In acute appendicitis, which imaging finding is classically present on CT scan?

In acute appendicitis, the classic imaging finding on a CT scan is an enlarged, non-compressible appendix. This characteristic appearance arises due to the inflammation and swelling of the appendiceal wall, which prevents it from being compressed during the imaging process.

The enlargement is typically associated with surrounding inflammatory changes, which can also include periappendiceal fat stranding, as well as possible fluid collections. The non-compressibility of the appendix is an important diagnostic feature, as it distinguishes acute appendicitis from other conditions where the appendix may be compressible.

Other imaging findings, such as air-fluid levels, may be present in different conditions like bowel obstruction or perforation, and free air under the diaphragm is usually indicative of perforation but is not a typical feature of acute appendicitis itself. An extracorporeal fluid collection does not pertain directly to acute appendicitis and would suggest different pathology. Thus, the presence of an enlarged, non-compressible appendix is the hallmark finding that supports the diagnosis of acute appendicitis on CT imaging.