CT of Small Bowel Obstruction

CT is commonly performed for clinically suspected small bowel obstruction (SBO), and sometimes for bowel obstructions already visualized on plain abdominal radiographs. CT can provide important additional information to guide clinical management and improve patient outcomes. We’ll examine the CT diagnosis of small bowel obstruction in more detail, dispelling some common myths along the way.

Recommended CT technique
The American College of Radiology (ACR) Appropriateness Criteria® for suspected small-bowel obstruction recommend CT abdomen and pelvis with intravenous contrast – but without oral contrast – for suspected high-grade small bowel obstruction based on clinical evaluation or initial radiography [1]. In fact, the ACR specifies: “Oral contrast should not be used if high-grade SBO is known or suspected. Oral contrast will not reach the site of obstruction, wastes time, adds expense, can induce further patient discomfort, will not add to diagnostic accuracy, and can lead to complications, particularly vomiting and aspiration.” Ironically, a recent survey of academic emergency departments found that almost 70% of programs were administering oral contrast for suspected SBO, despite the ACR recommendation [2]. Oral contrast use can add roughly two hours to patient preparation time for CT, even when antiemetics are given [3], so omitting unnecessary oral contrast can improve ED length of stay.

CT identifies small bowel obstruction
The CT diagnosis of small bowel obstruction relies on the same definition of “dilated” small bowel encountered in other imaging modalities, including x-ray (plain radiography) and ultrasound. The normal small bowel diameter is less than 2cm; diameter greater than 3cm is uniformly abnormal. As progressively higher thresholds are applied, the specificity rises while sensitivity declines. A small bowel diameter of 2.5cm achieves the highest combination of sensitivity and specificity for the diagnosis of obstruction.

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CT determines the location – and sometimes the etiology – of obstruction.
In addition to dilated small bowel, a small bowel obstruction requires a physical point of obstruction, at which dilated bowel proximally gives way to non-dilated bowel distally. Radiographically, this location is the transition point. CT identifies this location and in some cases can distinguish the etiology, such as a mass lesion, adhesion, internal hernia, inflammatory bowel disease, or bowel volvulus. The transition point can be identified without the use of orally or intravenously administered contrast, as it requires only the measurement of small bowel diameter.

CT identifies time-sensitive complications of bowel obstruction, including ischemia and perforation.
Bowel obstructions can be complicated by time-sensitive catastrophes, such as perforation or intestinal ischemia resulting from a closed loop obstruction. Identification of these complications with CT guides patients toward rapid surgical intervention rather than bowel rest and decompression via nasogastric tube. Free air is readily identified on CT without the addition of any contrast agents. Air appears black on all CT window settings. The conspicuity of air can be increased by changing the CT window setting from the default soft tissue setting usually used for inspection of the abdomen. On this setting, intraperitoneal fat appears nearly black, potentially masking small extraluminal air collections. Two alternative window settings render air particularly evident – using bone or lung window settings, only air remains black in appearance. A more subtle alteration of the window setting using PACS (Picture Archiving and Communication System) controls to brighten fat can also improve recognition of free air. Ischemic changes on CT are discussed below.

CT contrast agents – A closer look
Why isn’t oral contrast needed for diagnosis of SBO? Oral contrast seems a logical requirement for detection of a small bowel obstruction, with oral contrast help ing to differentiate complete from partial SBO (Figure 1). However, in the case of obstruction, native “contrast agents” are already present within the bowel lumen: fluid (including ingested and secreted fluids) and ingested air. Simple fluids such as water provide excellent “neutral” contrast, appearing dark gray on abdominal CT soft tissue windows (Figures 2, 3). Air (-1000 Hounsfield units, the CT density unit) provides outstanding contrast because its density is so much lower than other body materials; the closest competitor is fat, at -50 to -100 Hounsfield units. For this reason, air has been used as a radiographic contrast agent for decades, as in the case of air contrast enema. As with plain radiographs, CT can depict bowel dilated with air and fluid. Without oral contrast, the sensitivity of CT for small bowel obstruction is around 88% [4].

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Why is intravenous contrast helpful in evaluation of SBO?
Addition of intravenous contrast does not significantly alter CT sensitivity or specificity for the diagnosis of small bowel obstruction [4]. However, intravenous contrast can provide clues to the presence of bowel ischemia. While some small bowel obstructions can be managed conservatively with bowel rest and decompression via nasogastric tube, some obstructions are associated with segmental bowel ischemia, which can lead to bowel gangrene, perforation, and increased patient morbidity and mortality without rapid surgical intervention. Blood flow may be impaired by a tight adhesive band, an internal hernia, or by volvulus of a bowel loop. A primary benefit of CT is identification of such ischemic complications. Intravenously injected contrast material “goes with the [blood]flow” – resulting in an increased density of well-perfused organs, a phenomenon called “enhancement.” Bowel with an intact blood supply will demonstrate a bright and thin wall following the administration of intravenous contrast. Poorly perfused bowel may fail to enhance or may be thickened in appearance (Figure 3). Failure of a segment of bowel to enhance with intravenous contrast is highly specific (100%), though with poor sensitivity (33% in one study) [5]. High density oral contrast agents such as Gastrografin® or barium can interfere with the assessment of enhancement of the adjacent bowel wall – providing another reason to avoid oral contrast agents in suspected small bowel obstruction.

The Bullets

• CT can identify small bowel obstruction and its significant time-sensitive complications, ischemia and perforation.
• Obstruction and perforation can be recognized without any contrast agents.
• Oral contrast is not needed and may hinder evaluation of ischemia.
• Intravenous contrast is a valuable tool to identify ischemic bowel.

REFERENCES

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1. ACR Appropriateness Criteria® Suspected Small-Bowel Obstruction. American College of Radiology. (Accessed September 9, 2015, at http://www.acr.org/~/media/832f100277004bc69a8c818c7c9bff33.pdf.)
2. Broder JS, Hamedani AG, Liu SW, Emerman CL. Emergency department contrast practices for abdominal/pelvic computed tomography-a national survey and comparison with the american college of radiology appropriateness criteria((R)). J Emerg Med 2013;44:423-33.
3. Garra G, Singer AJ, Bamber D, Chohan J, Troxell R, Thode HC, Jr. Pretreatment of patients requiring oral contrast abdominal computed tomography with antiemetics: a randomized controlled trial of efficacy. Ann Emerg Med 2009;53:528-33.
4. Atri M, McGregor C, McInnes M, et al. Multidetector helical CT in the evaluation of acute small bowel obstruction: comparison of non-enhanced (no oral, rectal or IV contrast) and IV enhanced CT. Eur J Radiol 2009;71:135-40.
5. Sheedy SP, Earnest Ft, Fletcher JG, Fidler JL, Hoskin TL. CT of small-bowel ischemia associated with obstruction in emergency department patients: diagnostic performance evaluation. Radiology 2006;241:729-36.