Colon: Imaging Approach and Differential Diagnosis

Embryology and Congenital Malformations

The ascending and transverse colon, along with the small intestine, are part of the embryologic midgut, which undergoes marked elongation beginning in the 6th week of fetal development. To accommodate this increased length, the midgut herniates into the base of the umbilical cord. During the 10th week, it returns to the abdomen while undergoing a complex series of rotations and fixations.

All portions of the fetal colon are suspended on mesenteries, but the mesenteries of the ascending and descending colon usually fuse to the retroperitoneal fascia, normally leaving these portions covered by peritoneum only on their anterior surface and are thus regarded as retroperitoneal organs.

Variations in these embryologic steps are relatively common and may have clinical consequences. Failure of rotation results in the cecum and ascending colon lying on the left side of the abdomen. Accompanying malrotations of the other portions of the midgut may result in neonatal or adult midgut volvulus or adhesive band small bowel obstruction. A left-sided cecum carries with it a left-sided appendix, and appendicitis may present with confusing left lower quadrant pain in these individuals.

The cecum and portions of the ascending colon often maintain a mesentery of variable length into adulthood. This makes the cecum more mobile and prone to twist on its mesentery, especially with gaseous distention of its lumen, and may result in cecal volvulus . The sigmoid mesocolon is also often long with a narrow base of attachment to the posterior abdominal wall, predisposing it to twist. Sigmoid volvulus often obstructs the lumen, compresses blood vessels, and may lead to ischemia and perforation.

Gross Anatomy

The cecum is the 1st part of the colon and is ~ 7 cm long. It receives the terminal ileum through the ileocecal valve whose "lips" usually contain abundant fat, allowing them to be identified as a useful landmark on CT scans. The appendix is a blind diverticulum, 6-15 cm in length, that has its own mesentery (the mesoappendix). The appendix always arises from the tip of the cecum but may lie in many locations with over 60% of patients having a retrocecal appendix.

The ascending colon extends from the 1st semilunar fold, at the ileocecal valve, to the transverse colon. Its vascular supply is from the right colic branches of the superior mesenteric artery and vein (SMA and SMV, respectively). The transverse colon includes the "radiologic" hepatic and splenic flexures and is supplied by middle colic branches of the SMA and SMV. The descending colon is supplied by the inferior mesenteric artery and vein (IMA and IMV, respectively) and is retroperitoneal. Despite frequent anastomoses between branches of the SMA and IMA, including the marginal artery (of Drummond) and arc of Riolan, the splenic flexure through the descending colon is a common site of hypoperfusion and ischemia and may be the result of a congenital deficiency of vascular anastomoses ("watershed area").

The sigmoid colon is quite variable in its length, redundancy, and location. The rectum is the final 15-20 cm of colon. The rectosigmoid junction is usually at the lumbosacral junction and lies in the extraperitoneal pelvis. The rectum has both a mesenteric (superior rectal branches of IMA and IMV) and systemic (middle and inferior rectal branches in the internal iliac vessels) vascular supply. Because of its dual blood supply, rectal carcinoma may metastasize to systemic sites (lungs, bones, etc.) as well as to the liver (via its IMV drainage), while colon carcinoma almost always metastasizes to the liver first.