MDCT of the Small Bowel

Grace A. Tye, MD; Terry S. Desser, MD

Disclosures

Appl Radiol. 2012;41(8):6-17. 

In This Article

Obstruction

Small-bowel obstruction (SBO) is a common cause of abdominal pain, accounting for 4% of all emergency room visits for abdominal pain and 20% of surgical admissions. Rapid diagnosis and identification of complicated cases, such as closed-loop obstruction, volvulus,or superimposed ischemia, are of critical importance, as these patients require emergent surgical management. At our institution, MDCTis routinely performed for the evaluation of suspected SBO, having essentially replaced the small-bowel follow-through examination and sometimes even plain radiography as the initial examination of choice.

MDCT findings in SBO include dilated loops of small bowel measuring >3 cm in diameter, with a transition to normal caliber or collapsed bowel loops seen distally. The "small bowel feces" sign has been identified as a specific but not sensitive sign for SBO, and describes the presence of solid material intermixed with gas bubbles (ie, the appearance of fecal material) within the small bowel, typically just proximal to the site of obstruction.[35–37] This finding can be helpful in identification of the transition point.[38] The fecal material should be present within dilated loops of bowel, however, as fecal material can be seen within the small bowel in nonobstructed bowel of cystic fibrosis patients as well as individuals with metabolic or infectious enteropathies. Furthermore, fecal material within the distal ileum can be seen in patients with an incompetent ileocecal valve, but no SBO.[37,39]

MDCT often facilitates the identification of the cause of an SBO, whether it is extrinsic or intrinsic to the bowel. The most common causeo f SBO in developed countries is adhesions, with the vast majority secondary to prior abdominal or pelvic surgery. Adhesions are not visualized on CT, with only a beak or sharp angulation in the bowel seen at the transition point from normal to dilated bowel (Figure 5). A diagnosis of adhesions as the cause of SBO can only be made when other causes have been excluded and an appropriate clinical history is present.[38] The reported accuracy of CT in the diagnosis of adhesive SBO is 70% to 95%.[38]

Figure 5.

Small-bowel obstruction due to adhesion. (A) Axial image shows dilated loops of small bowel proximal to a loop of decompressed small bowel in the pelvis (arrow). (B) Coronal reformatted image shows a linear soft tissue density and sharp angulation (arrow) at the transition point, representing the obstructing adhesive band.

The second most common cause of SBO is hernias, which are most commonly external but may also be internal. There are several typesof external hernias, including but not limited to inguinal, femoral, umbilical, ventral, obturator, lumbar, and Spigelian hernias, all of which occur at points of structural weakness in the abdominal or pelvic wall musculature. These areas of weakness may be iatrogenic in nature,such as in the cases of parastomal, incisional, and laparoscopic port site hernias. When small bowel protrudes into these areas of weakness and becomes entrapped, passage of enteric contents proximal to this may become obstructed.

Internal hernias are less common than external hernias, but are important to identify as they are associated with significant morbidity and mortality, yet are difficult to diagnose both clinically and radiographically. Internal hernias can occur in many locations, including paraduodenal, the foramen of Winslow, pericecal, and intersigmoid. Transmesenteric internal hernias are becoming increasingly common because of the increasing number of bariatric surgeries performed in which a mesenteric defect is created to accommodate a Roux loop. When small bowel herniates through this surgically created defect, causing an SBO, small bowel is seen in an abnormal position lateral to the colon, with omentum and small bowel located directly adjacent to the abdominal wall (Figure 6).[38]

Figure 6.

Small-bowel obstruction due to transmesenteric hernia in a patient following laparoscopic gastric bypass surgery. There is construction of bowel loops and a small bowel feces sign at the site of herniation (arrow), as well as encapsulated loops of bowel immediately adjacent to the anterior abdominal wall without intervening omental fat (arrowhead).

A rare cause of SBO is intussusception, responsible for only 1% of cases. A "bowel-within-bowel" appearance on CT is pathognomonic for an intussusception and describes the telescoping of a small-bowel loop, often with its associated mesenteric fat and vessels, into a downstream loop of bowel. Most small-bowel intussusceptions identified on CT are transient, cause no associated obstruction, and have no underlying tumor acting as a lead-point (Figure 7). Rather, the majority of entero-enteric intussusceptions may be idiopathic. In a minority of cases, however, a mass lesion may be acting as the lead-point for the intussusception. In these cases, the intussusception is more likely to cause significant SBO requiring surgical treatment (Figure 8).[40]

Figure 7.

Incidental short segment intussusceptions in a patient undergoing CT for abdominal trauma. (A) Axial image following intravenous contrast administration shows 3 loops with "bowel within bowel" appearance (arrowheads). (B) More inferior image shows an intussuscepted segment in long axis (arrow). Note the short length and relative lack of luminal expansion.

Figure 8.

Small bowel intussusception due to metastasis from melanoma. (A) Axial image shows "bowel within bowel" as well as telescoped mesenteric vessels and fat, indicative of intussusception (arrow). (B) Image slightly more inferiorly shows the intussusception in long axis (arrow). Note long segment length and luminal expansion characteristic of lead-point intussusception.

Tumors intrinsic and extrinsic to the bowel can also cause SBO, with some common causes including adenocarcinomas of the GI tract, pancreas, and ovary, carcinoid tumor and its associated desmoplastic reaction, and extraenteric nonHodgkin's lymphoma.[38]

Other rare intraluminal causes of SBO include gallstones that migrate from the gallbladder via a biliary-enteric fistula into the adjacent duodenum, typically lodging itself in the distal ileum to create the entity known as "gallstone ileus." Foreign bodies, bezoars, enteroliths,and inspissated fecal material in patients with cystic fibrosis can also cause SBO.[38]

MDCT plays a role not only in diagnosing the presence of an SBO and determining its cause, but also in identifying potential complications, one of the most important of which is a closed loop obstruction. A closed loop obstruction results when 2 points along the same length of small bowel are obstructed at a single point. This is most often caused by an adhesion, although internal and external hernias are also common causes. The segment of bowel between the 2 obstructed points is predisposed to volvulus, leading to venous outflow obstruction,and high risk of strangulation of the incarcerated segment of bowel. Findings of closed loop obstruction on MDCT depend on the length and angle of the incarcerated segment. Typically, a C-shaped, U-shaped, or coffee bean-shaped loop of bowel is seen, with radiating folds and accompanying mesenteric vessels seen converging to the point of obstruction, often called the "beak sign" (Figure 9). A "whirl sign" can be seen in the presence of a volvulus, describing the twisting of the mesentery and its vessels around the point of obstruction.[41,42]

Figure 9.

Closed-loop small-bowel obstruction. (A) Relatively long segment closed-loop small-bowel obstruction shows pattern of radiating folds with mesenteric vessels converging to a point (arrow). Mesenteric fluid and lack of bowel-wall enhancement imply superimposed ischemia. (B) Images of more proximal bowel show normally enhancing small bowel (arrow-heads) in segments remote from the closed loop.

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