The ileocecal region comprises the cecum, appendix, ileocecal valve, and terminal ileum. Because of the complex topography of this region, it is sometimes difficult to identify positional anomalies or inflammatory or neoplastic changes. The normal ileocecal valve has a variable appearance on 2D and 3D images.
Detection of flat lesions in the cecum is sometimes difficult, because a cecal mass may be confused with untagged stool or a normal ileocecal valve. Postsurgical findings, such as an inversion ligation appendectomy, which can lead to inversion of the appendiceal stump, are another source of misinterpretation. The ileocecal region is also a predilection site for inflammatory changes, such as appendicitis and terminal ileitis in Crohn disease. These changes require very close inspection of the cecum and its anatomical structures during CT colonography. Especially on bidirectional endoluminal 3D evaluation, the orifice of the appendix and the ileocecal valve should be identified in every patient to ensure that the entire large bowel really has been fully examined.
Among the various postsurgical conditions seen at CT colonography, an inverted appendiceal stump following appendectomy is a common finding and a potential source of misinterpretation.
Inverted Appendiceal Stump
In patients who have previously undergone appendectomy, the inverted appendiceal stump may be seen as a round, smooth, polypoid filling defect at the cecum near the orifice of the appendix, and so can potentially mimic a polyp. An inverted appendiceal stump and an adenomatous polyp both have a sessile polypoid morphology on both 2D and 3D views (Figs. 4.100 and 4.101). On 2D images, both lesions have a homogeneous structure with soft-tissue attenuation. Of course they will not take up any tagging material. After intravenous administration of a contrast agent there may be contrast enhancement. When the patient changes position, neither lesion shows mobility. Hence, it is not always possible to distinguish with certainty between a polyp and an inverted appendix stump on morphological criteria. A history of previous appendectomy (inversion ligation) and absence of the appendix on 2D images can help. If there is uncertainty concerning a polypoid lesion at the cecum, an endoscopic examination should be performed.
Among the neoplastic lesions of the cecum, the most common are adenomatous polyps, carcinomas, and lymphomas. The CT criteria for identifying these pathologies are identical to the general criteria used for their identification in the remainder of the colon and are described in more detail under their respective headings above. In general, because of the proximity of the cecum to the small bowel, the presence of residual fecal matter is common, complicating the detection and interpretation of lesions.
Untagged stool particles adhering to the bowel wall may either obscure or mimic a flat lesion (Fig. 4.102). Tumorous masses involving the cecum may in some instances be confused with residual stool or with the ileocecal valve. Because of the larger diameter of the cecum and the ascending colon, malignant tumors do not lead to obstruction until they have reached an advanced stage (Fig. 4.103).