DOI:10.2214/AJR.04.0815
AJR 2005; 185:1036-1044
© American Roentgen Ray Society
Imaging of Acute Small-Bowel Obstruction
Savvas Nicolaou1,
Brian Kai2,
Stephen Ho3,
Jenny Su4 and
Karim Ahamed5
1 Department of Radiology, Vancouver General Hospital, 899 W 12th Ave.,
Vancouver, BC, V5Z 1M9, Canada.
2 University of British Columbia, Vancouver, BC, Canada.
3 Department of Radiology, Gastrointestinal Radiology, Vancouver Hospital &
Health Sciences Centre, Vancouver, BC, Canada.
4 Department of Internal Medicine, University of British Columbia, Vancouver,
BC, Canada.
5 Department of Diagnostic Radiology, University of Alberta, Edmonton, AB,
Canada.
Received May 24, 2004;
accepted after revision November 19, 2004.
Address correspondence to S. Nicolaou
(snicolao{at}vanhosp.bc.ca).
Abstract
OBJECTIVE. The objective of this pictorial essay is to review the
different imaging techniques used for diagnosing small-bowel obstruction.
CONCLUSION. Small-bowel obstruction is a common presentation, for
which safe and effective management depends on a rapid and accurate diagnosis.
Conventional radiographs remain the first line of imaging. CT is used
increasingly more because it provides essential diagnostic information not
apparent from radiographs. MRI may play a role in the future as technology
improves and it becomes more readily available.
Introduction
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
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.
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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. 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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Contrast Studies
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).
Sonography
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.
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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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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
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. 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. 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. 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.
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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].
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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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MRI
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. 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.
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Conclusion
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.
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Abstract
Introduction
or 
shape. If
orientation of loop is at right angle to axial plane, appearance will vary
depending on slice.