Distinguishing between an empyema and an abscess that abuts the chest wall can be difficult. Since an abscess originates in the lung and grows spherically i.e. like a ball, when it touches the chest wall, the angle between the abscess and the lung will be acute. An empyema originates in the pleural space, with the visceral and parietal pleura forming an envelope around the fluid collection, resulting in an obtuse angle at the interface of the empyema and chest wall. Abscesses will be round (roughly!) in all 3 planes. Empyemas conform to the shape of the pleural space and may appear round in one plane and crescentic in another. These are rough guidelines, and it can sometimes be a struggle to determine if angles are acute or obtuse or if the lesion is "round enough" to be an abscess. In this case, a large amount of pus was drained from the pigtail catheter.