A fluid collection (often colloquially expressed in the medical vernacular as a collection) is a non-specific term used in radiology to refer to any focal loculation of liquid in the body, usually within a pre-existing anatomical space/potential space e.g. peritoneal, pleural, subdural. The term is deliberately employed when the more specific nature of the collection is not yet known. The fluid may be pus, blood, serous, chyle etc.
Free fluid (most commonly used in the phrase: peritoneal free fluid) is the antonym of fluid collection, and is used when the fluid is not loculated.
Fluid collections include:
NB this is not intended to be an exhaustive list
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Terms used in radiology