Small-Bowel Obstruction Imaging and Diagnosis: Practice Essentials, Radiography, Computed Tomography (2024)

CT scanning is recommended when the clinical findings and the initial plain radiographs are inconclusive or when strangulation is suspected. CT scans clearly demonstrate abnormalities of the bowel wall, the mesentery, the mesenteric vessels, and the peritoneum. ^{[11, 19, 18, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29]}

CT scanning should be performed with intravenous contrast enhancement. Intraluminal contrast material may not be necessary, because fluid and gas accumulation in the bowel may provide sufficient contrast; however, oral administration of a 1-2% barium sulfate suspension or a 2% water-soluble contrast agent 30-120 minutes before scanning may be useful for accurately locating the site and degree of obstruction. ^{[20, 7]}

The diagnosis of obstruction is based on the identification of a dilated proximal loop and a collapsed distal loop of small bowel (see the images below).

A bowel diameter in excess of 2.5 cm is regarded as abnormal, but this criterion is insufficient to differentiate mechanical obstruction from an adynamic ileus.

The small-bowel feces sign is an uncommon but reliable sign of a mechanical obstruction if it is seen in conjunction with bowel dilatation (cystic fibrosis, infectious and metabolic enteropathies, and reflux through the ileocecal valve can result in this finding in nondilated bowel). The feces sign occurs when feces and gas intermingle and are observed proximal to the obstruction. ^[30]

The feces sign is most helpful in identifying the site of obstruction, which will usually be just distal to the area of feculent-appearing material. ^[30] Some reports suggest that the sign is associated with low-grade obstructions, which usually do not require surgical intervention; however, one report suggests that it may be be associated with higher-grade obstructions and bowel ischemia. ^[31]

The level of obstruction may be determined by identifying the transition from dilated to collapsed loops of bowel; the degree of collapse and the amount of residual content in the distal bowel beyond the obstruction are useful to note; the passage of contrast material into the distant collapsed segment indicates that the obstruction is partial or incomplete ^[32]

CT scans do not usually help in identifying an adhesive band. The diagnosis of adhesions is based on the abruptly changing caliber of the small-bowel lumen in the absence of radiologic evidence of another obstructive cause. Adhesions most frequently involve the terminal ileum, usually in association with the undersurface of an abdominal scar, the site of previous surgical intervention, or an inflammatory focus. ^{[21, 8]}

High-density intra-abdominal free fluid (>10 HU) on CT has been determined to be significantly associated with the need for surgical intervention in patients with adhesive small-bowel obstruction. In a study by Matsushima et al, sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of high-density intra-abdominal free fluid to predict need for surgical intervention were 83.9%, 65.3%, 75.4%, 76.2%, and 75.6%, respectively. ^[33]

CT findings in a closed-loop obstruction depend on the length, the degree of distention, and the intra-abdominal orientation of the closed loop. Findings may include a U- or C-shaped loop of small bowel and a radial configuration of the mesentery, with stretched vessels converging on the site of a torsion. Tightly twisted mesentery is occasionally depicted as the whirl sign. At the site of the obstruction, collapsed loops are round, oval, or triangular. On longitudinal imaging, the beak sign appears as a fusiform tapering at the site of the obstruction. ^{[20, 21, 28]}

In a retrospective study of 148 patients with closed-loop small-bowel obstruction, increased attenuation of bowel wall and mesenteric vessels on non-contrast-enhanced CT had a specificity for bowel ischemia or necrosis of 100%, according to de Kok et al. ^[2]

The features of strangulation on CT scans may include evidence of small-bowel obstruction, a circumferential thickening of the bowel wall (with a high attenuation), the target sign, and congestion or hemorrhage in the mesentery attached to the closed loop. A serrated beak (see the image below) may be seen at the site of obstruction. Edema in the mesentery attached to the involved segment may result in a hazy appearance and diffuse engorgement, and the mesenteric vasculature may take an unusual course. After the intravenous administration of contrast material, the bowel wall may show delayed, poor, or no enhancement. In advanced cases, pneumatosis intestinalis may develop. ^[34] A large amount of ascites may be present. ^[21]

In determining strangulation in small bowel obstruction, Millet et al found that 2 CT findings can be particularly helpful in clinical practice: reduced bowl wall enhancement (specificity, 95%) and the absence of mesenteric fluid (sensitivity, 89%). In addition, bowel wall thickness had a sensitivity of 48% and a specificity of 83%). ^[29]

In a retrospective analysis of 164 unenhanced and contrast-enhanced CT studies from 158 consecutive patients with mechanical small-bowel enhancement, adding unenhanced CT to contrast-enhanced CT improved the sensitivity, diagnostic confidence, and interobserver agreement of the diagnosis of ischemia, a complication of mechanical small-bowel obstruction, on the basis of decreased bowel wall enhancement. ^[27]

The CT findings of malrotation include the distribution of the small bowel to the right side of the abdomen and of the colon to the left side of it. Abnormal orientation of the superior mesenteric vessels and aplasia of the uncinate process of the pancreas may be seen. If a volvulus occurs, its radiologic appearance is similar to that of any closed-loop obstruction. ^{[21, 35]}

CT scanning is useful in the diagnosis of external hernia at unusual sites, particularly in patients with obesity. The technique can demonstrate visceral hernial contents and complications such as vascular compromise. The diagnosis of an internal hernia is always based on radiologic findings, and CT scanning is useful in depicting the precise site, type, and contents of the hernia. ^{[19, 21]}

The CT features of a paraduodenal hernia include the following:

• A cluster of small bowel anterior and lateral to the pancreas

• A saclike mass of small-bowel loops

• Encapsulation of the small-bowel loops

• A mass effect on the posterior wall of the stomach

• Caudal displacement of the duodenojejunal junction

• Stretching and/or engorgement of the mesenteric vessels, which may be displaced to the left or right of the aorta or the inferior vena cava, respectively

  See Also

  Histology of the lower digestive tract

Less common CT findings include an established small-bowel obstruction, caudal or dorsal displacement of the transverse colon, and medial displacement of the ascending or descending colon. Similar features may be seen with the uncommon transmesenteric hernia, together with the additional finding of a deficiency in the omental fat overlying the herniated small bowel.

Any of the less-common causes of small-bowel obstruction can be demonstrated on CT scans, and many of them have specific radiologic features. In patients with Crohn disease, narrowing of the small-bowel lumen and mural thickening (see the image below) are well depicted. In the acute phase, the small bowel shows mural stratification, and, often, it has a target-like or double-halo appearance. With intravenous contrast enhancement, inflamed mucosa and serosa show marked enhancement, and the intensity of enhancement is correlated with the activity of disease. In chronic disease, mural stratification disappears and the diseased bowel has a typically uniform attenuation. Fat deposition in the bowel wall indicates inactive disease. ^[21]

On CT scans, the appearance of intussusception (see the image below) depends on the severity and duration of the condition. On cross-sectional imaging, the intussusception may appear as a target, with alternating layers of low and high attenuation, or it may be seen as a sausage-shaped or reniform mass. ^{[21, 36]}

When small-bowel obstruction is secondary to bowel adhering to an inflamed mass in appendicitis or diverticular disease, the CT findings may help in making the diagnosis. CT scans can demonstrate appendicitis and its complications, such as an inflammatory mass, abscess, or peritonitis. In diverticular disease, mural thickening may be demonstrated clearly together with edema of the mesentery and the complications of paracolic and pelvic abscess and peritonitis. ^[21]

In radiation enteropathy, CT scans demonstrate the features of small-bowel obstruction, as well as the abnormalities of an irradiated segment of small bowel, which, in most instances, lies within the pelvis. Mural thickening, luminal narrowing, and mesenteric fibrosis occur. ^[21]

In intestinal tuberculosis, CT scans may demonstrate only slight asymmetric mural thickening if the inflammation is mild; however, with gross disease, the bowel wall is thickened and an inflammatory mass is demonstrated. This mass has a heterogeneous appearance. Large regional lymph nodes with low-attenuation centers may also be demonstrated. ^[21]

In gallstone ileus (see the image below), CT scans may demonstrate the gallstone and gas within the shrunken gallbladder or biliary tree, in addition to small-bowel obstruction. ^[17]

The CT appearance of an intramural hematoma is nonspecific and depends on the age of the hematoma. Soon after hemorrhage occurs, the attenuation is low; however, as time passes, its attenuation increases. Once lysis of the clot begins, the attenuation again decreases. It may also be centripetal, which gives rise to the ring sign, with a crescent of high attenuation. ^[21]

If a bezoar is the cause of small-bowel obstruction, CT scanning may demonstrate the bezoar as a mass in the obstructed segment of bowel. The bezoar may be outlined by fluid in the proximally dilated small bowel, and the mass may be mottled as a result of air trapped within it. ^{[21, 37]}

Malignancy involving the small bowel may have a variety of CT appearances. Although rare, adenocarcinoma may be seen, particularly in the duodenum and proximal jejunum. The tumor is usually detected only at an advanced stage, and in patients with small-bowel obstruction, the mass may be seen as mural thickening with luminal compromise in a transitional zone between dilated and collapsed bowel. ^[21]

Primary non-Hodgkin small-bowel lymphoma rarely causes obstruction; however, nodal lymphoma may arise in the mesentery and invade the small bowel by means of direct infiltration to cause small-bowel obstruction. A mesenteric mass invading, kinking, or compressing the small bowel may be seen on CT scans. ^[21]

Small-bowel obstruction is a major complication of a carcinoid tumor, one largely caused by the desmoplastic reaction (see the image below) that may occur in the mesentery; therefore, mural thickening and retraction of bowel loops around a segment of mesentery may occur. Metastatic nodular masses may be present. ^[21]

Peritoneal carcinomatosis may be recognized by the demonstration of omental masses kinking or compromising the small bowel, thereby resulting in small-bowel obstruction. ^[21]

In all malignant tumors causing small-bowel obstruction, CT scans are useful in depicting local extent of the disease and the presence of distant metastases (when appropriate).

With the advent of multisection CT, CT angiography may provide a future alternative to angiography for assessing intestinal ischemia.

In one study of 44 patients, increased bowel-wall attenuation on unenhanced 64-section multidetector CT images was found to be a specific sign for ischemia complicating small-bowel obstruction. ^[38]

Degree of confidence

The reported sensitivity of CT scanning for detecting small-bowel obstruction is 78-100% in high-grade or complete obstruction. If the obstruction is partial or intermittent, the accuracy is low. ^{[20, 21, 39]}

CT can be helpful in excluding closed-loop small-bowel obstruction, according to Makar et al. In their study, sensitivity and specificity of MDCT for closed-loop small-bowel obstruction were 53% and 83%, respectively. ^[28]

If the entire small bowel is distended, a barium enema study should be performed to exclude a large-bowel obstruction and a patent ileocecal valve. A distended loop of small bowel may migrate from its expected anatomic location; therefore, determining the site of obstruction on the basis of a transitional zone alone may be misleading.

Carcinoid tumors, intestinal tuberculosis, and desmoid tumors all may have CT features similar to those of peritoneal carcinomatosis. Although the presence of ascites suggests a strangulated obstruction, intra-abdominal fluid may occur in less complicated forms of obstruction. Feces in the small bowel can mimic the mottled appearance of a bezoar if stasis above a complete blockage is severe. ^[21]