The sigmoid colon, rectum and anal canal

Computed tomography Axial CT scanning may also demonstrate the muscles of the pelvic floor (see Figs 6.4 and 6.6).

THE SIGMOID COLON, RECTUM AND ANAL CANAL

(Figs 6.8 and 6.9; see Fig. 5.20)

The sigmoid colon

The sigmoid colon is variable in length (12-75 cm, average 40 cm). It is covered by a double layer of peritoneum, supported on its own mesentery which is attached to the posterior and left lateral pelvic wall. It has the same sacculated pattern as the rest of the colon in young people. In older subjects it may appear featureless. The redundant sigmoid lies on the other pelvic structures.

Blood supply (see Fig. 5.23) The arterial supply is from the inferior mesenteric artery via the sigmoid arteries, which join in the arterial cascade of the large bowel. Venous drainage is to the portal system via the inferior mesenteric vein.

Lymph drainage This is via the blood supply to preaortic nodes around the origin of the inferior mesenteric artery.

The rectum

This is about 12 cm long. It commences anterior to S3 and ends in the anal canal 2-3 cm in front of the tip of the coccyx. It forms an anteroposterior curve in the hollow of the sacrum. The rectum has no sacculations or mesentery. Its upper third is covered by peritoneum on its front and sides; its middle third has peritoneum on its anterior surface only, and its lower third has no peritoneum. The lower part of the rectum is dilated into the rectal ampulla. In the resting state the mucosa of the rectum has three to four longitudinal mucosal folds, known as the columns of Morgagni. The taenia coli fuse in the rectum. Some longitudinal shortening of the rectum occurs, giving the rectum a slight S shape. This creates three lateral mucosal folds - left, right and left from above downward. These are known as the valves of Houston.

Posteriorly are the sacrum and coccyx. Anteriorly, loops of small bowel and sigmoid colon lie in the peritoneal cul-de-sac between the upper two-thirds of the rectum and the bladder or uterus. The lower third is related to the vagina in the female and to the seminal vesicles, prostate and bladder base in the male.

The rectum is surrounded by perirectal fat. Fascia known as the perirectal fascia surrounds the perirectal fat, and lateral to the perirectal fascia is the pararectal fat. The perirectal fascia separates the seminal vesicles and prostate from the rectum anteriorly.

2 22 ANATOMY FOR DIAGNOSTIC IMAGING

Fig. 6.8 Rectum: (a) anterior view and blood supply; (b) lateral view showing peritoneal reflections.

The anal canal (Fig. 6.9) This is directed posteriorly almost at right-angles to the rectum. It is a narrow, muscular canal. It has an internal sphincter of involuntary muscle and an outer external sphincter of voluntary muscle, which blends w i th levator ani. The internal (smooth muscle, involuntary) sphincter occupies the upper two-thirds of the anal canal. The external (striated, voluntary) sphincter occupies the lower twothirds. Thus, the sphincters overlap in the middle third, w i th the internal deep to the external. The junction of the rectum and anal canal is at the pelvic floor where the puborectal sling encircles it, causing its anterior angulation.

Anteriorly, the perineal body separates the anus from the vagina in the female and the bulb of the urethra in the male. Posteriorly, the anococcygeal body is between it and the coccyx, and laterally is the ischiorectal fossa.

The anal canal is lined by mucous membrane in its upper two-thirds and by skin in its lower third. The mucosa of the anal canal has several vertical folds. The mucocutaneous junction is known as the dentate line or Hilton's white line. This defines a division between arterial supply and venous and lymphatic drainage.

Blood supply (see Fig. 5.23) The superior, middle and inferior rectal (haemorrhoidal) arteries form a rich submucous plexus supplying the rectum and anal canal. The superior rectal artery is a branch of the inferior mesenteric artery. The middle and inferior arteries arise from the internal iliac artery. The plexus drains via a superior rectal vein to the inferior mesenteric vein and via middle and inferior veins to the internal iliac vein. This represents a communication between systemic and portal systems.

Lymph drainage The rectum and upper anal canal drain to pararectal nodes, and from here to preaortic nodes around the inferior mesenteric artery and to internal iliac nodes. The lower anal canal drains to superficial inguinal nodes.

Radiology of the sigmoid and rectum

Plain films (see Fig. 5.1) The sigmoid colon and rectum may be identified on plain radiographs outlined by air, faeces or both. The sigmoid colon has a characteristic S-shaped curve as it joins the descending colon to the rectum, which lies anterior to the sacrum.

Barium enema (see Fig. 5.20) This is the main radiological examination used to assess these structures. The best detail is attained by the doublecontrast technique, that is, the use of a small amount of barium to coat the mucosa and sufficient air or gas to distend the bowel. The valves of Houston are the lateral mucosal folds of the rectum and are usually less than 5 mm thick. The longitudinal columns of Morgagni are best identified after evacuation of barium, when the rectum is not distended. These measure 3 mm in width. On the lateral view, the postrectal space is measured between the posterior w a ll of the rectum and the anterior part of the sacrum at the level of S4. This measurement should not exceed 1 cm. The posterior impression of the pubococcygeal fibres of levator ani may be identified at the lower limit of the rectum. The anal canal may be identified if outlined w i th barium paste; it makes an acute, posteriorly directed angle w i th the rectum.

Computed tomography The rectum and perirectal tissues are readily assessed by CT. Visualization of the rectum may be improved by the administration of dilute rectal contrast or air. The rectal wall, perirectal fat and fascia and pararectal fat may be identified.

Magnetic resonance imaging MRI is also used to image the sigmoid colon and particularly the rectum. The ability to obtain coronal and sagittal images in addition to axial scans is a great advantage, particularly to assess tumour spread and to evaluate the mesorectal fat (see Figs 6.5 and 6.15).