Computed tomography (CT) has emerged as the most accurate noninvasive imaging tool for emergent evaluation of acute abdominal pain. The authors review the CT evaluation of bowel obstruction and other nontraumatic, emergent GI tract pathology. Characterization of the primary abnormality and the identification of ancillary findings permit accurate diagnosis and help guide and expedite appropriate management.
is a Postgraduate Year 5 Resident,
is a Research Assistant,
is the Vice Chair of Education, Chief of Body Imaging and
Professor of Clinical Radiology, and
is an Assistant Clinical Professor, Department of Radiology,
University of California, San Diego, CA.
Acute abdominal pain due to gastrointestinal (GI) tract
pathology is one of the most frequent causes of patient
presentation to the emergency room. Rapid diagnosis is essential to
expedite appropriate management. In recent years, computed
tomography (CT) has emerged as the most accurate noninvasive
imaging tool in this setting. Distinct advantages of CT are its
ability to directly visualize bowel wall and surrounding
structures. This review discusses the CT evaluation of bowel
obstruction and other nontraumatic, emergent GI tract pathologies.
Imaging strategies, CT findings, differential diagnosis, and
identification of potential complications are stressed.
Bowel obstruction is a common, potentially devastating cause of
acute abdominal pain, which occurs in approximately 20% of patients
with an acute abdomen.
The frequency is even higher in patients with prior abdominal
surgery. Important definitions are listed in Table 1. Imaging plays
an important role in diagnosis and management.
Imaging strategies and techniques
To our knowledge, a formal cost-benefit analysis delineating the
optimal imaging workup for patients with suspected bowel
obstruction has not been performed. The imaging algorithm proposed
here is based on the authors' anecdotal experience (Figure 1).
We believe that CT is the study of choice for suspected
small-bowel obstruction (SBO). Although it may seem reasonable to
begin with plain radiographs rather than with CT, CT has much
higher accuracy for: 1) the diagnosis of SBO, 2) the recognition of
its cause and potential complications, and 3) the identification of
other entities with similar clinical presentations.
Thus, if clinical suspicion warrants, CT should be performed
regardless of plain radiographic results. CT is particularly
valuable in patients without prior laparotomy, as these patients
are unlikely to have adhesions and may harbor an occult malignancy.
CT is also invaluable in patients with suspected strangulation,
known malignancy, or palpable abdomino-pelvic mass.
To help assess the bowel wall, intravenous (IV) contrast should
be administered, unless its use is contraindicated. Enteric
contrast is useful if symptoms are mild, as these patients may have
partial obstructions or other causes for their pain.
We believe that enteric contrast should be avoided in patients with
a more fulminant presentation and obvious intestinal distention.
In these patients, intraluminal fluid and gas provide effective
negative contrast. Moreover, enteric contrast may unnecessarily
delay CT or interfere with mural assessment.
In suspected large-bowel obstruction (LBO), a single preliminary
radiograph of the abdomen may be helpful.
If the radiograph shows marked colonic dilatation, a fluoroscopic
enema study, rather than a CT, should be considered, as the CT may
be difficult to interpret in this setting. Patients with modest or
no colonic dilatation should undergo CT. Rectal contrast may be
necessary if the colon is tortuous.
Patients with suspected low-grade or intermittent SBO in whom CT
is nondiagnostic may benefit from conventional or CT
Small-bowel obstruction is at least 3 times more common than
Luminal narrowing is usually severe; because small-bowel contents
are liquid, mild or moderate narrowing may be insufficient to
produce symptomatic obstruction. Small-bowel obstruction is usually
caused by extrinsic rather than intrinsic factors, and benign
causes are more common than malignant ones (Table 2, Figures 2
Prompt diagnosis and management are necessary, as untreated SBO
may be complicated by fluid shifts, electrolyte abnormalities, and
strangulation. The clinical examination is usually nonspecific.
Thus, imaging plays a crucial role. Imaging goals are summarized in
CT criteria for diagnosis of SBO are listed in Table 4.
The transition point itself is often not visualized, particularly
if the obstruction is caused by adhesions. Some authorities
recommend multiplanar reformatting to identify the transition
point, but the diagnostic value of reformation has not been proven.
An important ancillary finding is the "small-bowel feces" sign
(Figures 2 and 3): Fecal-like material in the segment of small
bowel just proximal to the obstruction. This sign suggests a
relatively long-standing obstruction and should be differentiated
from meconium equivalent in patients with cystic fibrosis (history
and evaluation of the pancreas may be useful). In cases of adynamic
ileus, both the transition point and small-bowel feces are absent,
and the colon is dilated.
Table 5 lists CT findings that differentiate low- and high-grade
obstructions. Findings favoring high-grade obstruction include
total collapse of the small bowel and colon distal to the
obstruction, marked dilatation proximal to the obstruction, and
triangular fluid collections within the adjacent mesentery (Figure
4). Identification of high-grade obstruction is important because
aggressive management is usually necessary.
CT findings that suggest closed-loop obstruction are summarized
in Table 6.
The appearance of the closed loop depends on its orientation and
length. If horizontally oriented, the closed segment typically
manifests as "C"-, "U"-, or comma-shaped loops. If vertically
oriented, it appears as radially distributed loops around a central
point (Figures 5 and 6). When long, a closed segment may appear as
a cluster of multiple fluid-filled loops. Dilated mesenteric
vessels (>3 mm diameter) characteristically converge to the
point of obstruction or twist. The entry and exit loops of bowel
may lie adjacent to one another and have a smoothly tapered,
triangular, or beaked appearance (beak sign) (Figure 5). The closed
loop is usually fluid-filled and gasless. The degree of dilatation
and quantity of intraluminal fluid may be conspicuously
disproportionate to that of more proximal segments; in the authors'
anecdotal experience, this disproportion can be a helpful clue to
the diagnosis. Closed-loop obstructions are usually treated as
surgical emergencies because of the high risk of strangulation.
Strangulation can occur with simple or closed-loop obstruction
but is more frequent in the latter.
CT findings suggesting strangulation can be classified as mural or
extramural (Table 7).
The mural target or halo sign (indicating submucosal edema) is best
seen with IV contrast (Figure 7), while mural hemorrhage (Figure 6)
is best seen without IV contrast. Focal or segmental loss of mural
enhancement, which some investigators believe is the most specific
sign of strangulation (Figure 5),
suggests impaired arterial inflow. Persistent mural enhancement
suggests impaired venous outflow. Pneumatosis and portomesenteric
gas suggest advanced bowel ischemia.
Pneumatosis may be difficult to identify if luminal contents are
heterogeneous. If pneumatosis is equivocal, identification of
portomesenteric gas can be extremely helpful.
Large-bowel obstruction is less common than SBO. Because
large-bowel contents are solid or semi-solid, mild to moderate
luminal narrowing may cause symptomatic obstruction. In contrast to
SBO, LBO is usually secondary to intrinsic rather than extrinsic
factors and malignant causes are more common than benign ones
(Table 2, Figures 8 through 11).
Partly because of the different causes and partly because they
produce fewer fluid and electrolyte abnormalities, LBOs tend to
present more insidiously than do SBOs. The major risk of LBO is
perforation. Perforation tends to occur at the site of greatest
dilatation, usually the cecum.
In cases of malignant obstruction, the perforation may occur at or
adjacent to the cancer site rather than in the cecum.
Whereas CT is the imaging study of choice for SBO, plain
radiographs and enema studies continue to play an important role in
the evaluation of LBO (Figure 8).
Radiographic findings depend on the site and cause of obstruction
and the competency of the ileocecal valve. If the ileocecal valve
is competent, the colon, and particularly the cecum, may markedly
dilate without significant small-bowel distention.
If incompetent, diffuse gaseous distention of both the small and
large bowel results, making differentiation from adynamic ileus
difficult. Distal LBOs may also mimic adynamic ileus. In this case,
films with the patient in the right-side-down decubitus or prone
positions may be helpful; gas entering the rectum and rectosigmoid
after repositioning excludes obstruction, while nondistention of
the rectum suggests obstruction. Cecal volvulus classically
manifests as a kidney-shaped mass in the left upper quadrant or
and sigmoid volvulus as an inverted "U" arising vertically or
obliquely from the pelvis or as three curved lines converging
toward the pelvis.
The CT technique for LBO is identical to that for SBO except
that rectal contrast or air insufflation may be necessary. Rectal
contrast should not be given, however, if colonic inflammation is
severe, or if peritoneal signs or perforation is present.
Other causes of acute, nontraumatic GI pain include
appendicitis, diverticulitis, ischemia, inflammatory bowel disease
(IBD), and infection. Each of these topics is complex, and thorough
reviews have been published.
Salient features are discussed below.
The optimal CT imaging technique for appendicitis is
controversial. We obtain 5-mm slices without oral or IV contrast on
a multidetector scanner from the liver dome (to visualize the lung
bases) to the symphysis pubis and then retrospectively reconstruct
overlapping 2.5-mm slices, if necessary. If the findings are
ambiguous or if the appendix is not visualized (about 25% of
cases), we give IV contrast and rescan the lower quadrants (Figure
12). In the setting of prolonged symptoms (eg, >72 hours),
pre-existing bowel disease, pregnancy, or young patient age, both
IV and oral contrast are administered initially. Other institutions
routinely give IV and/or enteric contrast in all cases.
The CT findings of appendicitis are summarized in Table 8
and include primary and secondary signs. Although most authors
recommend a 6mm outer-to-outer wall threshold measurement for
appendiceal dilatation, up to 42% of appendices 6- to 10-mm in
diameter are normal.
Thus, in the absence of other findings, a diameter of 6 to 10 mm
does not clinch the diagnosis. Periappendiceal fat stranding is
usually, but not invariably, present and may be absent in mild or
early appendicitis. In our anecdotal experience, thickening of the
right lateral conal fascia may be obvious even if fat stranding is
subtle. Thus, we feel that thickening of the lateral conal fascia
is a helpful finding. In equivocal cases, we recommend close
clinical observation and repeat imaging.
Complications of appendicitis occur more frequently with
prolonged symptoms and include perforation, peritonitis, abscess,
and sinus tract/fistula formation.
Diverticulitis occurs when a diverticulum becomes obstructed and
focally inflamed and then perforates. The perforation is typically
contained and causes a local inflammatory process that is
predominantly extracolonic or "peridiverticular." The greatest
concentration of diverticula is in the distal descending andsigmoid
colon; hence, most cases (95%) of diverticulitis are left-sided
; a minority of cases (5%) are right-sided.
For unknown reasons, right-sided diverticulitis has a predilection
for patients of Asian descent.
Diverticulitis of the small bowel is uncommon.
CT criteria for the diagnosis of diverticulitis are listed in
The most common and usually striking finding is pericolonic fat
stranding. Fat stranding is characteristically disproportionate to
the degree of wall thickening,
which tends to be mild (4 to 5 mm). Disproportionate fat stranding
is an important finding, as it may help suggest the diagnosis in
atypical presentations. At least one diverticulum is visualized in
>80% of cases of diverticulitis
; definitive CT diagnosis of diverticulitis may be difficult if
diverticula are not identified. The comma sign refers to
comma-shaped thickening of the root of the sigmoid mesocolon. The
centipede sign refers to a multitude of engorged, parallel
mesenteric vessels supplying the affected segment of bowel. Based
on anecdotal experience, we also rely on an unpublished "fuzzy
diverticulum" sign (Figure 13A). This sign refers to an ill-defined
diverticulum, larger than other visible diverticula, centered where
the fat stranding is most pronounced. This "fuzzy diverticulum"
presumably represents the causative diverticulum.
The most important differential consideration for left-sided
diverticulitis is adenocarcinoma, as it affects a similar
demographic group (in whom incidental diverticula may be present)
and causes relatively focal colonic involvement. Differentiation is
aided by ancillary findings (Table 10).
In particular, colonic thickening disproportionate to pericolonic
fat stranding and the presence of regional nodes suggest
malignancy. However, clinical and CTfindings may overlap (Figure
If findings are equivocal and tissue sampling is not planned, a
follow-up examination should be performed after resolution of acute
manifestations (usually approximately 4 weeks). Other differential
considerations include appendicitis, omental infarction, and
Complications of diverticulitis include intra- and extramural
abscesses, sinus tract and fistula formation (especially
colovesical fistula), bowel and ureteral obstruction, gross
intraperitoneal or retroperitoneal perforation, peritonitis, septic
thrombosis of portomesenteric veins, and liver abscess. Gross
perforation with free gas in the peritoneal or retroperitoneal
spaces usually requires immediate laparotomy. Contained
perforations with extramural abscesses >3 cm are usually treated
by drainage followed by surgery; abscesses <3 cm typically
resolve with conservative management.
Bowel ischemia can be divided into occlusive and nonocclusive
causes. Occlusive disease refers to obstruction of large arteries
(embolus, thrombus, trauma, compression, or infiltration) or veins
(thrombus). Nonocclusive ischemia is due to low-flow states,
small-vessel disease, splanchnic vasoconstriction, or a combination
of these factors. Nonocclusive ischemia tends to resolve
spontaneously and has a better prognosis than occlusive
Small-bowel ischemia is usually due to occlusion of the superior
mesenteric artery or vein (Figure 14)
or their branches and tributaries. The distribution and length of
involvement depend on the site of vascular obstruction. The
jejunum, ileum, and right colon may be affected and the duodenum
(supplied by the celiac axis) spared. Morbidity and mortality are
high, especially for arterial occlusions.
Nonocclusive ischemia of the small bowel is less common.
Possible causes include hypovolemia (Figure 15, "shock bowel"),
drugs, and vasculitis. These usually result in diffuse small-bowel
involvement without sparing the duodenum. Long segments of colon
may also be affected. Radiation enteritis, another cause of
nonocclusive ischemia, is not as common as radiation colitis, due
in part to the greater mobility of small-bowel loops.
As opposed to small-bowel ischemia, colonic ischemia is usually
due to nonocclusive factors.
Watershed areas (splenic flexure to descending colon and
rectosigmoid) are most frequently involved. However, any portion of
the colon can be affected, especially with embolic sources.
Radiation colitis affects bowel within the radiation field, most
commonly the rectosigmoid.
CT findings of bowel ischemia depend upon the presence of
reperfusion. In ischemia without reperfusion (active ischemia), the
bowel wall is hypo- or nonenhancing (Figure 5) and may be thin, and
the mesenteric vascular structures are diminutive. In ischemia with
reperfusion (Figure 14), the wall is thick, the mucosa and
muscularis/serosa are hyperemic (striated, halo, or target pattern)
and the mesenteric vessels are engorged. Engorged, hyperenhancing
vasa rectae may penetrate the involved bowel wall and stand out
against the edematous submucosa ("starry sky," Figure 14).
Ancillary findings suggestive of bowel ischemia include pneumatosis
and portomesenteric gas.
Inflammatory bowel disease (IBD)
Crohn's disease (CD) or ulcerative colitis (UC) may cause acute
abdominal pain. Although a history of IBD is usually known, new
presentations occur. In these cases, a combination of direct and
ancillary findings on CT often permits correct diagnosis.
The primary CT feature of acute IBD is mild-to-moderate bowel
wall thickening (mean of 10 to 11 mm for CD and 7 to 8 mm for UC).
The involved wall may hyperenhance. Typically, UC manifests a
stratified pattern (Figure 16),
whereas CD may manifest a stratified or homogeneous pattern (Figure
17). Mesenteric vessels may be engorged with either CD or UC.
Vascular engorgement in the setting of active CD has been termed
the comb sign and is analogous to the centipede sign of
The distribution of wall thickening is important. Crohn's
disease affects the small bowel alone in 20% of cases, the small
bowel and colon in 60%, and the colon alone in 20% of cases (right
more commonly than left colon)
; terminal ileal involvement and skip lesions are characteristic.
In contrast, UC causes continuous involvement proximally from the
rectum (Figure 17)
; the terminal ileum may be abnormal in a minority of cases of
long-standing diffuse UC ("backwash ileitis").
Fibrofatty proliferation around the terminal ileum ("creeping fat")
is diagnostic of CD (Figure 16) and helps differentiate CD from UC
and any disease of the terminal ileum. Fibrofatty proliferation
around the rectum also occurs but is nonspecific and can develop
with any chronic proctitis (UC, CD, or other). Sacroiliitis is more
characteristic of UC. Other extraluminal complications (fistula,
sinus tract, abscess, septic thrombosis of portomesenteric veins)
are distinctly more common in CD.
Gastrointestinal infection can cause severe abdominal symptoms.
Acute infections of the small bowel are usually subtle on CT and
may have no CT imaging abnormalities. Colonic infections are
usually obvious because mucosal ulceration and submucosal edema
cause detectable colonic thickening. If thickening is severe, the
colon may assume an accordion-like configuration (Figure 18).
Transmural inflammation leads to adjacent fat stranding and/or
ascites. Although obvious, CT findings are usually nonspecific and
overlap with noninfectious causes of colitis. However, some
features aid in differential diagnosis. For example, Yersinia,
myco-bacteria, and amebiasis have a predeliction for the ileocecal
region. Cytomegalo- virus (CMV) and Clostridium difficile (Figure
18) often cause diffuse colonic edema, and the degree of mural
thickening may be severe.
Although immunocompromised patients are susceptible to the
normal spectrum of GI infections, some infections are particularly
characteristic in this population. These opportunistic pathogens
include mycobacteria (tuberculosis and Mycobacterium avium
intracellulare), viruses (CMV), parasites (Cryptosporidium) and, in
the setting of neutropenia, typhlitis (neutropenic colitis, Figure
CT plays an increasingly important role in the evaluation of the
acute abdomen. Characterization of the primary abnormality and the
identification of ancillary findings permit accurate diagnosis and
help guide and expedite appropriate management.