Normal Anatomy of the Colon and Rectum

10.1055/b-0034-91855

Normal Anatomy of the Colon and Rectum

Thorough knowledge of the anatomy and the regular two-dimensional (2D) and three-dimensional (3D) CT morphology of the large bowel is essential for the interpretation of CT colonography. In virtual colonoscopy, as in conventional colonoscopy, anatomical landmarks may be used as a guide for orientation in the colon (Figs. 4.1 and 4.2). Identifying and documenting anomalies in the position or length of the colon provides important information for subsequent conventional endoscopy or surgical intervention; these should be noted in the radiologist′s report along with the location of any pathologies.

CT Morphology of the Bowel Wall

The wall of the large bowel has three typical structures that are always identifiable at CT colonography: the taeniae, the semilunar folds, and the haustra.

**Anatomical segments of the colon:** Cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum.

**Normal anatomy of the colon on axial 2D views.** a Global 3D view of the colon for spatial orientation with marking of the corresponding planes (1–3). b The corresponding axial CT image at the level of the left flexure (1) shows the ascending colon on the right, the transverse colon in the middle, and the descending colon on the left. c The corresponding axial CT image at the level of the upper iliac crest (2) shows the ascending colon on the right, the lowest point of the transverse colon in the middle, and the descending colon on the left. d The corresponding CT image at the level of the lesser pelvis (3) shows the cecum on the right, the sigmoid colon on the left, and in the middle of the image the rectosigmoid junction.

**Taeniae, semilunar folds, and haustra.** This endoluminal 3D view clearly shows the taeniae as discrete longitudinal indentations in the bowel lumen with the semilunar folds (plicae semicirculares) and the haustra between them.

Taeniae. The taeniae are three 1-cm wide bands of longitudinally oriented smooth muscle running in the colonic wall. They are more prominent in the proximal colonic segments, become thinner further distally, and finally disappear in the lower sigmoid colon. At CT colonography the taeniae are most readily identified in the transverse colon and in the ascending colon, where they appear as slight longitudinal indentations in the lumen (Fig. 4.3).

Semilunar folds. The plicae semicirculares (also known as semilunar folds) are crescent-shaped colonic folds that should be understood, not as a rigid structure, but as variable functional entities. For this reason, the semilunar folds may appear different on supine and prone scans of the same patient. The semilunar folds are normally thin, delicate structures with soft-tissue attenuation (Fig. 4.4). They can be readily identified at CT colonography on both 3D and 2D views. Patients with diverticular disease may have prominent or bulbous folds due to muscular hypertrophy. Complex fold structures are typically found at flexures and loops (Fig. 4.5) or on the base of the cecum (Fig. 4.6). If bowel distension is inadequate, there may be contact between opposing folds which can result in artifacts (“kissing folds”) and make proper colonic evaluation more difficult.

Haustra. The haustra are smooth outpouchings of the colonic wall between the semilunar folds and the taeniae. Because of the three taeniae there are also three rows of haustra, each sited between two taeniae. The depth of the haustra depends on the distension of the colon. When distension is inadequate, the haustra are deeper.

**Three-dimensional view of the semilunar folds.** a On combined 2D/3D views, the semilunar folds are depicted as thin, crescent-like structures. b The virtual dissection view clearly shows three rows of semilunar folds and the haustra, separated by the three taeniae.

Wall thickness and characteristics. Assessment of wall thickness is especially important when neoplastic or inflammatory changes are present, as these can give rise to circumscribed or diffuse thickening of the wall. The wall of a normal colon is very thin (less than 2 mm thick) when fully distended (Fig. 4.7). On 2D views, the bowel wall is depicted as a narrow, soft-tissue-attenuation border between the gas-filled lumen and pericolic fat tissue. Normal bowel wall takes up contrast material slightly (Fig. 4.7). If distension is inadequate and the lumen is narrowed, the colonic wall will naturally be thicker. In this situation reliable evaluation of the wall thickness is limited since the physiological wall, if undistended or collapsed, can appear thickened (up to 5 mm thick; Wiesner et al. 2002) and irregular, mimicking pathological changes (Fig. 4.8). For this reason, in CT colonography wall thickening in the large bowel should only be diagnosed and described as pathological if the colon is well distended. The large bowel has a smooth border with the surrounding homogeneously hypodense pericolic and perirectal fatty tissue.

Diagnostic Criteria at CT Colonography

Normal Large-Bowel Wall

3D morphology:

Folds, haustra, taeniae
Smooth, regular surface
Watch out for: Image noise on low-dose scans simulating granular surface irregularity

2D structure:

Very thin when fully distended (no thicker than 1–2 mm)
Thin layer, soft-tissue attenuation
Watch out for: Increased wall thickness due to underdistension of segments

**Complex fold structures in the sigmoid colon mimicking a polypoid colonic mass: “flexural pseudotumor.”** a The axial 2D image shows a circumscribed thickening of the bowel wall with soft-tissue attenuation at the inner curvature of a sigmoid loop (arrow). b The corresponding endoluminal 3D image shows several closely spaced semilunar folds (arrow).

**Complex fold structure at the cecum.** Depiction of a complex fold structure at the cecum in a combined coronal 2D/3D view (arrow). The folds are not parallel, but converge. The distended appendix is also visible (arrowhead).

**Normal colon wall on 2D views.** With good colonic distension, normal bowel wall is extremely thin and barely visible. It enhances moderately after intravenous administration of a contrast agent, becoming somewhat better defined. Fecal tagging may result in a thin coating of contrast on the bowel wall. a Nonenhanced 2D view of the colon wall. b After intravenous administration of a contrast agent. c 2D view of the colon wall after fecal tagging.

**Variability of wall thickness with the degree of distension.** a With the patient supine, the rectum is poorly distended. When the lumen is narrow, the colon wall becomes thicker (arrow). The perirectal fat tissue is normal. b With the patient prone, the rectum is well distended, showing clearly that the bowel wall is not thickened.

Mobility:

Retroperitoneal bowel segments (ascending colon and descending colon) do not move when the patient changes position
Bowel segments with a mesocolon (cecum, transverse colon, sigmoid colon) may move about on their mesentery

Intravenous contrast: