Summarize the radiology report findings into an impression within 3 sentences.
    INPUT: A single portable semi-erect chest radiograph is obtained . Lung volumes are low . A retrocardiac opacity has improved since ___ . The right lung is clear . There is no effusion or pneumothorax . Cardiomegaly is mild .
    OUTPUT:
    
Retrocardiac opacity most likely represents atelectasis and is improved since ___ .