CT Enterography: Principles, Trends, and Interpretation of Findings

Report
CT Enterography:
Principles, Trends, and
Interpretation of
Findings
A Norouzi MD
Fall 2013
Introduction
 With the improved resolution of multidetector computed
tomography (CT), CT enterography has become an
important method of choice for evaluating small bowel
disorders.
 Although capsule endoscopy provides better mucosal
visualization, it does not allow visualization of
abnormalities outside the bowel lumen.
 Moreover, capsule endoscopy cannot be performed when
the presence of a stricture is suspected; the endoscopic
capsule may become lodged at the diseased segment
and cause obstruction.
 CT enterography is noninvasive, easy to perform, and
allows visualization of extraenteric structures as well as
of the bowel wall.
 Its usefulness for detecting small bowel tumors is
established; in addition, CT enterography with modified
protocols has proved useful for detecting occult
gastrointestinal (GI) tract bleeding in hemodynamically
stable patients.
 In the American College of Radiology appropriateness
criteria, CT enterography is rated as the most
appropriate imaging modality for evaluating known
Crohn disease in patients with an acute exacerbation or
suspected complications.
 The main disadvantages of the technique are the
requisite exposure of patients to ionizing radiation, of
particular concern in young patients; the frequent need
for repeated imaging; and the necessity of using
intravenous contrast material
 Although capsule endoscopy is reported to have high
sensitivity for evaluation of the small bowel mucosa, CT
enterography provides better visualization of the entire
small bowel wall and allows detection of extraenteric
disease as well.
 Additional disadvantages of capsule endoscopy for
evaluating early-stage disease include a predictable
higher rate of false-positive findings due to mucosal
breaks and small bowel erosions, which may occur even
in healthy patients, as well as the risk of possible
impaction of the capsule proximal to a diseased bowel
segment.
 MR enterography recently has been gaining in popularity,
especially for follow-up imaging of pediatric and young
patients with established inflammatory conditions of the
small bowel .
 The lack of ionizing radiation is an advantage of MR
enterography over CT enterography, especially when
evaluating patients with known renal dysfunction.
However, MR enterography is more time consuming,
more costly, and more variable in regard to image
quality than CT enterography.
 In particular, patients who have difficulty holding their
breath may be better examined with CT.
 The results of a prospective study by Schmidt et al
show that, in addition to the superior spatial resolution
that CT offers, interobserver agreement is better and
sensitivity for the detection of active disease is higher
with CT.
 The radiation dose resulting from a CT enterographic
examination should be similar to that from a routine
abdominopelvic venous phase scan.
 The radiation dose can be reduced by using the dose
modulation option available on most new scanners.
 In addition, the use of advanced reconstruction
techniques, such as iterative reconstruction, enables
further dose reduction.
 In our experience, simply reducing the voltage to 80 or
100 kV allows a significant dose reduction in patients
with an average or below average body mass index.
 In our practice, we regularly perform CT enterography
with a mean estimated dose of 4–8 mSv.
CT Enterographic Technique
 Patients undergoing CT enterography are asked to withhold
all oral intake, starting 4 hours before the examination.
 To improve visualization of the mucosa and achieve better
bowel distention, a negative oral contrast agent is
administered.
 Our current regimen with regard to the timing of
administration of oral contrast agents involves the ingestion
of a total of 1.35 L over 1 hour: 450 mL at 60 minutes, 450
mL at 40 minutes, 225 mL at 20 minutes, and 225 mL at 10
minutes before scanning.
 After the oral contrast agent is ingested, a bolus of
intravenous contrast material (125 ml ) followed by 50 mL
of saline solution is administered with a power injector at a
rate of 4 mL/sec.
 Helical scanning is performed from the diaphragm to the
symphysis pubis, beginning 65 seconds after the
administration of intravenous contrast material, and
includes a single (venous) phase for the evaluation of
known or suspected Crohn disease or dual (arterial and
venous) phases for the evaluation of mesenteric vessels,
GI tract bleeding, and suspected tumors.
 Scanning parameters include a section thickness of
0.625 mm and interval of 0.625 mm.
 At present, CT enterography is most commonly used for
diagnosing and assessing
 inflammatory bowel disease,
 localizing sites of GI tract bleeding,
 detecting small bowel neoplasms.
Crohn Disease
 Crohn disease predominantly involves the small bowel,
particularly the terminal ileum, but is also commonly
associated with extraintestinal manifestations.
 Small bowel involvement in Crohn disease is typically
transmural, with characteristic skip lesions .
 The main diagnostic purpose of CT enterography in the
setting of Crohn disease is to differentiate active
inflammatory strictures from fibrotic strictures in order
to guide therapy.
 Active bowel disease is more likely to be treated
medically, whereas surgery and strictureplasty may be
considered in the setting of fibrotic strictures.
Skip lesions in Crohn disease.
Axial CT enterographic section
depicts two inflammatory small
bowel strictures (arrows)
separated by a segment of
normal distended small bowel
(arrowheads), characteristic
findings of Crohn disease.
 CT features of active Crohn disease include
 mucosal hyperenhancement,
 wall thickening (thickness > 3 mm),
 mural stratification with a prominent vasa recta (comb
sign),
 mesenteric fat stranding,
 all of which are exquisitely demonstrated at CT
enterography
 The capability of CT enterography for depicting
extraenteric disease allows the simultaneous diagnosis
of complications associated with Crohn disease, such as
obstruction, sinus tract, fistula, and abscess formation.
Axial CT enterographic section
shows mucosal
hyperenhancement (black
arrow) and mural stratification
(white arrow) of the terminal
ileum, an appearance that
contrasts markedly with that of
nondiseased ileal segments
Axial CT enterographic section
from another patient shows
mesenteric hypervascularity
(arrowheads) adjacent to the
involved bowel segment.
Coronal volume-rendered CT enterographic sections from two patients (a
and b) demonstrate prominence of the vasa recta, or “comb sign” (arrows).
 The term mural stratification denotes the visualization of
bowel wall layers at CT.
 At CT enterography, the edematous bowel wall has a
trilaminar appearance, with enhanced outer serosal and
inner mucosal layers and an interposed submucosal
layer of lower attenuation .
 However, this feature is not specific to Crohn disease; it
is seen also in other inflammatory bowel diseases and
even in some cases of bowel ischemia.
 In active Crohn disease, increased attenuation of the
mesenteric fat is often seen.
 This appearance is due to the transmural extension of
inflammation across the serosa and to engorgement of
the hyperemic vasa recta surrounding the inflamed
bowel segment.
 Prominence of the vasa recta adjacent to the inflamed
loop of bowel is termed the “comb sign” .
 This sign, along with increased mesenteric fat
attenuation, is the most specific CT feature of active
Crohn disease.
 Findings that might be seen in inactive longstanding
Crohn disease include submucosal fat deposition,
pseudosacculation, surrounding fibrofatty proliferation,
and fibrotic strictures.
 Involvement of the mesenteric border of the affected
bowel segment in the inflammatory process, with
associated asymmetric fibrosis and pseudosacculation of
the antimesenteric border, is a hallmark .
 CT enterography has high sensitivity for the detection of
bowel strictures occurring as a complication of Crohn
disease.
 Reversible strictures produced by active disease
demonstrate mucosal hyperenhancement, mural
stratification, fat stranding, and engorgement of the
vasa recta; however, a lack of enhancement and loss of
stratification might be seen in the presence of
transmural fibrosis .
 It is important to differentiate between reversible and
irreversible strictures because the former warrant
medical management, whereas the latter may require
surgical intervention.

Inflammatory bowel stricture in a patient with active Crohn disease. (a) Axial
CT enterographic section shows involvement of a long segment of the terminal
ileum in an inflammatory stricture represented by mucosal hyperenhancement
(arrowhead) and bowel wall thickening with mural stratification (arrow). (b)
Coronal volume-rendered CT enterographic section shows luminal narrowing
(arrow) and proximal dilatation (*) of the small bowel, findings indicative of
obstruction.
 Submucosal fat deposition
in longstanding Crohn
disease. Axial CT
enterographic section shows
abnormally low attenuation
of the bowel wall (arrowheads), a finding consistent
with submucosal fat
deposition.
 Large bowel involvement
in Crohn disease. Axial
CT enterographic
sections obtained in
different patients show
luminal narrowing and
dilatation in a segment
of the ascending colon
(arrow in a), a perianal
abscess (arrow in b)
 rectosigmoid
involvement (black
arrowhead in c), and a
comb sign (white
arrowheads in c).
 Small bowel capsule endoscopy is usually contraindicated
when inflamed or fibrotic strictures cause narrowing of the
intraluminal diameter to less than 1 cm, a condition in
which there is an increased risk of capsule retention leading
to bowel obstruction.
 The rate of capsule retention in patients with both Crohn
disease and small bowel stricture is relatively high (13%) .
 To avoid this complication, patency capsule endoscopy or CT
enterography may be performed to evaluate the patency of
the small bowel before capsule endoscopy is undertaken.
 CT enterography is useful also for detecting reactive
lymphadenopathy in the mesentery adjacent to stenosed
bowel segments; this finding is an important indicator of
active disease.
Capsule impaction in bowel stricture due to Crohn disease. Anteroposterior
radiograph (a) and axial CT enterographic section (b) demonstrate the
impaction of an endoscopic capsule (black arrow) within the lumen of a
thick-walled diseased ileal segment (white arrow in b).
 The excellent spatial resolution and multiplanar imaging
capability of CT enterography make it the modality of
choice for evaluating complications such as enteroenteric
fistula, enterovesical fistula, and interloop abscess.
 CT enterography resulted in accurate detection of
fistulas in 94% of cases .
 A review of multiplanar reformatted images improves
visualization of fistulas and aids in preoperative
planning.
 Fistulas usually appear as enhancing linear extraluminal
tracts connecting bowel loops and may or may not
contain fluid .
Fistula formation in Crohn disease. Coronal volume-rendered CT
enterographic sections obtained in two patients depict ileo-ileal fistulas
(arrowheads in a) and an ileocolonic fistula (arrow in b).
 Abscesses often are seen in the setting of Crohn disease.
They appear as extraluminal fluid collections without
communication with the bowel lumen.
 Because the contents of an abscess may have an
appearance and attenuation similar to those of
intraluminal contrast material, it is crucial that a lack of
communication between the collection and the bowel
lumen be established before an abscess is diagnosed.
 A multiplanar review of images in oblique coronal and
sagittal planes is useful in difficult cases, especially in
patients with a paucity of abdominal fat.
Abscess formation in Crohn disease. Axial (a) and coronal volume-rendered (b) CT
enterographic sections show a large fluid collection (arrows) with attenuation similar to
that of enteric contrast material. Lack of communication between the collection and the
bowel, as well as the marked difference in caliber between the two, helps confirm an
extraluminal location of the collection. Note the evidence of active Crohn disease (* in a)
in the terminal ileum.
Ulcerative Colitis
 Ulcerative colitis is characterized by a continuous pattern
of bowel wall involvement, starting from the rectum,
without evidence of skip lesions.
 Ulcerative colitis predominantly involves the large bowel
but may extend to the terminal ileum, a condition
referred to as “backwash ileitis” .
 Extraintestinal manifestations may occur but are
uncommon.
Ulcerative colitis with backwash ileitis. Axial CT enterographic sections show
continuous involvement of the large bowel (white arrrows) and backwash
ileitis (black arrow in b)
 Because the CT appearance of the bowel often is normal early in the
course of the disease, CT is not performed for initial diagnostic
evaluations .
 Colonoscopy remains the primary diagnostic method for routine
evaluations of the colon .
 However, when specific features are present, CT enterography can
be helpful for achieving an accurate diagnosis and excluding small
bowel involvement.
 For example, severe progressive ulcerative colitis may result in
pseudopolyp formations that may be seen at CT.
 In addition, features such as mural thinning, pneumatosis, and
perforation may be detected in cases of toxic megacolon. In cases
of longstanding disease, features such as rectal narrowing and
presacral fat accumulation can be seen at CT.
 CT also aids in the diagnosis of complications such as rectal
carcinoma.
Long-term complications of ulcerative colitis. Axial (a) and coronal reformatted (b) CT
enterographic sections depict an enhancing, cauliflower-shaped, polypoid soft tissue mass
(arrows) that protrudes into the rectal lumen, a finding consistent with adenocarcinoma,
which was surgically proved.
GI Tract Bleeding
 Bleeding in the GI tract has many possible causes, including
ulcers, vascular malformations, and tumors.
 Patients in whom the presence of GI tract bleeding is
suspected usually undergo upper and lower GI tract
endoscopy for initial evaluation.
 If the result of endoscopy is negative or inconclusive,
multiphase CT may be helpful.
 Triple phase (arterial, enteric, and delayed phase) CT
enterography, in particular, was recently reported to be of
value for detecting GI tract bleeding and identifying the
source.
 Active small bowel bleeding at multiphase CT enterography
is observed as a gradual accumulation of contrast material
within the bowel lumen.
 Angiodysplasia is the most common cause of occult GI
tract bleeding.
 The structural abnormality usually appears as an avidly
enhancing plaque or nodule during the enteric phase
and fades during the delayed phase. Less commonly, it
might manifest as a focal area of enhancement or an
associated early draining vein during the arterial phase.
 Other causes of GI tract bleeding that may be detected
at CT enterography include various types of vascular
malformations, neoplasms, and Meckel diverticulum .
 Small bowel tumors such as leiomyoma and
gastrointestinal stromal tumor (GIST) also may be
sources of occult GI tract bleeding.
Angiodysplasia. Axial (a) and coronal reformatted (b) CT enterographic
sections demonstrate a tuftlike area of enhancement in the medial cecal wall
(arrow), a finding suggestive of angiodysplasia.
Meckel diverticulum. (a) Coronal reformatted CT enterographic section depicts a flaskshaped ileal outpouching with a discernible fundus (arrow) and neck (arrowhead),
findings suggestive of a Meckel diverticulum. (b) Axial CT enterographic section from a
patient with lower GI tract bleeding shows an enhancing pouchlike formation in the bowel
wall (arrow), a finding that represents ectopic gastric mucosa.
Small Bowel Neoplasms
 Small bowel neoplasms are relatively rare and usually
produce nonspecific clinical signs and symptoms that
result in a late diagnosis.
 They are an important cause of obscure GI tract
bleeding, and they sometimes lead to bowel obstruction.
 CT has demonstrated a fairly high level of accuracy in
the evaluation of small bowel tumors , including GIST,
adenocarcinoma, carcinoid, lymphoma, and metastases.
 When performed with bowel-loop distention with a lowdensity contrast material, intravenous administration of
an iodinated contrast agent, and an optimized
acquisition protocol, CT enterography is a reliable
method for diagnosing and staging small-bowel
neoplasms.
 Benign GISTs may arise in any part of the small bowel,
whereas malignant GISTs usually are seen in the distal
small bowel.
 Most GISTs are predominantly extraluminal , although a few
have been described as endoluminal .
 Most have been found in the stomach, followed by the small
bowel.
 Other, less likely locations include the colon and esophagus.
 It is often difficult to differentiate between benign and
malignant GISTs at imaging.
 CT findings of lesion size larger than 5 cm, heterogeneous
enhancement, gastric location, associated metastases, and
a cystic-necrotic component were significantly more
frequent among tumors with a high mitotic index.
GIST. Axial (a) and coronal volume-rendered (b) CT enterographic sections depict a
large exophytic mass with central low attenuation (arrows) arising from an ileal loop. The
appearance and location of the mass are highly suggestive of GIST.
 In a series of 1260 small bowel cancers, carcinoid
tumors were found to be the most common primary
small intestinal cancers identified histologically, followed
by adenocarcinomas and lymphomas .
 A carcinoid tumor often appears as an intensely
enhancing mucosal polyp or an enhancing carpet lesion
with apparent wall thickening.
 The ileum is reported to be the most frequent location of
carcinoid involvement of the small bowel.
 Mesenteric carcinoid tumors often lead to a desmoplastic
reaction that produces a spiculated masslike
appearance.
Carcinoid tumors. (a) Axial CT enterographic section obtained in a 51-year-old man
shows a small, enhancing lesion in the terminal ileum (arrow), an incidental finding that
was proved at pathologic analysis to be a carcinoid. (b) Axial CT enterographic section
obtained in another patient demonstrates a mesenteric mass with a spiculated margin
(arrows), a finding representative of a mesenteric carcinoid.
 Adenocarcinoma may manifest as annular narrowing, a
discrete tumor mass, or an ulcerative plaque and is most
frequently observed in the duodenum.
 It may lead to progressive small bowel obstruction,
intussusception, or, rarely, perforation.
 Lymphoma is more likely to occur in the ileum than in
other small bowel segments.
 It might manifest as a nodular filling defect, discrete
polyp, infiltrating lesion, or exoenteric mass, and it is
usually associated with significant lymphadenopathy .
 With good bowel distention, the nodular thickening that
predominantly involves the terminal ileum in cases of
small bowel lymphoma is well demonstrated at CT
enterography .
 Lymphomatous involvement of the terminal ileum may
lead to the development of intussusception.
Small bowel lymphoma. Axial (a) and coronal volume-rendered (b) CT
enterographic sections demonstrate multiple focal areas (arrows) of nodular
wall thickening involving distal ileal loops. These findings are compatible with
lymphoma, which was subsequently proved.
 Intestinal involvement in metastatic cancer is common.
The prevalence of different malignancies varies;
however, intestinal involvement, mostly in the form of
diffuse peritoneal carcinomatosis, has been reported in
5%–10% of patients with neoplasms such as breast
cancer and malignant melanoma.
 Intestinal metastases may be secondary to
intraperitoneal seeding, hematogenous spread, or direct
extension from an adjacent visceral or colonic
malignancy.
Celiac Disease
 Celiac disease is a chronic inflammatory disorder of the
small bowel that is induced in genetically susceptible
people by the irritant gluten and, possibly, various
environmental cofactors.
 The disorder may be asymptomatic or characterized by
diverse symptoms of malabsorption with varying
severity .
 The burden of undetected celiac disease is high.
 Researchers have calculated that the ratio of diagnosed
cases of celiac disease to cases that are as yet
undiagnosed is as high as 1:7.
 CT enterography can be useful in evaluating patients
with celiac disease, especially when the clinical signs and
symptoms are nonspecific.
 The CT findings in celiac disease include small bowel
dilatation, small bowel intussusception, villous atrophy,
and jejunization of the ileum as reflected by a decrease
in jejunal folds in contrast with the increasing fold
pattern seen in the ileum.
 An accurate diagnosis can be made with visualization of
the reversed jejunoileal fold pattern and at least two
other characteristic features (fold thickening, jejunal
atrophy, or dilatation)
 Associated imaging findings include increased splanchnic
circulation, transient small bowel intussusception,
mesenteric and retroperitoneal lymphadenopathy, and
hyposplenism.
 Lymphomatous infiltration in the context of celiac
disease may be seen as polypoidal involvement of the
small bowel wall .
Celiac disease. (a, b) Axial (a) and coronal reformatted (b) CT
enterographic sections show jejunization of the ileal mucosa
(arrowheads).
Axial CT enterographic section from
another patient depicts multiple
polypoid lesions (arrowheads) in the
small bowel and jejunization of the
ileal mucosal pattern, features
consistent with subsequent
histologic findings of T-cell
lymphoma occurring as a
complication of celiac disease.
Mesenteric Ischemia
 CT enterography is frequently used for the evaluation of
mesenteric ischemia in a subgroup of patients with
nonspecific symptoms.
 CT enterography performed with an optimized multiphase or
dual phase technique may directly depict the cause of
mesenteric ischemia.
 It is critical that radiologists understand the underlying
disease process so as to accurately interpret the findings
and diagnose the cause of mesenteric ischemia.
 The three main causes of mesenteric ischemia are
(a) arterial occlusion or compromise,
(b) venous occlusion,
(c) low flow states such as poor cardiac output or hypovolemia
 Superior mesenteric artery occlusion accounts for more
than half of all cases of mesenteric ischemia , and
nonocclusive ischemia secondary to low flow states
accounts for 30%.
 Venous occlusion is the least common cause of
mesenteric ischemia.
 New treatment methods based on minimally invasive
techniques (eg, intra-arterial thrombolysis or
angioplasty, anticoagulation therapies) make the CT
detection of arterial or venous occlusion important.
 Most cases of acute mesenteric ischemia are due to
emboli at the origin of the middle colic artery branch of
the superior mesenteric artery, or in the proximal part
3–10 cm from the origin.
 Multidetector CT, with its increased temporal resolution
and decreased section thickness, allows significant
improvements in the evaluation of mesenteric vessels,
detection of fat stranding, and characterization of bowel
wall changes, including thickening or edema (which,
although nonspecific, are the most commonly reported
findings).
 The thickened segment of bowel may demonstrate
hyperenhancing mucosa, a sign of reactive hyperemia
due to reperfusion injury, another important finding.
 The involved loop of bowel may demonstrate an absence
or delay of enhancement in the presence of interrupted
blood flow or increased venous pressure.
 Dilatation of the involved bowel segment due to
aperistalsis is another described finding .
 Pneumatosis is less commonly seen .
 In addition to visualization of bowel wall changes, CT
enterography allows improved evaluation of the
mesenteric vasculature and helps determine the
underlying cause of vascular compromise .
 Small bowel ischemia.
Axial CT enterographic
section obtained in the
arterial phase shows an
abnormal segment of
small bowel with wall
thickening and
hypoenhancement (white
arrows), findings
indicative of ischemia
due to thromboembolic
occlusion (black arrow)
of the superior
mesenteric artery
Other Findings
 Although CT enterography is a dedicated protocol for the
diagnosis of small bowel diseases, incidental findings
with this modality may be significant.
 Such findings might include not only abnormalities
internal to the GI tract but also extraluminal, peritoneal,
mesenteric, and omental abnormalities; hepatic and
pancreatic masses; and disease involving other organs.
 Mesenteric
lymphangioma. Axial CT
enterographic section
obtained for evaluation
of Crohn disease depicts
a nonenhancing
mesenteric lesion
(arrows) with attenuation
similar to that of fluid, an
incidental finding
consistent with
mesenteric
lymphangioma.
Summary
 CT enterography is an invaluable technique for the evaluation of
various small bowel diseases.
 Its main uses are in the assessment of inflammatory bowel disease,
occult GI tract bleeding, and mesenteric ischemia.
 An optimal technique with adequate bowel distention is required for
obtaining accurate diagnostic results.
 A single (venous) phase technique is used for bowel evaluation, and
a dual phase technique is used for evaluation of mesenteric
ischemia. A triple phase protocol is currently being optimized for the
evaluation of GI tract bleeding.
 CT enterography also has been shown to be more cost-effective in
the long-term assessment and follow-up of patients, especially
those with established Crohn disease.
 MR enterography is being used more frequently because of the
advantage of lack of radiation. However, the superior spatial
resolution of CT enterography still makes it the initial imaging
modality of choice in many adult patients.

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