AJR 2003; 181:125-130
© American Roentgen Ray Society
Contrast-Enhanced Helical CT of Choledocholithiasis
Frank H. Miller1,
Caroline M. Hwang1,
Helena Gabriel1,
Lori A. Goodhartz1,
Anees J. Omar2 and
Willis G. Parsons, III3
1 Department of Radiology, Northwestern University Medical School, 676 N. St.
Clair St., Ste. 800, Chicago, IL 60611.
2 Department of Medicine, Evanston Hospital, 2650 Ridge Ave., Evanston, IL
60201.
3 Department of Gastroenterology, Northwestern University Medical School,
Chicago, IL 60611.
Received February 12, 2002;
accepted after revision December 4, 2002.
Address correspondence to F. H. Miller.
Introduction
Choledocholithiasis is a common problem seen in as many as 6–12% of
patients undergoing cholecystectomy. Common bile duct stones may be
asymptomatic but can often lead to biliary colic, cholangitis, jaundice, or
pancreatitis. In patients in whom choledocholithiasis is highly suspected,
transabdominal sonography is the most commonly used initial examination to
screen for gallbladder disease because of its low cost and high accuracy.
However, in patients with symptoms that are not specific, such as abdominal
pain, or with symptoms that are suspicious for pancreaticocholedochal disease,
helical CT performed with IV contrast material is widely used for the initial
evaluation. Consequently, radiologists need to recognize the imaging findings
of common bile duct stones on contrast-enhanced CT scans because that is often
the first opportunity they have to diagnose a common bile duct stone.
Although endoscopic retrograde cholangiopancreatography (ERCP), MR
cholangiopancreatography, and endoscopic sonography are more sensitive than
CT, these modalities are usually not the first imaging tests performed for
evaluating common bile duct stones. In this pictorial essay, we illustrate the
findings of choledocholithiasis on contrast-enhanced helical CT and compare
them with findings on ERCP.
Contrast-Enhanced Helical CT
Researchers
[1–8]
have extensively studied the use of conventional (nonhelical) CT in the
detection of choledocholithiasis and have found the sensitivity of
conventional CT to be superior to that of sonography for diagnosing
choledocholithiasis [3,
7,
8]. Sensitivities for the
detection of common bile duct stones on conventional CT have ranged from 25%
to 90%, with most studies reporting sensitivities in the ninth decile
[1–8].
However, sensitivities for direct depiction (i.e., excluding indirect signs
like ductal dilatation from criteria) of common bile duct stones have not
exceeded 75% [1]. Recent
studies examining the use of unenhanced helical CT in the identification of
common bile duct stones have reported higher sensitivities of 65% and 88%
[1,
9]. However, IV
contrast-enhanced helical CT is performed in patients with abdominal
complaints more often than is unenhanced CT.
Radiologists can determine the presence or absence of bile duct stones on
the basis of the four criteria previously described by Baron
[2,
6,
10]
(Fig. 1). With the target sign,
the stone is seen as a central density surrounded by hypoattenuating bile or
ampullary soft tissue (Figs.
2A,
2B). With the rim sign, one can
see a faint rim of increased density along the margin of a low-density area
(Fig. 3). A calculus with
increased density that is surrounded by a crescent of hypoattenuating bile is
suggestive of the crescent sign (Figs.
4A,
4B,
4C). Indirect signs, such as
the abrupt termination of the duct or ductal dilatation, can be helpful but
are not considered definitive for the diagnosis of stones. The criteria used
to determine the presence of a stone can vary. In cases in which a given stone
appears on multiple slices, more than one sign may be used to describe the
stone; the criteria change depending on which slice is used to evaluate the
stone (Figs. 1 and
4A,
4B,
4C).
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Fig. 1. —Illustration of stones in common bile duct as seen on
contrast-enhanced CT; appearance of stone can vary depending on slice of
examination. On left, dotted lines passing through common bile duct represent
scan obtained at each level, and images on right show CT appearance of stones.
Slice 1 shows fluid attenuation in lumen of bile duct without stone. Slice 2
shows target sign. Crescent and rim signs are shown at slices 3 and 4,
respectively. Slice 5 shows stone completely filling duct. (Modified and
reprinted with permission from
[10])
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Fig. 2A. —84-year-old man with common bile duct stone exhibiting target
sign. On contrast-enhanced helical CT scan, choledocholithiasis is represented
as central density (arrowhead) surrounded by hypoattenuating ampulla
of Vater (arrow). In this particular scan, stone is seen as
heterogeneous with center showing lower density.
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Fig. 2B. –84-year-old man with common bile duct stone exhibiting target
sign. ERCP image confirms presence of stone (arrow) in common bile
duct, which was pushed superiorly before it was surgically removed.
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Fig. 3. —50-year-old man with stone in distal common bile duct
presenting as rim sign on contrast-enhanced helical CT scan. Faint rim of
increased density (arrow) is visible along peripheral margin of
low-density calculus.
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Fig. 4A. —88-year-old woman with common bile duct stone displaying
crescent and target signs. On contrast-enhanced CT scan, stone (straight
arrow) with density of soft tissue is surrounded by crescent-shaped lower
density bile (curved arrow).
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Fig. 4B. —88-year-old woman with common bile duct stone displaying
crescent and target signs. Contrast-enhanced CT scan obtained at level
inferior to A shows same stone (arrow) completely surrounded
by bile (target sign). Note how appearance of stone changes on different CT
slices.
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When identified on CT, stones can be characterized as calcified
(Fig. 5), soft tissue
(Fig. 6), or low density
(Fig. 7). Stones that show
calcific density on CT are easiest to detect but are not the most frequently
observed type of stone. Experience documented in the literature has shown that
even small stones composed of calcium bilirubinate are detectable on
conventional CT, whereas those composed primarily of cholesterol show lower
density and thus are harder to identify
[4]. Because only 20% of stones
display homogeneous high density, depiction of noncalcified stones on helical
CT is subject to problems similar to those encountered on conventional CT
[2]. Consequently, close
scrutiny of the bile duct is required to detect these lower-attenuation
stones.
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Fig. 5. —33-year-old woman with calcified stone (arrow) in
common bile duct seen on contrast-enhanced CT scan. Because of their density,
calcified stones are easier to detect than—but are not as common
as—cholesterol stones.
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Fig. 6. —90-year-old woman with common bile duct stone. On
contrast-enhanced CT scan, stone (curved arrow) appears as
soft-tissue density in distal portion of common bile duct. Note difference in
appearance of soft-tissue stone from calcified gallstones (straight
arrows). Dilated intrahepatic duct is indicated by arrowhead.
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Fig. 7. —91-year-old woman with stone in common bile duct seen as
subtly increased density (straight arrow) in hypoattenuating bile on
contrast-enhanced CT scan. Stone has similar attenuation to surrounding bile,
making diagnosis difficult. Note incidentally discovered low-density gallstone
(arrowhead) in markedly thickened gallbladder (curved
arrow).
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Use of the appropriate CT technique can optimize detection of soft-tissue
and low-attenuation stones. The difficulty in detecting soft-tissue stones is
that they may have the same attenuation as the bile duct wall. A thin
collimation of 2.5–5.0 mm can help prevent volume averaging with the
adjacent bile [11]. However,
the 15–25% of stones that are isoattenuating with bile are not
detectable on helical CT despite the use of thin collimation
[12]. Overlapping
reconstructed axial CT scans obtained at 2-mm increments can also be helpful.
This technique allows better visualization of a thin rim or crescent of the
lower-attenuation bile between the stone and the wall of the bile duct.
Because most stones are composed primarily of cholesterol (resulting in an
attenuation similar to that of bile), Baron
[13] has recommended using the
highest kilovoltage setting to increase the chances of distinguishing these
stones from the bile.
In a prospective study by Neitlich et al.
[1] comparing unenhanced
helical CT with ERCP in the detection of choledocholithiasis, the authors
found that using bile window settings (i.e., adjusting the window level
setting to the mean attenuation of the common bile duct and the window width
to 150 H) improved the detection of the stones because of better contrast
between the bile and the soft tissue. When viewing scans acquired in patients
with suspected stones, magnifying the scans allows better visualization of the
common bile duct. When using cine mode or scrolling through CT scans on a PACS
(picture archiving and communication system), the bile duct can be traced from
scan to scan. Obtaining multiplanar reformatted oblique coronal images through
the common bile duct can also be helpful for visualizing the duct.
Administration of IV contrast material may be beneficial for revealing
subtle ductal dilatation. In addition, ductal wall enhancement may help
delineate an impacted, nearly isodense stone. Despite these advantages, the
use of IV contrast material has some drawbacks that can decrease the
sensitivity of enhanced helical CT compared with the sensitivity attainable
with unenhanced helical CT. A major limitation of the contrast-enhanced
helical CT as a tool for detecting choledocholithiasis is that the oral and IV
contrast material may obscure the stones and make recognition more difficult
[1,
12]. Distinguishing enhancing
bile duct mucosa from common bile duct stones can be challenging and result in
both decreased sensitivity and specificity of contrast-enhanced helical CT.
The enhancing mucosa can easily appear to be a stone, although the longer the
enhancement in a segment lasts, the more likely it is that a stone is not
present. This finding is not absolute; cholangitis associated with a stone can
cause enhancement of a long segment of the bile duct. Enhanced mucosa seen on
CT in patients with other abdominal diseases such as HIV cholangiopathy may be
confused with stones (Fig. 8).
In some patients, especially those with cholangitis, the use of unenhanced CT
should be considered to prevent mistaking the contrast-enhanced mucosa for a
stone.
Other sources of false-positive findings for stones include enhancing
vessels and intraluminal masses, the latter requiring further investigative
studies. Increased density seen within the common bile duct due to sludge or
artifacts can also cause false-positive findings
(Fig. 9). Dilated bile ducts,
a sign of obstruction and an indirect sign of choledocholithiasis, can also
mislead radiologists. The location, size, and composition of the stone affect
CT findings and can be the cause of false-negative results. Proximal stones in
minimally dilated ducts are often not suspected and can be difficult to
detect, even retrospectively. Oral contrast material may also obscure small
distal stones, especially near the ampulla of Vater. The high density of
barium may obscure stones; therefore, water can be a more effective oral
contrast agent in patients thought to have common bile duct stones. Calculi
with a density similar to that of bile can also be extremely difficult to
diagnose. In addition, unusual stone morphology can be misleading, such as an
elongated stone resulting from bile duct cast (Figs.
10A,
10B).
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Fig. 9. —25-year-old woman who had undergone cholecystectomy and
presented with abdominal pain. On contrast-enhanced CT scan, area of increased
density (arrow) presumably caused by artifact is seen in
hypoattenuating bile, a finding that is suggestive of stone and led to
false-positive diagnosis. On ERCP images (not shown), no stone was
identified.
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Fig. 10A. —51-year-old woman with common bile duct cast stone in
cirrhotic liver that was missed on CT. Contrast-enhanced CT scan shows faint
subtle increase in density (arrow) in common bile duct that was
initially thought to represent enhancing mucosa.
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Fig. 10B. —51-year-old woman with common bile duct cast stone in
cirrhotic liver that was missed on CT. ERCP image shows cast (arrows)
in common bile duct that gastroenterologist classified as atypical stone.
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Endoscopic Retrograde Cholangiopancreatography
ERCP continues to be considered the most definitive method for diagnosing
common bile duct stones. The accuracy of MR cholangiopancreatography
approaches that of ERCP, but it has the disadvantage of being nontherapeutic.
For ERCP, the patient is placed under conscious sedation, and a contrast
medium is injected into the bile and pancreatic ducts. The presence of a stone
causes a filling defect to appear on the images obtained. In addition,
therapeutic options such as sphincterotomy, stone extraction, stricture
dilation, stent placement, and, if required, tissue sampling can be performed
at the time of the examination. However, ERCP has an associated complication
rate of approximately 5%; complications include pancreatitis, perforation,
hemorrhage, and cholangitis. Consequently, ERCP should probably be reserved
for patients with choledocholithiasis who require therapy or for those who
have a high probability of having choledocholithiasis but have had negative
findings on initial imaging modalities. MR cholangiopancreatography may be
useful in some patients as a noninvasive means of diagnosis after CT has been
performed.
Conclusion
ERCP continues to be an established and effective means for diagnosing and
treating choledocholithiasis. However, its invasiveness and high cost make
other imaging modalities more desirable for use as the initial screening test
in patients believed to have stones in the common bile duct. ERCP should be
reserved for therapeutic purposes or for those patients with a high likelihood
of stones being present despite negative findings on prior studies.
In patients with abdominal complaints or elevated levels of liver enzymes,
contrast-enhanced multidetector CT is the most commonly performed examination.
Although the IV contrast material may limit the sensitivity of CT,
radiologists can detect stones in the common bile duct on contrast-enhanced CT
by recognizing the important features of choledocholithiasis while remaining
aware of the potential pitfalls that accompany the use of contrast
material.
Acknowledgments
We thank Mary Costello and Ashish Varma for their assistance in the
collection of data as well as David Botos, Danyell Kimbrough, and Michelle
Naidich for their help in the preparation of the manuscript.
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Introduction
Contrast-Enhanced Helical CT
