Pleural empyemas usually occur secondary to underlying pneumonia, with pneumococcus the most common cause in pediatric patients. Empyema may be suspected if there is the failure of resolution of infectious/febrile symptoms in a child with pneumonia and a persistent pleural effusion on a chest x-ray. 

If available, ultrasound evaluation of pleural fluid collections is usually preferred over CT as a first-line modality in pediatric patients. 

While this case demonstrates typical features for pleural empyema.  A pleural fluid infection cannot be excluded on imaging alone even if the effusion appears simple on ultrasound.