is Resident in Radiology, and
is an Associate Professor of Radiology, Department of Radiology,
Stanford University School of Medicine, Stanford, CA.
Historically, the length and convoluted course of the small
bowel have presented formidable diagnostic challenges to
radiologists. The volumetric and multiplanar images that can now be
obtained routinely with multidetector computed tomography (MDCT)
scanners have greatly expanded the ability of CT to systematically
evaluate the small bowel luminal contents, with the added bonus of
being able to evaluate the bowel wall, mesentery, and associated
vasculature. This article will summarize the use of MDCT in a
variety of pathologic entities that involve the small bowel.
Small bowel MDCT imaging protocol
At our institution, all examinations of the abdomen are done on
8-, 16-, or 64-detector scanners. Sample MDCT scan parameters for
small bowel studies on our 16-slice scanner (LightSpeed 16 scanner,
GE Healthcare, Waukesha, WI) are shown in Table 1. The basic
principle of MDCT protocol design is to scan with thin collimation,
reconstruct thick slices for viewing, and reconstruct thin slices
with overlap for multiplanar reformations.
Luminal contrast media
Positive oral contrast media that contain either dilute iodine
or barium are useful for differentiating bowel segments from
extraluminal collections, such as abscesses and fistulous tracts.
But high-density media can mix unevenly with fluid bowel contents,
and can also obscure features of the enhancing bowel wall. In
3-dimensional imaging studies of the vasculature, high-density
segments must be painstakingly edited out. Thus, in our practice,
we prefer to use water to distend the small bowel, and we do not
routinely administer positive oral contrast media unless an abscess
or fistula is suspected. We administer 750 mL of water 20 minutes
prior to scanning, with an additional 250 mL given to the patient
just before the scan to distend the stomach. Limitations of
intraluminal water include rapid transit and quick absorption,
which can result in poor distention at the time of imaging. VoLumen
(E-Z-EM Inc., Lake Success, NY), an ultra-low-density (0.1%, 15 to
30 HU) barium suspension that contains sorbitol, has been reported
to improve distention of distal bowel segments.
We routinely use intravenous contrast for evaluating enhancement
patterns of the bowel wall and mesenteric vasculature, except when
an intramural hemorrhage is suspected. An injection rate of 2 to 3
mL/sec is adequate for most indications. For angiographic studies,
an injection rate of 4 to 5 mL/sec is required.
The duodenum is retroperitoneal, with the exception of the first
2 centimeters of the first portion (within the hepatoduodenal
ligament) and the last few millimeters of the fourth part (at the
ligament of Treitz). This transition of the fourth portion from a
retroperitoneal to an intraperitoneal position creates the
paraduodenal recesses of Landzert (on the left) and Waldeyer (on
the right), which are potential sites of internal hernias.
The jejunum and ileum are entirely intraperitoneal.
Normal wall thickness of the fully distended small bowel should
not exceed 2 mm. If the bowel is collapsed or if a slice is
obtained within the plane of the valvulae conniventes, this can
give a false appearance of focal concentric bowel wall thickening.
However, true pathologic bowel wall thickening should be contiguous
along a segment (Figure 1).
The gastroduodenal branches of the celiac axis supply the first
and second portions of the duodenum, and the inferior
pancreatoduodenal branches of the superior mesenteric artery (SMA)
supply the third and fourth portions. The jejunum and ileum are
supplied by, respectively, the jejunal and ileal branches of the
SMA. Multidetector CT has largely supplanted catheter angiography
as the principal method to evaluate the mesenteric vasculature. The
mesenteric arcades and vasa recta are exquisitely seen in the
coronal plane when thin collimation and volume rendering is
utilized (Figure 2).
Small bowel diverticula are outpouchings that typically extend
from the mesenteric side of the bowel wall. Duodenal diverticula
are the most common, with most of these acquired and asymptomatic.
Intraluminal "windsock" diverticula, by contrast, are congenital
and may be symptomatic. Multidetector CT multiplanar rendering can
clearly show the orientation of these lesions and can assist in
Meckel's diverticula are the most common type of congenital
abnormality of the small bowel, with a prevalence of approximately
2%. They arise from the antimesenteric side of the bowel wall
typically within 100 cm of the ileocecal valve and result from
incomplete closure of the intestinal end of the embryonic
omphalomesenteric duct. On CT, they are most commonly found at or
near the midline and range from 1 to 10 cm in length. They may
present clinically with diverticulitis, intussusception, or
Small bowel obstruction
Small bowel obstruction (SBO) accounts for 20% of surgical
admissions for acute abdominal pain. The causes of SBO can be
divided into 3 broad categories, including extrinsic, intrinsic,
and intraluminal etiologies (Table 2).
In Western societies, the most common causes of SBO are adhesions,
Crohn's disease, and neoplasia, while hernias are the most common
cause of SBO in developing countries.
Prompt diagnosis is crucial because missed or delayed
recognition can result in life-threatening complications, such as
ischemia or perforation. Plain films are often equivocal, and
further evaluation with CT is usually indicated to elucidate the
site and cause of obstruction as well as to detect potential
complications of closed-loop obstruction or strangulation.
Exogenous oral contrast is usually not necessary because the fluid
retained in obstructed bowel loops provides excellent distention.
In fact, positive oral contrast decreases the conspicuity of bowel
wall enhancement, making ischemia more difficult to detect.
In mechanical small bowel obstruction, CT shows dilated bowel
segments (>25 mm diameter) proximal to collapsed loops distally.
The specific point of transition may be seen as a small triangular
"beak" immediately beyond the dilated segment. A "small bowel
feces" sign may also be seen immediately proximal to the transition
zone and can be helpful in identifying the site of obstruction.
This sign is defined as gas bubbles and particulate matter within
dilated small bowel segments that mimic colonic fecal matter and
that result from stasis and water absorption.
Although it is fairly specific for SBO, it has also been reported
in cystic fibrosis and other entities.
Identifying the complication of closed-loop obstruction is
critical, as these patients are at high risk for developing
ischemia (strangulation) as a consequence of impaired arterial or
venous flow. CT findings of closed-loop obstruction depend on the
orientation and length of the incarcerated segment and include a
dilated, fluid-filled segment with a "C"- or "U"-shaped
configuration, and a radial or "spoke-wheel" arrangement of
mesenteric vessels (Figure 4).
With superimposed volvulus, CT may show engorged mesenteric vessels
converging toward the abnormal segment, often with delayed
enhancement. Often coronal images may exhibit these features when
they are not apparent in the axial plane. A "whirl" sign (Figure
5), which represents rotation of the mesentery in small bowel
volvulus, may be seen in the axial plane but is best appreciated in
the coronal or sagittal planes, depending on the axis of rotation.
Superimposed ischemia in SBO is usually due to impaired venous
outflow in a closed-loop obstruction.
Acute small bowel ischemia can result from occlusive (arterial
or venous) causes as well as nonocclusive causes, such as
hypoperfusion. Most cases of acute mesenteric ischemia are due to
thromboembolic disease obstructing the proximal portion of the SMA.
Embolic disease related to atrial fibrillation is the most common
etiology for SMA occlusion. Venous occlusion may result from a
hypercoagulable state and/or in the postsurgical setting but can
also occur as a result of infiltrative, neoplastic, or
inflammatory/infectious conditions. As described above, closed-loop
obstruction may result in bowel ischemia if the veins are twisted,
stretched, or compressed.
The use of routine MDCT protocols of the abdomen can often
reveal small bowel ischemia in patients with nonspecific symptoms
or suspected SBO. However, if bowel ischemia is the specific
question, mesenteric CT angiography (CTA) is preferred at our
institution to determine the presence, site, distribution, and
cause. Three-dimensional volume-rendered processing of vascular
structures is a helpful adjunct to primary interpretation of
2-dimensional images and allows immediate recognition of the
vascular abnormalities, even in the presence of dense
calcifications (Figure 6). The postprocessed volumetric images are
often preferred by our clinicians, since one image presents a
synthesis of the findings of hundreds of source images.
CT findings in acute mesenteric ischemia are variable, depending
on the etiology and degree of damage to the bowel wall.
The most common CT finding in acute bowel ischemia is bowel wall
thickening, which is present in 26% to 96% of reported cases,
due to mural edema, hemorrhage, and/or superinfection of the
ischemic bowel wall. However, this is also the least specific
finding, and the degree of bowel wall thickening does not correlate
with the severity of ischemic bowel wall damage.
Acute arterial occlusions may actually produce thinning of bowel
wall segments because of the loss of motor tone in the ischemic
wall musculature. The key CT findings in mesenteric ischemia are
summarized in Table 3 and include mesenteric fat stranding,
mesenteric fluid, ascites, submucosal edema with low attenuation in
the bowel wall, and increased attenuation due to hyperemia,
intramural hematoma, or hemorrhagic infarction. Segmental absence
of bowel enhancement is an especially helpful sign of focal
ischemia (Figure 7). Hyperemia and hyperperfusion of an ischemic
segment that primarily affect the mucosa and submucosa may produce
a "target" sign. Pneumatosis and porto-mesenteric venous gas are
less common and are usually late findings in acute ischemia (Figure
8), but they also have a variety of benign etiologies.
In mesenteric ischemia caused by a low-flow state
(nonobstructive ischemia), CT may show diffusely narrowed arteries
as well as distal branches that are fewer in number and of
decreased caliber. "Shock bowel" is a reversible, nonocclusive
ischemia that is thought to be a reperfusion injury related to
resuscitation following severe hypotension. CT findings include
diffuse small bowel wall thickening and hyperattenuation, with
persistent increased enhancement and slow washout (Figure 9).
Chronic mesenteric ischemia results in a characteristic clinical
syndrome of chronic abdominal pain, especially after meals, and is
frequently associated with weight loss. It most frequently affects
elderly patients and is secondary to long-standing atherosclerotic
disease. Mesenteric CTA has become a common screening technique for
Findings may include arterial plaque formation, luminal narrowing,
and collateral vessel formation.
Crohn's disease is a chronic granulomatous disease that can
affect any portion of the gastrointestinal tract in a noncontiguous
fashion (socalled skip lesions) and most frequently involves the
terminal ileum. The recent introduction of wireless capsule
endoscopy (WCE) into clinical practice has led to the detection of
the early small bowel mucosal disease, such as aphthoid ulcers,
even in the setting of a negative enteroclysis.
It is likely that the even the less-sensitive small bowel
follow-through examination will diminish in importance in this new
era of WCE. By contrast, CT can depict changes of Crohn's disease
beyond the mucosa and beyond the scope of WCE.
CT is useful in determining the presence, extent, and severity
of Crohn's disease, to assess disease activity, and to detect
Findings vary depending on whether the disease is in its acute or
chronic phase. The most common finding is wall thickening (up to 2
cm), which is often associated with luminal narrowing. During the
acute stage, the mural stratification of the bowel wall is
maintained, often with a target (or "double halo") sign produced by
concentric rings that represent enhancing mucosa, lower attenuation
submucosa, and higher attenuation muscularis propria (Figure 10).
The outer contours of the bowel wall are often irregular during the
acute stage, and disease activity has been correlated with the
intensity of contrast enhancement in the inflamed mucosa and
With disease progression, mural stratification is lost and there is
homogenous wall enhancement in association with fibrosis. Both
inflammatory and fibrotic changes tend to occur on the
antimesenteric side of the bowel.
Inflammatory hyperemia of the mesenteric vasculature creates the
"comb" sign, which also suggests active disease (Figure 11).
The engorged vasa recta creates a series of parallel opacities that
mimic the teeth of a comb. Coronal images often beautifully display
the mesenteric hypervascularity. In Crohn's disease, local
mesenteric lymph nodes are increased in number but small in size
(usually <1 cm). Both of these findings can be used to help
distinguish Crohn's disease from neoplastic processes.
The separation of bowel loops caused by fibrofatty proliferation of
the central mesentery ("creeping fat") and perirectal fat
deposition are other common signs. Because of edema and
infiltration with inflammatory cells, attenuation measurements of
the involved fat are often increased.
Complications of Crohn's disease (such as fistulae, abscesses,
and extramural extension to adjacent structures) often dictate
patient management and are well depicted with MDCT.
Three-dimensional reconstructions of fistulous tracts can be
particularly helpful in surgical planning.
Small bowel neoplasms are rare, representing <25% of all
gastrointestinal neoplasms. Adenocarcinoma, carcinoid tumor,
lymphoma, and gastrointestinal stromal tumor (GIST) are the most
common tumors of the small bowel and may be distinguished by
location and imaging characteristics. Most small bowel tumors tend
to be small and present with nonspecific symptoms, such as
abdominal pain or bleeding. For detecting and evaluating small
bowel endoluminal masses, the distension with neutral or negative
contrast agents is key. Thin collimation and coronal or sagittal
reformations can help resolve uncertain findings and bring
additional findings to light.
Adenocarcinoma, the most common primary malignancy of the small
bowel, usually appears as asymmetric focal wall thickening or as an
enhancing intraluminal mass. The duodenum is the most common site,
followed by the jejunum. Associated luminal narrowing can lead to
obstruction. Local infiltration of the mesenteric fat can be seen.
Less commonly, an "apple-core" type lesion can occur. Tumors of the
duodenum can invade the adjacent pancreas, obstructing the
pancreatic duct, creating an appearance that can be
indistinguishable from pancreatic cancer.
Carcinoid tumors are of neuroendocrine origin and are the second
most common small bowel malignancy. Unlike adenocarcinoma, these
more commonly affect the distal small bowel. The primary tumors are
often very small and are slow-growing, appearing as small
hypervascular areas of nodular bowel wall thickening or submucosal
masses. In approximately 30% of cases, multiple sites are
A characteristic desmoplastic response can occur, with soft tissue
infiltration and retraction of the adjacent mesentery.
Calcifications are frequently present. Bulky lymphadenopathy can
occur, with central low attenuation signifying necrosis. Pulmonary,
hepatic, and osteoblastic skeletal metastases are common.
Small bowel lymphoma can occur either as a primary disease or
secondary to nodal spread of extraintestinal lymphoma. Four major
patterns of small bowel lymphoma have been described: 1) a focal
intramural or intraluminal mass; 2) multiple nodules; 3) diffuse
involvement of a small bowel segment with wall thickening and
aneurysmal dilatation of the lumen; and, 4) rarely, an exophytic
mass with ulceration.
Lymphoma is a soft tumor, rarely leading to obstruction even with
marked luminal narrowing. Focal masses may act as lead points in
intussusception. Bulky lymphomatous masses in the mesentery may
encase mesenteric vessels, producing the characteristic "sandwich"
Gastrointestinal stromal tumors, formerly known as leiomyomas or
leiomyosarcomas, are mesenchymal tumors that arise from the smooth
muscle cells within the small bowel wall. They may be submucosal,
subserosal, or intraluminal, occurring as solitary or multiple
lesions. While GISTs can be benign or malignant, imaging features
do not easily distinguish their behavior unless obvious metastatic
disease is present. Submucosal GISTs often appear as smoothly
marginated, round filling defects, but the more common subserosal
GISTs are typically large, bulky, exophytic masses with ulceration
that displace adjacent structures (Figure 12).
Metastatic disease may spread to the small bowel via
intraperitoneal seeding, hematogenous dissemination, lymphatic
channels, or direct extension. The most common malignancies that
seed the small bowel intraperitoneally are gastrointestinal (GI)
primaries, and, in women, ovarian or uterine carcinoma.
Hematogenous spread to the small bowel usually manifests as
multiple small nodules along the mesenteric border at the insertion
of the vasa recta. Melanoma, lung, and breast are the most common
primary cancers (Figure 13).
Trauma and perforation
Contusion, intramural hematoma, or small bowel perforation may
result from blunt abdominal trauma. As MDCT use becomes routine for
assessing trauma patients, these injuries are being recognized with
increasing frequency. Findings on CT are typically subtle and
include focal bowel wall thickening and adjacent mesenteric
stranding. The attenuation of any interloop fluid should be
measured. Water-attenuation fluid suggests bowel perforation,
whereas high attenuation (>30 HU) suggests hemorrhage.
Penetrating injuries to the small bowel typically produce
extraluminal air. Hypotension and subsequent resuscitation may give
rise to a "shock bowel" pattern of dramatic mucosal
hyperenhancement, as described above.
Foreign body ingestion is an uncommon but fascinating cause of
small bowel perforation. Most foreign bodies are thought to pass
unimpeded through the intestine, but sharp bodies (such as
toothpicks, or small fish or chicken bones) may lead to
perforation. Clinical manifestations vary from pain, nausea, and
vomiting to fever and peritonitis with local abscess formation. CT
diagnosis may be extremely difficult because the object ingested is
typically unknown. Bowel wall thickening, adjacent fat stranding,
and localized extraluminal gas are common-but nonspecific-findings.
In a recent series described by Coulier et al,
direct visualization of the calcified foreign body was the most
definitive sign and was greatly facilitated by thin, overlapping
sections and multiplanar reformations, which allowed the shape of
the object to be recognized.
Multidetector CT facilitates the evaluation of a variety of
small bowel processes. Water is preferred for luminal distention in
conjunction with intravenous contrast administration. For the
highest-quality multiplanar reformations, imaging should be
performed with narrow detector collimation and overlapping
reconstruction. Imaging in both the arterial and venous phases of
enhancement increases total radiation dose but may be helpful for
the assessment of vascular pathology. Coronal images may be
particularly helpful in assessing small bowel obstruction and
inflammatory bowel disease.