A fluid collection is any abnormal pooling of fluid outside of the normal fluid-containing structures of the body. On ultrasound, fluid collections appear as anechoic or hypoechoic areas, and their appearance can provide clues about their contents. Simple, anechoic collections may represent seromas or transudates, while complex collections with internal echoes, septations, or debris may indicate blood, pus, or other proteinaceous fluid. Common locations for pathological fluid collections include the peritoneal cavity, the pleural space, the pericardial sac, and around surgical sites. Ultrasound is not only the primary method for detecting fluid collections but also the most common tool used to guide needle drainage of these collections.
Sonography Term