As we saw before, a simple transudative pleural effusion is anechoic. On the other hand, presence of echogenicities in the effusion is suggestive of complex/exudative pleural effusion. The term ‘plankton sign’ is often used to indicate debris appearing as swirling, punctiform echoes in such effusions and may indicate infection or hemothorax depending on the clinical context. Rarely, it can be seen in simple effusions after aggressive diuretic therapy. As the collection progresses, fibrin is activated and septations or loculations appearing as thin hyperechoic lines begin to form within the fluid collection, resembling a ‘spider web’. This is an important finding because effusions with septae are more likely to require a surgical intervention rather than a chest tube drainage and antibiotics alone.

Following are several examples of complex pleural effusion/empyema.