Extravasation is a commonly used term by radiologists to indicate that blood is going outside a blood vessel. It is also used in a more general way to denote that a liquid or contrast is outside of a space where it is normally located. I have seen this term used across many different types of tests and areas of the body.
Most commonly, this term is used when there is active bleeding. An example would be a liver injury from trauma. On CT, contrast Extravasation would look like bright contrast in the area of injury indicating active bleeding. Contrast given through a vein makes the blood vessels look white. If that contrast is located outside a blood vessel, than that implies bleeding.
Another example would be an injury to a blood vessel in the leg after a gun shot wound. The CT would be done after giving contrast though a vein. The blood vessels will look white on the CT because contrast is within the blood vessels. If there is an injury to a blood vessel from a gun shot wound, than the white contrast will go outside the blood vessel. This will alert the clinical team that there is active bleeding and urgent treatment is necessary.
I also see the term Extravasation used when liquid material other than blood is located outside a lumen or compartment. An example would be a CT looking for a leak from the esophagus. Normally, the swallowed barium contrast stays in the tube which goes from the throat to the stomach. Contrast will go outside the esophagus if there is a tear. This can be referred to as Extravasation as well.
Another example would be after surgery to the colon. Sometimes surgeons like to check to make sure there is no leak after they remove a piece of the colon and hook up the remaining colon. If the contrast placed into the colon leaks out than this is called extravasation. Similarly, contrast which leaks out the stomach after surgery or an ulcer being present can be referred to as extravasation.
Extravasation is therefore a way of saying that blood or other liquid is located outside the lumen of a vessel or other compartment in the body. It is usually abnormal, most common when there is bleeding. It can also imply a leak of the bowel or an injury of the bladder to name a few.