Colon and Appendix

The aganglionic segment is usually located in the upper rectum or rectosigmoid junction, with the proximal large bowel becoming grossly dilated over many years of partial obstruction. The diagnosis is usually best made by contrast enema studies (Fig. 5-2), even in the neonate, but definitive diagnosis requires rectal biopsy. The stricture is usually identified at contrast enema in the rectosigmoid regions with variable colonic distention proximal to the stricture. The aganglionic segment may not appear as an abrupt transition but rather as an irregular serrated or “sawtooth” appearance, which is characteristic of the disease (Fig. 5-3). Some patients develop an associated colitis, termed “Hirschsprung-associated enterocolitis,” that is characterized by foul-smelling diarrhea. These patients are at risk of perforation because of acute colitis, but subacute forms exist, and contrast enema studies may show the characteristic rectal sawtooth finding.

Ultrasound (US) sometimes shows a hypoechoic cystic mass with a thick wall, which has an echogenic outer layer and hypoechoic inner layer. On BE the cyst produces a mass effect of adjacent bowel and on CT appears as a nonenhancing mass, compressing or displacing the adjacent bowel, which may contain simple fluid, hemorrhage, or proteinaceous fluid (Fig. 5-4). On MRI the enteric cysts are usually hyperintense on T2-weighted imaging, reflecting their cystic nature (Fig. 5-5). Because many duplication cysts contain ectopic gastric mucosa, a Tc-99m pertechnetate radionuclide study can often show radionuclide uptake, which can also be observed within a Meckel diverticulum for the same reason.

The colonic mucosal disease always starts in the rectum and extends proximally to a variable degree, sometimes affecting the whole colon (pancolitis) and sometimes even a short segment of ileum, so-called backwash ileitis. The extent of colonic involvement typically directs the symptoms and seriousness of the disease. Some patients have only one short episode confined to the rectum, and others have recurring disease, sometimes with a life-threatening toxic megacolon. This usually necessitates aggressive medical or surgical therapy, often requiring an emergency total colectomy as the diffusely involved colonic mucosa becomes so thickened, friable, and distended that perforation is imminent.

The imaging features depend on the stage of the disease (Table 5-2), and many of these features are shared by Crohn disease (Table 5-3). They can sometimes be identified by plain abdominal radiography as thickened haustra, particularly when caused by a pancolitis (Fig. 5-5). Severe disease can almost certainly be recognized on plain radiography as can toxic megacolon, with marked colonic distention resulting from ileus (Fig. 5-6) and wall and mucosal thickening (Fig. 5-7). The wall thickening is sometimes referred to as “thumb-printing” because of the polypoid soft tissue nature of the mucosal edema and thickening. BE demonstrates typical features but has largely been replaced by optical colonoscopy for diagnosis. When BE is performed, the features depend on the severity and acuity of disease. In acute disease a variable length of colon (starting in the rectum) shows a granular mucosal pattern representing edema and ulcer formation, sometimes of the whole colon (Fig. 5-8), which may also affect the last few centimeters of the terminal ileum (“backwash ileitis”) (Fig. 5-9). “Collar-button ulcer” formation has been described, which represents acute ulceration of the colon with submucosal extension (Fig. 5-10), with further ulceration prevented by the relatively impermeable bowel wall. This sign is three times as common in UC as in Crohn disease. Pseudopolyp formation, which can also be recognized in Crohn disease, can occur with more chronic disease and represents areas of reparative mucosa between areas of ulceration (Fig. 5-11). As the disease progresses, the affected length of colon becomes featureless and shortened, termed “lead piping” (Fig. 5-12).

The presence and extent of disease are more readily evaluated by CT than by contrast enema, whose features also mirror the clinical disease. These include bowel wall thickening and luminal narrowing with a variable amount of pericolonic edema and mesenteric vascular hyperemia (Fig. 5-13). The degree of inflammation may be reflected by ¹⁸F-fluorodeoxyglucose (FDG) uptake at PET imaging (Fig. 5-13). Mural stratification or double-halo sign, which is common with an acute presentation, is a nonspecific sign for inflammatory bowel disease representing hyperemic mucosa and serosa with intervening submucosal edema (Fig. 5-14). If the acute disease worsens, the mucosa becomes progressively thickened and inflamed (Fig. 5-15), which can ultimately lead to a toxic megacolon, whose CT features include a distended colon with profusely thickened mucosal tissue, sometimes referred to as an “accordion pattern” (see Fig. 5-52).

More chronic features of the disease at CT are similar to those observed at BE with colonic shortening and luminal fibrotic narrowing. However, CT may identify widening of the presacral fat space and a characteristic “fat halo” sign consisting of inner mucosal and outer muscularis enhancement with a nonenhancing middle submucosal ring. This ring is composed of increased submucosal fat deposition (Fig. 5-16). There may be mesenteric adenopathy, but this is less common in UC than in Crohn disease. Stricture formation may be seen but is also less common than in Crohn disease (Figs. 5-12 and 5-17).

In addition to perforation from toxic megacolon, the greatest risk to patients with chronic UC is the late development of adenocarcinoma of the colon, for which they have an increased risk of 5% to 30% over the general population (Fig. 5-18). The risk increases by 10% for each decade of disease. Patients with more extensive disease may therefore undergo prophylactic colectomy.

As in the small bowel, transmural inflammation and thickening of the bowel wall occur during the acute phase with pericolonic inflammatory change, which can be almost impossible to distinguish from other forms of acute colitis, especially if a large segment of colon is involved. Repetitive inflammatory disease causes deep ulceration with localized perforation and abscess formation or fistulization with adjacent small bowel. Because these repetitive bouts of acute disease heal, luminal narrowing and stricture formation often occur. Perianal disease with fistula and abscess formation is common in Crohn disease.

The imaging features are similar to those described in the small bowel, although the imaging of colonic Crohn disease can, at times, mimic those of UC. However, there are a number of distinguishing features that help to differentiate the two diseases (Table 5-3). Plain radiography may demonstrate mucosal thickening (Fig. 5-19) or toxic megacolon. Contrast enema studies (usually barium) may demonstrate involvement of the whole colon (which is therefore difficult to distinguish from UC), but this is uncommon. More commonly a variable segment of the colon is affected (Fig. 5-20, A), and the disease may or may not involve the rectum. Acute disease at CT may present with mural stratification similar to that in the small bowel, representing mucosal and serosal hyperemia with submucosal inflammation or simple mural thickening and mesenteric edema (Fig. 5-20, B). Aphthous ulceration is characteristic of Crohn disease (Fig. 5-21). As in the small bowel, the mucosa in active disease enhances avidly after the administration of IV gadolinium. Although CT is easier and faster to perform, many patients with Crohn disease are young and may require repetitive assessment of the extent of their disease, so avoiding the radiation dose from multiple CT images is preferable. Therefore MRI is often advised, and newer MR enterographic techniques, particularly of the small bowel, have proved highly effective for evaluating the extent of disease (Fig. 5-22). Furthermore, the extent of perianal disease is best imaged with MRI, which can outline the relationship of inflammatory disease to the internal and external anal sphincters; this is important to determine whether surgical repair is needed.

Other differentiating features of UC include a propensity for Crohn disease to fistulize with the small bowel, with adjacent organs, or to the skin. This can be assessed by CT (Fig. 5-23), BE (Figs. 5-24 and 5-25), or MRI (Fig. 5-26). Abscess formation is also recognized in Crohn disease rather than UC (Fig. 5-27). An increased risk of small and large bowel malignancies, predominantly adenocarcinomas, is associated with Crohn disease (Fig. 5-28).

Typhlitis is readily identified at CT by circumferential cecal wall thickening, sometimes marked, that is thought to be caused by a combination of infection, hemorrhage, and ischemia. Bowel wall enhancement in common with other colitides is present, and there is often pericolonic inflammatory change (fat-stranding). Typhlitis may affect a variable length of the ascending colon, and the adjacent terminal ileum or appendix may be affected (Fig. 5-33).

Most disease is segmental in the colonic “watershed” areas, with the splenic flexure (i.e., arterial vascular transition between the superior mesenteric and inferior mesenteric arteries) and rectosigmoid region (i.e., junction between the inferior mesenteric and hypogastric arteries) most at risk. More extensive disease is, however, well recognized, particularly if the superior mesenteric artery is also affected and a pancolitis ensues, which is difficult to differentiate from other colitides.

Plain radiography may demonstrate an ileus, sometimes confined to the left colon. As the disease progresses, bowel wall thickening develops (Fig. 5-34) with a toxic megacolon if severe (Fig. 5-35). BE is now rarely performed, but results demonstrate thickened folds and ulceration, either linear or with mucosal sloughing. Healing can lead to stricture formation (Fig. 5-36). The findings are now usually made by CT and are similar to other forms of colitis (inflammatory bowel disease, infectious colitides, and radiation colitis if the radiation field included the colon). The disease is suggested in the appropriate clinical setting and by the left-sided distribution of the colonic changes (Fig. 5-37). Severely affected patients show colonic pneumatosis as the gas permeates the damaged mucosa, which can then enter the mesenteric venous system and be recognized as mesenteric venous gas (particularly at CT) and ultimately intrahepatic portal venous gas. Occasionally, ischemia occurs proximal to an obstructing colonic stricture, such as colonic adenocarcinoma. The obstruction causes marked distention of the proximal colon, compromising its vascular supply or directly invading mesenteric vasculature (Fig. 5-38).

The findings are most often visualized by CT, particularly because patients are often serially imaged to evaluate for any local cancer recurrence or metastatic disease, although MRI often detects more subtle changes (Fig. 5-40). Associated inflammatory changes are often observed in the surrounding mesentery (fat-stranding). This is commonly seen in the presacral region after radiation for rectal cancer preceding attempted surgical removal of the tumor. Radiation changes can sometimes be difficult to differentiate from local recurrence, but awareness that the patient has undergone radiation should alert the radiologist that the changes are benign rather than malignant recurrence. Increasingly, PET imaging is used to differentiate postradiation changes from recurrent disease (Fig. 5-41).

The imaging findings may lag behind the clinical features, and patients may have pronounced clinical disease without obvious imaging findings. When observed, the imaging features are similar to most other colitides. Plain radiography often demonstrates ileus and, as the disease progresses, nodular haustral thickening, often over a long segment because the disease usually presents as a pancolitis. There may be polypoid mucosal thickening representing the pseudomembranes, but this is not often observed (Fig. 5-45). The disease can progress readily to frank toxic megacolon. However, it is optimally evaluated by CT, which demonstrates bowel wall thickening, mucosal enhancement, often with a mural stratification (or target sign representing unenhanced thickened submucosa surrounded by enhancing mucosa and muscularis propria), pericolonic edema, and mild ascites. The bowel wall thickening is often pronounced, more so than in other colitides, with the thickened haustra giving the appearance of an accordion pattern (also found with CMV colitis) over a relatively long segment of bowel (Fig. 5-52), representing oral contrast material trapped between the bulbous-thickened haustra.

Mycobacterium avium intracellulare (MAI) infection is an atypical mycobacterial infection most commonly seen in patients in the latter stages of acquired immunodeficiency syndrome (AIDS). It can also be a multisystem disease, often producing multiple enlarged hypoattenuating nodes in the abdomen and retroperitoneum. MAI can be difficult to distinguish from the more common forms of TB but should be strongly considered in a patient with AIDS.

TB most commonly affects the ileocecal region when involving the GI tract. In the acute phase, there is an edematous thick-walled inflammatory mass of the terminal ileum and cecum, which can be identified at BE or CT imaging (Fig. 5-54