A 55-year-old man on ICU with ARDS has been ventilated for 7 days. He develops a fever and a new right-sided chest opacity on CXR that could be either pneumonia or an empyema/parapneumonic effusion. Bedside ultrasound shows a complex echogenic fluid collection with septations. What feature on ultrasound distinguishes an empyema from a simple parapneumonic effusion?