Introduction	Choose Top of pageABSTRACTIntroduction <<Conventional RadiographyContrast StudiesSonographyCTMRIConclusionReferencesCITING ARTICLES

The morbidity and mortality associated with acute small-bowel obstruction continue to be significant. It accounts for 12-16% of all surgical admissions in patients with acute abdominal conditions [1]. Small-bowel obstruction is caused by postoperative adhesions in 70% of all cases [2]. Other common causes include hernias, neoplasms, and Crohn's disease [1, 2]. The important question in small-bowel obstruction management lies in determining whether early laparotomy is required or whether a trial of nonoperative management should be instituted [1]. Clinical examination findings and laboratory values are often nonspecific and unreliable at differentiating simple mechanical obstruction from strangulated bowel. Imaging in the acute setting plays a key role. It can indicate the location, degree, and cause of an obstruction and assess for the presence of ischemia [3]. This pictorial essay aims to review the various imaging techniques used in establishing the diagnosis of acute small-bowel obstruction.

Conventional Radiography	Choose Top of pageABSTRACTIntroductionConventional Radiography <<Contrast StudiesSonographyCTMRIConclusionReferencesCITING ARTICLES

Abdominal radiography in conjunction with the clinical examination is diagnostic in only 50-60% of cases [1]. Radiographs have been shown to be sensitive for high-grade but not low-grade obstructions [1]. Signs of small-bowel obstruction on radiographs include distended loops of bowel greater than 3 cm, collapsed colon, differential air-fluid levels, and thickened bowel wall (Figs. 1A and 1B). The string-of-pearls sign may also be identified (Figs. 1A and 1B). It is caused by slow resorption of intraluminal air leaving small bubbles trapped between the folds of the valvulae conniventes. Except for inguinal hernias [3] and gallstone ileus (Figs. 2A, 2B, and 2C), the cause of obstruction is often indiscernible on radiographs. Strangulation may be indicated by edematous folds, pneumatosis intestinalis (Fig. 3A), and gas in the portal vein (Fig. 3B), but these features are rarely seen. If a high clinical suspicion of obstruction exists, additional imaging is required even if radiographs are reported to show normal findings. Despite its limitations, conventional radiography continues to be the initial imaging examination for patients with suspected small-bowel obstruction because of its sensitivity in revealing high-grade obstruction [1], wide availability, and relatively low cost.

Contrast Studies	Choose Top of pageABSTRACTIntroductionConventional RadiographyContrast Studies <<SonographyCTMRIConclusionReferencesCITING ARTICLES

Oral contrast studies such as a small-bowel follow-through can offer additional information regarding the degree of obstruction. Findings suggestive of obstruction include dilated loops of small bowel and a delayed transit time of barium through a transition point [3]. Limitations of small-bowel follow-through include the length of time required to perform the study, dilution of barium because of excess residual intraluminal fluid, and the inability of patients to drink the barium in an acute setting [3].

Enteroclysis allows areas that are nondistensible or fixed to be more easily identified [4]. Enteroclysis is performed by intubating the small bowel and infusing contrast material, essentially bypassing the stomach. In the subacute setting, enteroclysis is very accurate in diagnosing low-grade and intermittent obstructions [4] and can serve as an adjunct to CT if more information, such as how much contrast material is making its way through the obstruction, is required [4] (Fig. 4).

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Fig. 1A —Small-bowel obstruction on radiography. Supine abdominal radiograph in 45-year-old woman with adhesional small-bowel obstruction shows multiple dilated loops of small bowel. Valvulae conniventes appear prominent. In appropriate clinical context, this would be diagnostic of small-bowel obstruction.

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Fig. 1B —Small-bowel obstruction on radiography. Upright abdominal radiograph in 56-year-old woman with adhesional small-bowel obstruction shows multiple air-fluid levels (arrows) and string-of-pearls sign (arrowhead).

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Fig. 2A —48-year-old woman presenting with gallstone ileus. Upright abdominal radiograph shows multiple air-fluid levels. Pneumobilia (arrow) is present, as is string-of-pearls sign (arrowheads).

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Fig. 2B —48-year-old woman presenting with gallstone ileus. CT scan through upper abdomen shows air in gallbladder (arrow) and proximal cystic duct.

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Fig. 2C —48-year-old woman presenting with gallstone ileus. CT scan obtained inferior to B shows calcified impacted gallstone (arrow) in distal jejunum with proximal dilated loops of bowel.

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Fig. 3A —Strangulation. Supine abdominal radiograph in 46-year-old woman with ischemic colitis shows linear radiolucency (arrows) along wall of bowel, which is consistent with pneumatosis intestinalis. Dilated loops of small bowel are also present.

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Fig. 3B —Strangulation. Right-side-up decubitus abdominal radiograph in 69-year-old woman shows multiple branching radiolucencies (arrows) in periphery of liver shadow, which is indicative of portal venous gas. Dilated loops of small bowel are also present, which is consistent with small-bowel obstruction.

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Fig. 4 —Enteroclysis. 54-year-old woman with adhesional small-bowel obstruction. Spot film from enteroclysis shows small-bowel loop narrowing (arrow) due to postoperative adhesion.

Sonography	Choose Top of pageABSTRACTIntroductionConventional RadiographyContrast StudiesSonography <<CTMRIConclusionReferencesCITING ARTICLES

On sonography, small-bowel obstruction is suspected if multiple dilated (> 3 cm), fluid-filled loops are seen (Figs. 5A and 5B). The obstructing cause can occasionally be visualized if it is a tumor or hernia. The presence of aperistalsis, fluid-filled bowel distention, and wall thickening supports infarction in the appropriate clinical context [5]. Bowel wall perfusion can also be assessed by Doppler sonography.

Sonography has been reported to have a sensitivity of 89% compared with 71% for conventional abdominal radiography in diagnosing small-bowel obstruction and is superior in its ability to identify features of strangulation and to predict the location and cause of obstruction [5]. Although not routinely used, sonography may be indicated in critically ill patients because transfer of the patient to the examination table may be time-consuming and difficult [5].

CT	Choose Top of pageABSTRACTIntroductionConventional RadiographyContrast StudiesSonographyCT <<MRIConclusionReferencesCITING ARTICLES

If an acute obstruction is suspected, CT is the technique of choice for several reasons. First, it does not require oral contrast material because the retained intraluminal fluid serves as a natural negative contrast agent. Second, when compared with enteroclysis, CT is rapid, noninvasive, and readily available [3]. Finally, it also allows extramural areas that would not be visible on contrast studies to be assessed.

The diagnosis of small-bowel obstruction on CT involves identifying dilated loops of bowel proximally with normal-caliber or collapsed loops distally. A small-bowel caliber of greater than 2.5 cm is considered dilated [6]. If a transition point is detected, the diagnosis is more certain [6]. The transition point often resembles a beak and is described as the beak sign (Figs. 6A, 6B, and 6C). This finding has been shown to be present in 60% of simple small-bowel obstruction cases [7]. Other reliable features include the string-of-pearls sign (Fig. 7A) and the “small-bowel feces” sign (Fig. 7B). The small-bowel feces sign is a result of stasis and mixing of small-bowel contents and is present in 82% of cases of small-bowel obstruction [2, 3]. Occasionally, visualization of an adhesional band is possible, although it is rare to be able to do so (Figs. 8A and 8B).

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Fig. 5A —Sonography features of small-bowel obstruction. Both cases are due to postoperative adhesions. Abdominal sonogram in 40-year-old woman shows dilated, fluid-filled loop of small bowel with prominent valvulae conniventes (arrows).

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Fig. 5B —Sonography features of small-bowel obstruction. Both cases are due to postoperative adhesions. Abdominal sonogram in 62-year-old man shows thickened small-bowel wall (arrows). Real-time scanning showed small bowel to be hyperperistaltic.

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Fig. 6A —Small-bowel obstruction secondary to adhesions. Axial CT scan through lower abdomen in 54-year-old woman with small-bowel obstruction secondary to adhesions shows multiple fluid-filled loops of small bowel (arrows).

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Fig. 6B —Small-bowel obstruction secondary to adhesions. CT scan obtained inferior to A shows transition point (arrows) with dilated bowel proximally and collapsed bowel distally. No pathologic process is visualized at transition point, and transition is smooth. This obstruction was found to be adhesional in nature.

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Fig. 6C —Small-bowel obstruction secondary to adhesions. Axial contrast-enhanced CT scan through mid abdomen of 55-year-old man with small-bowel obstruction secondary to adhesions shows multiple fluid-filled loops with tapering transition point (arrows), otherwise known as beak sign.

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Fig. 7A —Small-bowel obstruction secondary to Crohn's disease. Axial CT scan through lower abdomen of 44-year-old woman with small-bowel obstruction secondary to Crohn's disease shows multiple fluid-filled loops of small bowel (arrows) and CT equivalent of string-of-pearls sign on radiography.

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Fig. 7B —Small-bowel obstruction secondary to Crohn's disease. Axial CT scan through lower abdomen in 28-year-old woman with Crohn's disease shows partially solid material intermixed with air within distal small bowel (arrows), similar in appearance to feces in colon; this finding is called the “small-bowel feces” sign.

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Fig. 8A —58-year-old woman with small-bowel obstruction secondary to adhesions. Axial CT scan through lower abdomen shows dilated proximal loop (arrow) and collapsed distal loop (arrowhead).

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Fig. 8B —58-year-old woman with small-bowel obstruction secondary to adhesions. CT scan obtained inferior to A shows narrowing of involved loop of bowel (arrows). Adhesion is inferred to be causing narrowing given history of previous abdominal surgery and given neither masses nor extrinsic processes are seen to result in narrowing. Multiple dilated loops of small bowel are also seen.

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Fig. 9 —26-year-old woman with vasculitis and small-bowel obstruction. Axial contrast-enhanced CT scan through mid abdomen shows thickened loops of small bowel and target sign (arrows). Free fluid (arrowhead) is also seen.

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Fig. 10A —66-year-old woman with diagnosis of ischemic bowel. Axial contrast-enhanced CT scan through mid abdomen shows multiple dilated air- and fluid-filled loops of small bowel. There is evidence of pneumatosis intestinalis and lack of bowel wall enhancement (thin arrow) as compared with normally enhancing loop (thick arrow). Also seen is intraperitoneal free fluid (arrowhead). Round radiodensity seen in one loop of small bowel is surgical drain.

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Fig. 10B —66-year-old woman with diagnosis of ischemic bowel. CT scan obtained inferior to A shows air in mesentery (arrowhead), and lack of bowel wall enhancement (arrows) is again seen.

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Fig. 10C —66-year-old woman with diagnosis of ischemic bowel. CT scan obtained superior to A shows air in intrahepatic portal venous vasculature (arrow).

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Fig. 11 —57-year-old woman with small-bowel volvulus. Axial CT scan through upper pelvis shows whirl sign (arrow) signifying volvulus. Volvulus can result if loop of bowel is able to rotate around its mesentery. If loop sits in axial plane, it will appear as ⊂ or ∪ shape. If orientation of loop is at right angle to axial plane, appearance will vary depending on slice.

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Fig. 12A —64-year-old man with small-bowel obstruction secondary to incarcerated right inguinal hernia. CT scan shows incarcerated right inguinal hernia resulting in small-bowel obstruction. Left and right arrows point to dilated loop of small bowel with engorged mesentery (middle arrow).

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Fig. 12B —64-year-old man with small-bowel obstruction secondary to incarcerated right inguinal hernia. Inferior transverse CT image obtained at level of symphysis pubis reveals incarcerated thick wall loop of small bowel within right inguinal canal (arrow).

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Fig. 13 —80-year-old man with small-bowel obstruction secondary to adenocarcinoma of large bowel. Coronal reformatted 2-mm-thick CT view of abdomen reveals small-bowel obstruction is caused by thick annular constricting mass lesion involving hepatic flexure of large colon (thin arrows) resulting in proximal dilatation of cecum (thick arrow) and small bowel (arrowheads). Pathology revealed colonic adenocarcinoma.

CT has a sensitivity of 81-94% and a specificity of 96% for diagnosing high-grade obstructions [2]. However, when all grades of small-bowel obstructions are taken into account, the reliability of CT decreases dramatically (sensitivity of 64% and specificity of 79%) [2]. Therefore, CT is not the ideal technique for diagnosis of low-grade or subacute obstructions and should be complemented by a contrast study, ideally enteroclysis [6].

The most important information that CT can provide the surgeon is whether there is an associated strangulation. The sensitivity of contrast-enhanced CT for intestinal ischemia has been reported to be as high as 90% [1]. There are various signs that have been associated with ischemia [3, 7], although their usefulness is debatable. These include, first, thickened bowel wall (Fig. 9); second, ascites (Fig. 9); third, the target sign, a trilaminar appearance of the bowel wall resulting from IV contrast enhancement of the mucosal and muscularis layers, plus submucosal edema (Fig. 9); fourth, poor or absent enhancement of bowel wall on IV contrast-enhanced scans (Figs. 10A and 10B); fifth, pneumatosis intestinalis and gas in mesenteric or portal veins (Fig. 10C); sixth, the whirl sign, a twisting of the mesenteric vasculature signifying a volvulus (Fig. 11); seventh, tortuous engorged mesenteric vessels (Fig. 12A); eighth, mesenteric hemorrhage; and, finally, increased attenuation of bowel wall on noncontrast scans.

Although these signs are individually insufficiently sensitive, they are quite suggestive of ischemia when used together [7]. CT is also useful in differentiating small-bowel obstruction from ileus and determining the cause of obstruction (Figs. 11, 12A, and 12B).

Multiplanar reformations are now being used in difficult cases. Multiplanar views may help identify the site, level, and cause of obstruction when axial findings are indeterminate [7] (Figs. 13, 14A, and 14B). CT enteroclysis, a relatively new investigational tool for diagnosing small-bowel obstruction, can also be used with multiplanar reconstructions to overcome the unreliability of CT for diagnosing low-grade obstructions. CT enteroclysis has a greater sensitivity and specificity (89% and 100%, respectively) than CT alone (50% and 94%, respectively) [1]. At the same time, the 3D imaging provides precise localization of the pathology [1].

MRI	Choose Top of pageABSTRACTIntroductionConventional RadiographyContrast StudiesSonographyCTMRI <<ConclusionReferencesCITING ARTICLES

MRI provides rapid, accurate identification of small-bowel obstruction [8] and assists in the determination of cause without exposing the patient to radiation. MRI also utilizes intraluminal air as a natural contrast agent and is not limited by previous administration of barium. The diagnosis of small-bowel obstruction on MRI is similar to CT and involves identifying dilated loops of bowel proximal to the obstruction, a distinct transition point, and normal-caliber or collapsed bowel distally. Multiplanar capabilities of MRI allow visualization of the cause of small-bowel obstruction (Figs. 15A, 15B, and 15C). Rapid scanning with MRI using the HASTE sequence can, within seconds, evaluate small-bowel obstruction with a high degree of accuracy [8]. Furthermore, it avoids artifacts related to peristalsis and breathing that have limited the diagnostic yield of MRI in small-bowel obstruction in the past [8]. MRI is unlikely to replace CT for evaluating small-bowel obstruction because of longer scanning time and inferior resolution [8]; however, as availability and technology in MRI continues to improve, it has the potential to be an excellent diagnostic method for evaluating small-bowel obstruction.

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Fig. 14A —49-year-old man with incarcerated abdominal hernia. Axial CT scan shows defect in lower abdominal wall (arrow) that has incarcerated lower abdominal hernia within it (arrowhead).

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Fig. 14B —49-year-old man with incarcerated abdominal hernia. Sagittal reformatted MDCT view depicts defect in lower abdominal wall (long thick arrow) and incarcerated lower abdominal hernia. Within hernia sac, thick wall loop of small bowel and free fluid (arrowhead) are noted with dilated loops of small bowel proximal (short thick arrow) to incarcerated small-bowel loop. Free fluid (star and thin arrow) is also present in abdomen, which is an associated finding in small-bowel obstruction.

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Fig. 15A —80-year-old man with small-bowel obstruction secondary to adenocarcinoma of large bowel. Transverse gadolinium-enhanced T1-weighted image (TR/TE, 400/10) obtained with fat saturation shows narrowing of large bowel (arrow) caused by mass (small arrowhead) with resultant proximal small-bowel obstruction (large arrowhead).

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Fig. 15B —80-year-old man with small-bowel obstruction secondary to adenocarcinoma of large bowel. Coronal single-shot fast spin-echo T2-weighted image (1,800/103) reveals same constricting mass seen in A but with intermediate signal (thin arrows). Resultant proximal dilatation of large (thick arrow) and small (arrowheads) bowel is visualized.

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Fig. 15C —80-year-old man with small-bowel obstruction secondary to adenocarcinoma of large bowel. Subsequent coronal image reveals numerous proximal dilated loops of small bowel (arrows), which is consistent with diagnosis of small-bowel obstruction.

Conclusion	Choose Top of pageABSTRACTIntroductionConventional RadiographyContrast StudiesSonographyCTMRIConclusion <<ReferencesCITING ARTICLES

Small-bowel obstruction is a common presentation, for which safe and effective management depends on rapid and accurate diagnosis. Imaging can be diagnostic and helpful in guiding management. Abdominal radiography remains the first-line imaging examination because it is readily available, is cheap, and can be done serially to follow clinical progression. CT can provide additional information such as confirmation of an obstruction, degree and site of an obstruction, presence of ischemia, and the cause of the obstruction. MRI may have a role in the future evaluation of small-bowel obstruction as the technology improves and it increases in availability.