(Radiographics. 2001;21:341-355.)
© RSNA, 2001
Helical CT in the Diagnosis of Small Bowel Obstruction1
Akira Furukawa, MD,
Michio Yamasaki, MD,
Kenji Furuichi, MD,
Kenji Yokoyama, MD,
Tamotsu Nagata, MD,
Masashi Takahashi, MD,
Kiyoshi Murata, MD and
Tsutomu Sakamoto, MD
1 From the Department of Radiology, Shiga University of Medical Science, Seta Tsukinowa-cho Otsu, Shiga 520-2192, Japan (A.F., M.Y., K.F., M.T., K.M.); the Department of Radiology, Shin Kori Hospital, Hirakata, Japan (K.Y.); the Department of Radiology, Ijinkai Takeda Hospital, Kyoto, Japan (T.N.); and the Department of Radiology, Koga Public Hospital, Koga, Japan (T.S.). Recipient of a Certificate of Merit for a scientific exhibit at the 1999 RSNA scientific assembly. Received May 2, 2000; revision requested June 5 and received August 1; accepted August 3. Address correspondence to A.F. (e-mail: akira@belle.shiga-med.ac.jp).
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Abstract
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With recent technologic developments, the role of computed tomography (CT) in the diagnosis of bowel obstruction has expanded. CT is recommended when clinical and initial radiographic findings remain indeterminate or strangulation is suspected. This modality clearly demonstrates pathologic processes involving the bowel wall as well as the mesentery, mesenteric vessels, and peritoneal cavity. CT should be performed with intravenous injection of contrast material, and use of thin sections is recommended to evaluate a particular region of interest. CT is reported to have a sensitivity of 78%–100% for the detection of complete or high-grade small bowel obstruction but may not allow accurate diagnosis in cases involving incomplete obstruction. In such cases, the use of adjunct enteroclysis is indicated. Furthermore, multiplanar reformatted imaging may help identify the site, level, and cause of obstruction when axial CT findings are indeterminate. CT can also demonstrate findings that indicate the presence of closed-loop obstruction or strangulation, both of which necessitate emergency exploratory laparotomy. Unfortunately, these pathologic conditions may be missed, and patients with suspected severe obstruction or bowel ischemia in whom CT and clinical findings are widely disparate must also undergo laparotomy. In general, however, CT allows appropriate and timely management of these emergency cases.
Index Terms: Intestines, CT, 74.1211 • Intestines, hernia, 74.157, 74.158 • Intestines, stenosis or obstruction, 74.723, 74.724 • Intussusception, 74.732
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LEARNING OBJECTIVES
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After reading this article and taking the test, the reader will be able to:
- Discuss the use of CT in the diagnosis of small bowel obstruction.
- Recognize the CT features of various lesions and conditions that can cause small bowel obstruction.
- Identify the CT findings in small bowel obstruction that indicate conditions requiring prompt surgical intervention.
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Introduction
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Bowel obstruction is a relatively common condition, with diagnosis based on clinical signs, patient history, and radiographic findings. Once bowel obstruction is diagnosed, the site and cause of obstruction and the presence of strangulation must be determined to ensure appropriate treatment. Typically, conventional radiography is the first imaging procedure used in patients with bowel obstruction. However, the accuracy of this modality in determining the presence of obstruction is still only 46%–80% (1–5). Its accuracy in diagnosing the site and cause of obstruction and the presence of strangulation is even lower. In patients with indeterminate radiographic findings, radiography with intraluminal injection of contrast material has generally been the next step. Enteroclysis is particularly helpful in depicting and grading the severity of partial obstruction and demonstrating the sites of multifocal incomplete obstructions; however, it is contraindicated in patients with acute and complete or high-grade bowel obstruction and in those with strangulation or suspected perforation. Its use should also be avoided in patients with markedly diminished intestinal peristalsis (5).
With recent technologic developments, the role of computed tomography (CT) in the diagnosis of bowel obstruction has expanded (1–3,5–14). CT clearly depicts pathologic processes involving not only the bowel wall but also the mesentery, mesenteric vessels, and peritoneal cavity. In this article, we discuss and illustrate the use of CT in determining the presence, level, degree, and cause (extrinsic lesions, intrinsic lesions, intussusception, intraluminal lesions) of bowel obstruction and in identifying any associated strangulation. We also discuss the management of small bowel obstruction.
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CT Technique
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Intraluminal administration of contrast material may not be necessary because the fluid and gas inherent in the bowel provide sufficient contrast. The low density of intestinal fluid allows the evaluation of bowel wall enhancement after intravenous administration of contrast material. However, oral administration of either 1.2% barium or 2% iodinated water-soluble contrast material 30–120 minutes before scanning is preferred for correct identification of the anatomy and intestinal level and for evaluation of the presence and degree of obstruction (1). The abdomen and pelvis are usually scanned with a collimation of 7–10 mm, and a pitch of 1.0–1.5 is used with helical CT (8–10,15–19). Use of thinner sections or overlapping reformatted images obtained at the suspected obstruction site may aid in making the correct diagnosis. Intravenous administration of contrast material is preferred because it highlights the abdominal viscera and lesions of the bowel as well as the pathologic process around the bowel, which causes obstruction or ileus (1). Bowel wall enhancement patterns are quite helpful in diagnosing bowel ischemia associated with obstruction. In addition, intravenous contrast material demonstrates the presence of thrombi in the superior mesenteric artery or vein that are causing an adynamic ileus and bowel infarction, which mimics bowel obstruction at clinical examination (1). From 100 to 150 mL of contrast material (300–370 mg of iodine per milliliter) is usually administered with a power injector at a rate of 2–3 mL/sec.
Multiplanar reformatted imaging at a workstation is a new technology (20) that is of increasing interest in the diagnosis of small bowel obstruction. Volume data of the abdomen are acquired with a helical technique during a single breath hold, usually with a collimation of 5 mm. Thinner collimation for better spatial resolution is possible with a multidetector CT scanner. Axial, sagittal, coronal, and curved multiplanar reformatted images are created at a workstation from the acquired volume data. Multiplanar views may help identify the site, level, and cause of obstruction when axial findings are indeterminate (Fig 1). The clinical efficacy of this technique is now under investigation.
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Figure 1a. Adhesive small bowel obstruction. (a, b) Sagittal (a) and coronal (b) multiplanar reformatted images clearly depict a site of obstruction (arrow). (c) On an axial CT scan obtained with a helical multidetector CT scanner, the obstruction is again depicted (arrow), but not as clearly as on the reformatted images.
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Figure 1b. Adhesive small bowel obstruction. (a, b) Sagittal (a) and coronal (b) multiplanar reformatted images clearly depict a site of obstruction (arrow). (c) On an axial CT scan obtained with a helical multidetector CT scanner, the obstruction is again depicted (arrow), but not as clearly as on the reformatted images.
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Figure 1c. Adhesive small bowel obstruction. (a, b) Sagittal (a) and coronal (b) multiplanar reformatted images clearly depict a site of obstruction (arrow). (c) On an axial CT scan obtained with a helical multidetector CT scanner, the obstruction is again depicted (arrow), but not as clearly as on the reformatted images.
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Diagnosis of Bowel Obstruction
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Identification of dilated proximal bowel and collapsed distal bowel is diagnostic for bowel obstruction (6,8). In general, a small bowel with a caliber greater than 2.5 cm is considered dilated (11). If a transition zone between the dilated proximal and collapsed distal bowel is detected, the diagnosis is more certain (Fig 2). However, the degree of bowel dilatation alone is not a reliable criterion for distinguishing bowel obstruction from an adynamic ileus (6). The "small bowel feces" sign (12) is another less common but reliable indicator of small bowel obstruction. Gas bubbles mixed with particulate matter are observed in dilated small bowel loops proximal to an obstruction (Fig 3). In contrast, when distension of the entire small bowel is observed without colonic collapse, the most probable diagnosis is an adynamic ileus, and a contrast material enema is recommended to rule out colonic obstruction. The cause of the ileus (eg, abscess, pancreatitis, peritonitis, mesenteric ischemia) may be demon-strated at CT. The reported sensitivity of CT in the detection of small bowel obstruction ranges from 78% to 100% for complete or high-grade obstruction (1,8,11,13,14). For incomplete obstruction, particularly low-grade or intermittent obstruction, the diagnostic accuracy of CT may not be sufficient (1,14). In such cases, enteroclysis remains the best method for evaluating the presence and degree of bowel obstruction.
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Figure 2. Adhesive small bowel obstruction. Abdominal CT scan shows the transition zone (arrowhead), distended proximal bowel loops (thick arrow), and collapsed distal bowel loops (thin arrow). No adhesive band is actually seen at the transition zone; however, no other specific cause of obstruction is identified either, which suggests the diagnosis of adhesive bowel obstruction.
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Figure 3. Small bowel feces sign. CT scan shows gas bubbles mixed with particulate matter in dilated small bowel loops (arrow). This finding is frequently seen proximal to the site of obstruction.
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Level of Obstruction
The level of obstruction is determined by identifying the site of the transition zone and comparing the relative lengths of dilated versus collapsed bowel (Fig 4). Because distended bowel loops tend to migrate from their expected anatomic location, determination of the obstructed site on the basis of the intraabdominal location of the transition zone alone may be misleading.
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Figure 4a. Mechanical small bowel obstruction in a patient with a history of abdominal surgery (partial gastrectomy). (a) CT scan of the upper abdomen shows dilated proximal bowel loops. (b, c) CT scans through the midabdomen (c obtained at a lower level than b) demonstrate a transition zone at the undersurface of the abdominal scar (arrow). (d, e) CT scans obtained caudad to a-c (e obtained at a lower level than d) show several collapsed bowel loops, mostly in the pelvic cavity. Adhesive bowel obstruction at the distal jejunum was confirmed at surgery.
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Figure 4b. Mechanical small bowel obstruction in a patient with a history of abdominal surgery (partial gastrectomy). (a) CT scan of the upper abdomen shows dilated proximal bowel loops. (b, c) CT scans through the midabdomen (c obtained at a lower level than b) demonstrate a transition zone at the undersurface of the abdominal scar (arrow). (d, e) CT scans obtained caudad to a-c (e obtained at a lower level than d) show several collapsed bowel loops, mostly in the pelvic cavity. Adhesive bowel obstruction at the distal jejunum was confirmed at surgery.
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Figure 4c. Mechanical small bowel obstruction in a patient with a history of abdominal surgery (partial gastrectomy). (a) CT scan of the upper abdomen shows dilated proximal bowel loops. (b, c) CT scans through the midabdomen (c obtained at a lower level than b) demonstrate a transition zone at the undersurface of the abdominal scar (arrow). (d, e) CT scans obtained caudad to a-c (e obtained at a lower level than d) show several collapsed bowel loops, mostly in the pelvic cavity. Adhesive bowel obstruction at the distal jejunum was confirmed at surgery.
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Figure 4d. Mechanical small bowel obstruction in a patient with a history of abdominal surgery (partial gastrectomy). (a) CT scan of the upper abdomen shows dilated proximal bowel loops. (b, c) CT scans through the midabdomen (c obtained at a lower level than b) demonstrate a transition zone at the undersurface of the abdominal scar (arrow). (d, e) CT scans obtained caudad to a-c (e obtained at a lower level than d) show several collapsed bowel loops, mostly in the pelvic cavity. Adhesive bowel obstruction at the distal jejunum was confirmed at surgery.
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Figure 4e. Mechanical small bowel obstruction in a patient with a history of abdominal surgery (partial gastrectomy). (a) CT scan of the upper abdomen shows dilated proximal bowel loops. (b, c) CT scans through the midabdomen (c obtained at a lower level than b) demonstrate a transition zone at the undersurface of the abdominal scar (arrow). (d, e) CT scans obtained caudad to a-c (e obtained at a lower level than d) show several collapsed bowel loops, mostly in the pelvic cavity. Adhesive bowel obstruction at the distal jejunum was confirmed at surgery.
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Degree of Obstruction
Complete versus partial obstruction of the small bowel is determined by the degree of collapse and the amount of the residual contents in the portion of the bowel distal to the obstructed site. Passage of the contrast material through the transition zone to the collapsed distal bowel always indicates incomplete obstruction.
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Causes of Obstruction
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The causes of small bowel obstruction include extrinsic lesions, intrinsic lesions, intussusception, and intraluminal lesions (Table 1). The most common cause of obstruction is adhesion (up to 75% of cases), followed by hernias and neoplasm (21–23). Awareness that a patient has a history of surgery or irradiation or of a disease that may disseminate throughout the peritoneal cavity is important for correct CT interpretation and diagnosis.
Extrinsic Lesions
Adhesion.—Adhesions are responsible for more than one-half (50%–75%) of all small bowel obstructions (21–23). Eighty percent of adhesions arise after surgery, 15% are due to peritonitis, and the remainder are either congenital or of uncertain cause (5). Adhesive obstruction can be single, multiple, or extensive. The CT diagnosis of adhesions must be based on an abrupt change in bowel caliber without evidence of another cause of obstruction because the adhesive band itself is not identified at CT (Fig 2) (1,6,13). Adhesions involve the undersurface of the abdominal scar, the area of enteric surgical intervention, or the region of an inflammatory focus and are more frequently seen in the ileum (Fig 5) (5,24).
Hernia.—Hernia is the second most frequent cause of small bowel obstruction, with a reported prevalence of approximately 10% (21–23). However, the frequency of hernia as a cause of bowel obstruction has been decreasing, possibly owing to early surgical repair (25). The classification of hernias is shown in Table 2. The nomenclature used for each specific type of hernia indicates the anatomic location of its orifice.
External Hernia.—External hernia is produced by prolapse of the viscera through a defect in the abdominal or pelvic wall. It usually involves a specific site of congenital weakness or previous surgery (26). Although visible or palpable hernias account for 95% of obstructive hernias, CT is useful in detecting hernias in unsuspected sites and in obese patients. Another advantage of CT is its capacity to demonstrate the herniated viscera and associated complications (Figs 6–9).
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Figure 8. Obturator hernia. CT scan obtained at the level of the obturator foramen shows a small bowel loop herniating through the pelvic floor between the right pectineus and external obturator muscles (arrow).
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Internal Hernia.—Internal hernia is a herniation of the bowel loops through a developmental or surgically created defect of the peritoneum, omentum, or mesentery or through an adhesive band. Although they are less common than external hernias, the diagnosis is always based on radiologic findings (Figs 10, 11) (26).
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Figure 10a. Hernia through the foramen of Winslow. (a) Conventional abdominal radiograph demonstrates gas-containing bowel loops gathered in the center of the upper abdomen between the liver and stomach (arrows). (b, c) CT scans of the upper abdomen (c obtained at a lower level than b) demonstrate the distended small bowel loops in the lesser sac, occupying the space between the stomach and liver (arrows).
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Figure 10b. Hernia through the foramen of Winslow. (a) Conventional abdominal radiograph demonstrates gas-containing bowel loops gathered in the center of the upper abdomen between the liver and stomach (arrows). (b, c) CT scans of the upper abdomen (c obtained at a lower level than b) demonstrate the distended small bowel loops in the lesser sac, occupying the space between the stomach and liver (arrows).
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Figure 10c. Hernia through the foramen of Winslow. (a) Conventional abdominal radiograph demonstrates gas-containing bowel loops gathered in the center of the upper abdomen between the liver and stomach (arrows). (b, c) CT scans of the upper abdomen (c obtained at a lower level than b) demonstrate the distended small bowel loops in the lesser sac, occupying the space between the stomach and liver (arrows).
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Figure 11a. Right paraduodenal hernia. Contrast-enhanced CT scans demonstrate the proximal jejunum herniating into the mesentericoparietal fossa of Waldeyer via the orifice behind the superior mesenteric artery just below the transverse portion of the duodenum (solid arrow in b). The herniated loops are distended in the hernial sac (open arrows in b). A small amount of ascites is also present (arrowhead).
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Figure 11b. Right paraduodenal hernia. Contrast-enhanced CT scans demonstrate the proximal jejunum herniating into the mesentericoparietal fossa of Waldeyer via the orifice behind the superior mesenteric artery just below the transverse portion of the duodenum (solid arrow in b). The herniated loops are distended in the hernial sac (open arrows in b). A small amount of ascites is also present (arrowhead).
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Other Extrinsic Causes.—A wide variety of neoplastic, inflammatory, and vascular lesions may cause bowel obstruction, either by direct luminal compression or by producing a desmoplastic reaction of the bowel wall through the serosa. The most common cause is advanced peritoneal carcinomatosis, in which multiple transition zones of nodular wall thickening are demonstrated at CT (Fig 12) (5). Carcinoid tumors, mycobacterial infection, and desmoid tumors may have radiographic features similar to those of peritoneal carcinomatosis.
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Figure 12a. Mechanical bowel obstruction caused by tumors disseminated in the peritoneal cavity. CT scans obtained at the level of the renal hilum (a) and more caudally (b) demonstrate dilated bowel loops and multiple transition zones (arrow). The obstructions are incomplete and of moderate grade.
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Figure 12b. Mechanical bowel obstruction caused by tumors disseminated in the peritoneal cavity. CT scans obtained at the level of the renal hilum (a) and more caudally (b) demonstrate dilated bowel loops and multiple transition zones (arrow). The obstructions are incomplete and of moderate grade.
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Intrinsic Lesions
Intrinsic intestinal lesions such as neoplasms, inflammatory lesions, vascular lesions, and hematoma may cause bowel wall thickening and result in obstruction. Because the contents of the small bowel are liquid, obstruction occurs only when lesions grow large enough to cause significant luminal narrowing or by means of the mechanism of intussusception. Adenocarcinoma, Crohn disease, and radiation enteropathy are the most frequent intrinsic lesions to cause bowel obstruction (5). Intramural hematoma and eosinophilic gastroenteritis are rare causes. Most intrinsic bowel lesions causing obstruction are seen at the transition zone and manifest as localized mural thickening at CT (Figs 13–15) (8,11,12).
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Figure 13. Mechanical small bowel obstruction at the ileum in a patient with Crohn disease. CT scan obtained after intravenous injection of contrast material shows dilated proximal loops (arrows) and a transition zone (arrowhead). At the transition zone, the bowel wall demonstrates thickening and enhancement, findings that indicate an active lesion.
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Figure 14. Small bowel obstruction caused by adenocarcinoma of the ileum. CT scan demonstrates a transition zone (arrow) and distended proximal bowel (arrowheads). Luminal thickening is identified at the transition zone, a finding that indicates the presence of a mucosal lesion, which was known to be adenocarcinoma.
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Figure 15a. Duodenal obstruction caused by intramural and paraduodenal hematoma due to rupture of an aneurysm at the inferior pancreaticoduodenal artery. (a, b) Contrast-enhanced CT scans demonstrate distended proximal bowel and hematoma around the duodenum (arrowhead in b) causing duodenal obstruction. (c) Image from a barium study shows remarkable luminal narrowing and obstruction at the lower two-thirds of the duodenum (arrows). (d) Superior mesenteric arteriogram demonstrates the ruptured aneurysm (arrowhead).
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Figure 15b. Duodenal obstruction caused by intramural and paraduodenal hematoma due to rupture of an aneurysm at the inferior pancreaticoduodenal artery. (a, b) Contrast-enhanced CT scans demonstrate distended proximal bowel and hematoma around the duodenum (arrowhead in b) causing duodenal obstruction. (c) Image from a barium study shows remarkable luminal narrowing and obstruction at the lower two-thirds of the duodenum (arrows). (d) Superior mesenteric arteriogram demonstrates the ruptured aneurysm (arrowhead).
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Figure 15c. Duodenal obstruction caused by intramural and paraduodenal hematoma due to rupture of an aneurysm at the inferior pancreaticoduodenal artery. (a, b) Contrast-enhanced CT scans demonstrate distended proximal bowel and hematoma around the duodenum (arrowhead in b) causing duodenal obstruction. (c) Image from a barium study shows remarkable luminal narrowing and obstruction at the lower two-thirds of the duodenum (arrows). (d) Superior mesenteric arteriogram demonstrates the ruptured aneurysm (arrowhead).
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Figure 15d. Duodenal obstruction caused by intramural and paraduodenal hematoma due to rupture of an aneurysm at the inferior pancreaticoduodenal artery. (a, b) Contrast-enhanced CT scans demonstrate distended proximal bowel and hematoma around the duodenum (arrowhead in b) causing duodenal obstruction. (c) Image from a barium study shows remarkable luminal narrowing and obstruction at the lower two-thirds of the duodenum (arrows). (d) Superior mesenteric arteriogram demonstrates the ruptured aneurysm (arrowhead).
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Intussusception
Intussusception in adults is a relatively rare condition that accounts for less than 5% of cases of bowel obstruction (27). In contrast to cases involving infants, 80% of cases are associated with underlying causes such as neoplasm, adhesion, inverted Meckel diverticulum, foreign body, and prior history of abdominal surgery (5,22,23,28). Small bowel dismotility disorders such as celiac disease or scleroderma are also possible causes. CT depicts the collapsed, intussuscepted proximal bowel (intussusceptum) with the mesenteric fat and vessels lying within the wall of the distal bowel (intussuscipiens). On cross-sectional images, the intussusception has a target-like appearance. The underlying lesion may also be observed at the leading point (Fig 16) (29–31).
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Figure 16a. Small bowel obstruction caused by intussusception produced by an ileal lipoma. (a) Image from an enteroclysis study reveals a round mass in the proximal ileum. (b, c) CT scans demonstrate intussuscepted bowel and mesenteric fat with a target-like appearance in the wall of the distal ileum (arrow in b). Lipoma is identified at the leading point (arrowhead in c).
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Figure 16b. Small bowel obstruction caused by intussusception produced by an ileal lipoma. (a) Image from an enteroclysis study reveals a round mass in the proximal ileum. (b, c) CT scans demonstrate intussuscepted bowel and mesenteric fat with a target-like appearance in the wall of the distal ileum (arrow in b). Lipoma is identified at the leading point (arrowhead in c).
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Figure 16c. Small bowel obstruction caused by intussusception produced by an ileal lipoma. (a) Image from an enteroclysis study reveals a round mass in the proximal ileum. (b, c) CT scans demonstrate intussuscepted bowel and mesenteric fat with a target-like appearance in the wall of the distal ileum (arrow in b). Lipoma is identified at the leading point (arrowhead in c).
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Intraluminal Lesions
Bowel obstruction may be caused by gallstones, foreign bodies, bezoars, or meconium. Gallstone-induced bowel obstruction usually occurs in elderly patients, particularly women. The classic triad of a calcified stone in an ectopic location, gas in the shrunken gallbladder or biliary tree, and mechanical bowel obstruction (32) can be observed at conventional radiography, ultra-sonography (US), or CT (Fig 17). However, CT and US have a greater sensitivity in demonstrat-ing gallstones obstructing the bowel lumen (5, 33). A foreign body lodged in the small bowel is relatively rare (3.3% of cases) compared with a foreign body lodged in the esophagus (68%), stomach (11.6%), or colon (11.6%) (Fig 18) (34). Bowel obstruction caused by a foreign body usually occurs in children or mentally disturbed or disabled patients (5). If a foreign body is detected in the small bowel, an underlying obstructing lesion should be excluded. Bezoars are another, less common cause of small bowel obstruction. The persimmon fruit can become a nucleus for bezoar formation, and the condition is usually seen in patients who have undergone gastric outlet resection (35).
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Figures 17a. Gallstone ileus. (a) Abdominal CT scan demonstrates dilated proximal bowel and collapsed distal bowel loops, findings that indicate a mechanical bowel obstruction. A calcified gallstone is seen at the transition zone (arrow). (b, c) CT scans of the upper abdomen (c obtained at a lower level than b) demonstrate gas in the biliary tree and a collapsed gallbladder, findings that are diagnostic for gallstone ileus.
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Figure 17b. Gallstone ileus. (a) Abdominal CT scan demonstrates dilated proximal bowel and collapsed distal bowel loops, findings that indicate a mechanical bowel obstruction. A calcified gallstone is seen at the transition zone (arrow). (b, c) CT scans of the upper abdomen (c obtained at a lower level than b) demonstrate gas in the biliary tree and a collapsed gallbladder, findings that are diagnostic for gallstone ileus.
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Figure 17c. Gallstone ileus. (a) Abdominal CT scan demonstrates dilated proximal bowel and collapsed distal bowel loops, findings that indicate a mechanical bowel obstruction. A calcified gallstone is seen at the transition zone (arrow). (b, c) CT scans of the upper abdomen (c obtained at a lower level than b) demonstrate gas in the biliary tree and a collapsed gallbladder, findings that are diagnostic for gallstone ileus.
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Figure 18. Mechanical bowel obstruction caused by a foreign body. CT scan shows mechanical bowel obstruction caused by undigested food at the distal ileum (arrow). The small bowel feces sign and dilated proximal bowel loops are also identified.
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Closed-Loop Obstruction
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Closed-loop obstruction is a form of mechanical bowel obstruction in which two points along the course of the bowel are obstructed at a single site. The involved bowel loops can be single or multiple. Closed-loop obstruction is most often caused by an adhesive band and occasionally by an internal or external hernia. Because a closed-loop obstruction tends to involve the mesentery and is prone to produce a volvulus, it represents the most common cause of strangulation (1,6,15). A C-shaped, U-shaped, or "coffee bean" configuration of the bowel loop and the mesenteric vessel converging toward torsion are characteristic CT findings. Two adjacent collapsed, round, oval, or triangular loops, the "beak" sign, and the "whirl" sign are observed at the site of obstruction and torsion. The affected loops are usually filled with fluid (Figs 19–23) (1,6,15).
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Figure 21. Closed-loop obstruction. CT scan shows a U-shaped configuration of the bowel loops in the center of the pelvis, a finding that indicates closed-loop obstruction. The affected loops are filled with fluid.
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Figure 22. Closed-loop obstruction due to postoperative adhesion. CT scan demonstrates converging mesenteric vessels to the left of the affected loops at the site of torsion (arrow). As in Figure 21, the affected loops are filled with fluid. Dilated proximal bowel is also noted.
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Figure 23a. Closed-loop obstruction in a patient with intestinal torsion. CT scans show vessels in the mesentery and collapsed bowel loops creating the whirl sign (arrow in a). The beak sign is identified at the site of torsion (arrowhead in b).
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Figure 23b. Closed-loop obstruction in a patient with intestinal torsion. CT scans show vessels in the mesentery and collapsed bowel loops creating the whirl sign (arrow in a). The beak sign is identified at the site of torsion (arrowhead in b).
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Strangulation
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A strangulating obstruction is defined as a mechanical obstruction associated with bowel ischemia. This condition is seen in approximately 10% of patients with small bowel obstruction; however, the reported prevalence ranges from 5% to 42% (15,21,36). In the majority of cases, strangulation is found in a type of closed-loop obstruction. A strangulating obstruction has a mortality rate of 20%–37%, compared with a recently reduced rate of 5%–8% for a simple obstruction (16,37,38). This high mortality rate is mainly attributed to a delay in establishing the correct diagnosis. Reported CT findings indicating strangulation include thickening and increased attenuation of the affected bowel wall, the "target" or "halo" sign, serrated beaklike narrowing at the site of obstruction, pneumatosis intestinalis, and gas in the portal veins. Lack of bowel wall enhancement, asymmetric (decreased or increased) wall enhancement, and delayed bowel wall enhancement can be seen in the affected bowel loops after the administration of contrast material. Findings of haziness or obliteration of the mesenteric vessels and localized mesenteric fluid and hemorrhage are seen in the mesentery attached to the ischemic bowel loops (Figs 24–27) (15,17,39–43). Three or more of these findings are usually confirmed in patients with strangulation (15,39). Accurate detection of strangulation at CT remains controversial; however, several studies have reported a detection rate of 63%–100% (15,18,19,39).
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Figure 25. Strangulation in a patient with adhesive obstruction complicated by a mesenteric volvulus. On a contrast-enhanced CT scan, the walls of the bowel loops located in the right midabdomen do not enhance (arrow). Bowel infarction was confirmed at surgery.
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Figure 26. Strangulation in a patient with bowel ischemia caused by a mesenteric volvulus. Contrast-enhanced CT scan demonstrates lack of bowel wall enhancement in the affected loop (arrow). Mesenteric edema is also seen (arrowheads).
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Figure 27. Simple bowel obstruction. Contrast-enhanced CT scan shows enhancing intestinal folds mimicking bowel wall thickening (arrowheads). This finding should not be misinterpreted as a sign of bowel ischemia.
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Management of Small Bowel Obstruction
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It is generally accepted that acute and complete or high-grade obstruction requires immediate surgery, whereas partial obstruction can initially be managed conservatively unless there is an accompanying lesion that requires surgery (1,5, 6,18). The risk of strangulation increases over time in patients with acute and complete or high-grade obstruction. Follow-up CT or intraluminal contrast-enhanced gastrointestinal studies should be performed in indeterminate cases. Strangulation constitutes an emergency and has a high mortality rate, particularly if surgery is delayed 36 hours or more from onset; the condition has a mortality rate of 8% within 36 hours versus 25% after 36 hours (1,44). Although occasionally reversible, a closed-loop obstruction is also considered an emergency, even without ischemia at diagnosis, because it can quickly and unpredictably lead to strangulation. Emergency exploratory laparotomy is recommended in all patients with signs of ischemia or closed-loop obstruction at CT (18). Unfortunately, the presence of bowel ischemia or closed-loop obstruction can be missed even with state-of-the-art CT, and patients with an obvious discrepancy between CT and clinical findings in whom severe obstruction or bowel ischemia is suspected must undergo exploratory laparotomy (18).
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Conclusions
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CT is highly sensitive and specific in determining the presence of bowel obstruction and clearly demonstrates the site and cause of obstruction. The possibility of associated strangulation can be assessed with CT findings of bowel ischemia, particularly with contrast material administration or by pointing out the specific type of bowel obstruction (ie, closed-loop obstruction). CT is recommended for the evaluation of patients with suspected bowel obstruction, particularly when clinical and initial conventional radiographic findings remain indeterminate or strangulation is suspected.
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Abstract
LEARNING OBJECTIVES
