads terminating in the right ventricle and right atrium , in unchanged position from the prior study . Blunting of the right costophrenic angle on the AP view is not collaborated on the lateral view ; therefore no pleural effusion is thought to be present . A retrocardiac opacity is present and may represent a combination of atelectasis and aspiration . Pneumonia can not be ruled out in the correct clinical setting . There is no pulmonary edema and there is no pneumothorax . There is no free air .
    OUTPUT:
    
Retrocardiac opacity concerning for atelectasis versus pneumonia in the correct clinical setting .