The collection in the lower abdomen/pelvis does not have the typical appearances of a post-operative abscess, particularly the pattern of gas distribution and the multiple dense curvilinear structures. The findings are, however, typical of a retained surgical swab, known as a gossypiboma. The swab contains dense linear strands which are seen as the curvilinear structures, and organization of a granulation membrane leads to formation of a collection that resembles an abscess. The dispersed bubbles of gas are trapped within the cotton mesh, whereas a typical abscess due to a leak from an anastomosis would have more air in the non-dependent portion. The findings were confirmed at re-operation and the 'mass' was removed.

Note that surgical hemostatic material is another item that is often confused for abscesses, particularly as both are seen in post-operative imaging of patients who have undergone complex procedures with a stormy post-operative course. The best diagnostic tool is being told that it has been used in the request, but other tips include identifying a geographic distribution of gas bubbles, polygonal shapes, and a relative lack of change over serial imaging studies.