AJR 2002; 178:1139-1144
© American Roentgen Ray Society
Bowel Obstruction Revealed by Multidetector CT
Bharti Khurana1,
Stephen Ledbetter,
Jeffrey McTavish,
Walter Wiesner and
Pablo R. Ros
1 All authors: Department of Radiology, Brigham and Women's Hospital, 75 Francis
St., Boston, MA 02115.
Received October 4, 2001;
accepted after revision November 20, 2001.
Presented at the annual meeting of the American Roentgen Ray Society,
Seattle, April—May 2001.
Address correspondence to B. Khurana.
Introduction
Bowel obstruction is responsible for approximately 20% of surgical
admissions of patients with acute abdomen
[1]. The early diagnosis of
bowel obstruction is critical in preventing complications, particularly
perforation and ischemia. The accuracy of conventional radiography in
determining the presence of obstruction is 46-80%
[2]. CT has traditionally been
used to reveal the site, level, and cause of obstruction and to display the
signs of threatened bowel viability
[3]. This pictorial essay
presents CT imaging features of bowel obstruction with additional
consideration given to the role that multidetector CT may play in refining
it.
The value of CT in the evaluation of bowel obstruction is based on its
capability to provide information that aids in answering questions relevant to
the clinical treatment of patients with an acute abdomen
[3]: Can the obstruction be
confirmed? Where is the transition point? What is the grade of severity of the
obstruction? What is the cause of the obstruction? Can complications be
excluded? Oral contrast agents may not be necessary, because the fluid and gas
inherent in the dilated bowel provide sufficient contrast. IV administration
of contrast material is preferred for the evaluation of ischemia and other
abdominal viscera.
The first objective is to establish the presence of obstruction. The
criterion of small-bowel dilatation is defined as 2.5 cm calculated from outer
wall to outer wall [4].
Criteria are less well defined for the large bowel, although a caliber of
greater than 6 cm (9 cm in the cecum) should be considered dilated
[3]. Decompression of the
distal bowel is further confirmation of obstruction. In colonic obstruction,
the cecum is most distensible. If the ileocecal valve is incompetent, dilated
small-bowel loops may accompany a large-bowel obstruction. When CT findings
are equivocal for the presence of obstruction, it is often helpful to obtain a
delayed scan to look for the passage of contrast material.
The second objective is to identify the transition point. A systematic
approach begins at the rectum and proceeds proximally toward the cecum to
determine if the large or small bowel is involved. The transition point is
determined by identifying a caliber change between dilated proximal and
collapsed distal small-bowel loops. Identification of the transition point is
generally more difficult in jejunal obstruction; however, this information may
not be typically required by the surgeon. Multiplanar reformations may aid in
determining the site and level of obstruction (Fig.
1A,1B).
To be useful, multiplanar reformations should be generated from thin (1- to 3-
mm) source data, which are more easily obtained using a multidetector scanner
because it is capable of narrower collimation and faster scanning speed than a
single-detector scanner
[5].
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Fig. 1B. —53-year-old woman with abdominal distention and prior
appendectomy. Sagittal CT reformation clearly shows transition point
(arrow). Adhesions were confirmed as cause of bowel obstruction at
surgery.
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The third step is to grade the severity of obstruction. Complete versus
partial obstruction of the small bowel is determined by the degree of distal
collapse, proximal bowel dilatation, and transit of ingested contrast
material. Passage of contrast material through the transition zone into the
collapsed distal bowel indicates partial bowel obstruction.
Fourth, to determine the cause, it is important to distinguish whether the
obstruction involves the small or large intestine, because the causes,
symptoms, and treatment are often quite different. Adhesions are responsible
for more than half of all small-bowel obstructions, followed by hernias and
extrinsic compression due to neoplastic growths
[6]. The CT diagnosis of
adhesions is a diagnosis of exclusion (Fig.
2A,2B).
Multiplanar reformations allow the transition point to be viewed from a
variety of perspectives, increasing the diagnostic confidence once other
causes have been appropriately excluded. CT is excellent for detecting
external hernias and for characterizing the bowel and mesentery in the hernia
sac for the evaluation of strangulation and ischemia (Figs.
3A,3B,4A,4B,5A,5B).
Internal hernias originate from defects in the mesentery or peritoneum and may
be suspected when tightly grouped bowel loops are seen. In these instances,
the sac is not always readily apparent
(Fig. 6). Neoplasm is a
relatively unusual cause of small-bowel obstruction and is often associated
with extrinsic compression or local invasion by advanced intraabdominal
malignancies (Fig.
7A,7B),
rather than by primary small-bowel neoplasms. Common causes of large-bowel
obstruction are carcinoma (Figs.
8 and
9), sigmoid diverticulitis,
and volvulus. Intussusception in adults is a relatively rare condition causing
bowel obstruction. CT depicts the collapsed, intussuscepted proximal bowel
(intussusceptum) with the mesenteric fat and vessels telescoped into the lumen
of the distal bowel (intussuscipiens) (Fig.
10A,10B).
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Fig. 2B. —79-year-old woman after partial colectomy complicated by
ventral hernia. CT scan reveals site of obstruction as not within ventral
hernia but distal to hernia (arrow). Numerous adhesions were found at
surgery.
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Fig. 4B. —44-year-old woman who presented with abdominal pain and
distention after hysterectomy. CT scan obtained more inferiorly than A
reveals ventral hernia (arrow) as cause of obstruction. Fluid in sac
raised possibility of strangulation; surgery revealed patchy mucosal ischemia
but no transmural infarction.
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Fig. 5A. —64-year-old woman with ulcerative colitis and rectal cancer
examined after total proctocolectomy and ileostomy. CT scan shows loops of
dilated small bowel proximal to obstruction site.
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Fig. 5B. —64-year-old woman with ulcerative colitis and rectal cancer
examined after total proctocolectomy and ileostomy. CT scan shows parastomal
hernia (arrowheads) with transition point (arrow).
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Fig. 6. —59-year-old woman with no history of surgery who presented
with increasing abdominal pain. CT scan shows dilated small-bowel loops with
diffuse wall thickening and coning of mesentery (arrowheads) at site
of transition. Possibility of ischemia was considered on basis of CT
appearance; emergent surgery revealed internal hernia through which large
amount of ileum had prolapsed and volvulated. Ischemic necrosis was confirmed,
and approximately 1.2 m of small bowel was resected.
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Fig. 7A. —58-year-old woman with diabetes who presented with signs of
bowel obstruction. Surgery confirmed mucosal ischemia. CT scan shows annular
mass (arrowheads) in distal transverse colon, causing
obstruction.
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Fig. 7B. —58-year-old woman with diabetes who presented with signs of
bowel obstruction. Surgery confirmed mucosal ischemia. CT scan reveals
significant distention of cecum with wall thickening (arrow) and
trace of ascites.
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Fig. 8. —54-year-old man with diffuse abdominal distention.
Contrast-enhanced CT scan with rectal contrast shows cecal mass infiltrating
(arrow) and occluding ileocecal valve, resulting in bowel
obstruction. Notice fecaloid appearance of small-bowel contents (CT
small-bowel feces sign), likely caused by stasis and bacterial overgrowth.
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Fig. 9. —38-year-old man with history of testicular cancer.
Contrast-enhanced CT scan shows extensive retroperitoneal lymphadenopathy,
causing extrinsic compression (arrowheads), and small-bowel
obstruction.
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Fig. 10A. —55-year-old woman with intussusception. Contrast-enhanced CT
scan shows intussuscipiens (arrowheads) receiving intussuscepted
mesenteric fat (arrow) and dilated proximal bowel loop
(intussusceptum).
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The final but most crucial step is to differentiate a simple obstruction
from a complicated one, such as a closed loop or strangulated bowel
obstruction. In a closed loop or incarcerated small-bowel obstruction, a
U-shaped or radial configuration of fluid-filled dilated bowel loops is
typically seen, with mesenteric vessels converging toward the point of
obstruction. At the site of obstruction, there may be a whirl sign
(Fig. 11), a beak sign, or
triangular configuration of adjacent collapsed loops.
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Fig. 11. —30-year-old man who presented with recurrent emesis after
sigmoid colectomy for colon cancer. Contrast-enhanced CT scan shows dilated
small-bowel loops with whirl sign (arrow), caused by twisting of
mesenteric vessels suggestive of closed-loop obstruction. Surgery confirmed
volvulus with ischemia.
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Indications of strangulation include congestive changes or hemorrhage in
the mesentery associated with the affected loop, which usually manifests as
stranding within the mesenteric fat (Fig.
12A,12B).
Obstruction to venous outflow is the most common cause of ischemia in bowel
obstruction. This condition often results from an increase in intraluminal
pressure as a function of bowel distention. Simple obstruction with
progressive dilatation and rising intraluminal pressures can impair venous
drainage so that the bowel becomes edematous and leaks fluid into the lumen,
resulting in progressive dilatation, and into the peritoneal cavity, causing
ascites [7]. Intestinal
ischemia and infarction are the major causes of morbidity and mortality in
patients with bowel obstruction. In patients with strangulation, surgery
performed within 36 hr of the onset of symptoms has a reported mortality of
8%, whereas delay beyond 36 hr increases the mortality to a reported 25%
[7].
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Fig. 12A. — 57-year-old woman with history of small-bowel resection who
presented with signs of bowel obstruction. Axial CT scan shows several
thick-walled fluid-filled small-bowel loops oriented in radial configuration
(arrowheads) with site of prior surgery (metallic clip) at
center.
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Fig. 12B. — 57-year-old woman with history of small-bowel resection who
presented with signs of bowel obstruction. CT scan obtained more inferiorly
than A shows diffuse bowel-wall thickening (arrows) and
mesenteric congestion. Surgery confirmed small-bowel volvulus with ischemia
caused by adhesions.
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Multidetector CT may offer a distinct advantage over traditional
single-detector CT in the evaluation of mesenteric vasculature. Narrower
collimation coupled with shorter scanning times reduces motion artifact and
permits scanning during peak IV contrast enhancement. which improves the
quality of both axial and reformatted images. CT colonography (Fig.
13A,13B)
may be used for evaluating the entire colon before surgery in patients with
distal occlusive colorectal carcinomas
[8].
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Fig. 13A. —56-year-old woman with obstructing mass that precluded
conventional colonoscopy. Enlarged sagittal CT colonographic reformation shows
annular carcinoma (arrowheads) involving sigmoid colon.
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Fig. 13B. —56-year-old woman with obstructing mass that precluded
conventional colonoscopy. Perspective endoluminal CT colonographic image shows
significant luminal narrowing (arrow) caused by tumor.
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In conclusion, CT used with the comprehensive approach we describe not only
helps in making the correct diagnosis but also affects the outcome in patients
with bowel obstruction.
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Introduction
References
