1. Something created by humans usually for a practical purpose,
especially an object remaining from a particular period (e.g., caves
containing prehistoric artifacts).
2. A product of artificial character (as in a scientific test)
due usually to an extraneous (as human) agency.*
Mimbres Bowl, circa 1100 A.D. Southern New Mexico. [Courtesy Millicent Rogers Museum, Taos, New Mexico.]
Although this patient’s sonogram does not show typical
features of acute cholecystitis, the correct interpretation can
be deduced by understanding the principles of ultrasound imaging.
A sonographic image is formed by the reflection of the ultrasound
beam at the interface between two substances of differing acoustic
impedance. If the ultrasound beam does not encounter an acoustic
interface, no sound is reflected and the object appears black (sonolucent),
for example, a fluid-filled gallbladder. If there is a moderate
degree of reflection of the ultrasound beam at an acoustic interface,
there will be a gray area on the screen and the remaining ultrasound
beam continues deeper. If the beam encounters a very echogenic interface,
it is entirely reflected, producing a very bright signal and a dark acoustic shadow behind it. This is
the image produced by a gallstone within the gallbladder (Figure
Air almost entirely obscures an ultrasound image because the
ultrasound beam is unable to penetrate air-filled structures such
as bowel and lung. Sonography thus differs from conventional abdominal
radiography, in which air-filled structures are readily detected
because the radiopacity of air is very different from that of adjacent
soft tissues. Ultrasonography and abdominal radiography therefore
have complementary diagnostic capabilities.
Ultrasonography is subject to a variety of imaging artifacts. Artifacts, in general,
obscure or distort an imaging study because they produce nonanatomical findings.
However, in ultrasonography, artifacts are often helpful for image
interpretation. The acoustic shadow produced
by a gallstone is an example of a sonographic artifact. Although
it is nonanatomic, this visually distinctive artifact helps confirm
the diagnosis because it is characteristic of a very echogenic object
(a gallstone) within the sonolucent gallbladder (Figure 4).
*By permission. From Merriam-Webster’s
OnLine Dictionary ©2006 by
Merriam-Webster, Inc. (www.Merriam-Webster.com)
Under What Circumstances
Would the Gallbladder Not Have Its Typical Appearance?
In this patient, a sonolucent gallbladder could not be found.
Diffculty in finding the gallbladder occurs most often when a patient
has recently eaten. The gallbladder contracts and it is difficult
to visualize. When a patient has had multiple prior episodes of
cholecystitis, the gallbladder becomes scarred and shrunken and
difficult to image by ultrasonography. An antecedant cholecystectomy
is, of course, another cause of non-visualization of the gallbladder,
although not in this patient.
In this patient's sonogram, there is a bright crescent next to
the inferior surface of the liver in the region of the gallbladder
fossa (Figure 1). A similar finding occurs when the gallbladder
is filled with stones or is contracted around gallstones. It forms
two narrow curvilinear bands and casts an acoustic shadow—the wall-echo-shadow (WES) or “double
arc” sign (Figure 5).
The “wall-echo-shadow” (WES) or “double
arc” appearance of a gallbladder filled with gallstones
that cast an acoustic shadow.
However in this patient, there is only a single curved echogenic
line (Figure 6, arrow). This is similar
in appearance to the diaphragm/lung interface (D) seen
at the posterior surface of the liver. Both of these bright crescents
are, in fact, due to air interfaces.
Patient 5—Sonogram image A.
There is a bright crescent (arrow)
along the inferior margin of the liver (L) in the region of the
gallbladder fossa. A second bright crescent is seen at the air interface
between the lung and diaphragm (D).
Two sonographic artifacts associated
with air interfaces are seen in this patient’s sonogram
(Figure 7). Deep to the bright crescent, the sonographic image is
obscured by shadowing. This is not a black acoustic shadow like
that produced by a gallstone, but is instead a grayish shadow that
is called “dirty shadowing.”
Patient 5—Sonogram image C.
There are two artifacts associated with an air interface. Dirty shadowing (S) obscures sonographic
detail deep to the liver edge. Ring down artifact
is a bright streak (arrow) due to reverberation
of the ultrasound beam between two highly reflective interfaces.
A second artifact seen at highly echogenic interfaces is known
as “ring down” (Figure
7). It is due to reverberation of the ultrasound beam between two
reflective interfaces such as a layer of air or a metallic object.
This causes repeated reflection of the ultrasound beam back to the
transducer (Figure 8) and creates a bright white streak extending
down from the white crescent. The bright appearance of the ring-down
artifact is the opposite of the black acoustic shadow of a gallstone.
When the reverberations are more widely spaced, the bright artifacts appears
interrupted rather than continous (Figure 9).
Schematic diagram of reverberation artifact.
The sound beam reverberates between two highly reflective surfaces.
With each reverberation, an ultrasound signal is returned to the
ultrasound probe, which is registered as a signal coming from a
With “ring-down” artifact, the reverberations
are very closely spaced, producing a continuous white streak rather
than an interrupted series of bright spots.
(NB. The reverberations occur in the same path as the ultrasound
beam, not in the sine wave pattern drawn.)
Reverberal artifact due to bowel gas.
Bowel gas produces a bright sonographic signal (arrow) deep to the liver (L). The highly
reflective air collection produces a reverberation artifact (seen here)
or ring-down artifact.
One structure that can create an air interface adjacent to the
liver is an air-filled segment of bowel (Figure 9). However in this
patient, the gallbladder should still be detectable because he had
not had a cholecystectomy.
Another possibility is that the echogenic crescent in the gallbladder’s
expected location is caused by air within the gallbladder lumen
or wall. This is the correct explanation of the patient’s
sonographic findings. An air-filled gallbladder occurs in emphysematous cholecystitis.
Emphysematous cholecystitis occurs in 1% of patients
with acute cholecystitis. The patients are typically elderly, usually
male, and often have diabetes. The mortality rate is 15% (vs.
1.4% for cholecystitis). Emergency surgery is necessary
because of the high risk of perforation. The gas is due to infection
by gas-forming bacteria: clostridia (50%), coliform bacilli,
or gram-positive cocci (obligate or facultative anaerobes). Emphysematous
cholecystitis has a distinctive sonographic appearance, as is seen
in this patient.
The diagnostic test of choice for emphysematous cholecystitis
is an abdominal radiograph. Air within
the gallbladder appears as an oval-shaped lucency. An air/fluid
level may be seen on an upright abdominal radiograph. Air within
the gallbladder wall appears as a curved radioluent line.
In this patient, after the sonogram was performed, the radiologist
suggested that an abdominal radiograph be performed to confirm the
diagnosis. The radiograph showed air in the gallbladder and gallbladder
wall (Figure 10). The patient was taken to surgery and a dilated,
gangrenous, but nonperforated gallbladder was removed. Numerous
stones were found within the gallbladder.
Patient 5—Abdominal Radiograph.
The patient’s upright abdominal radiograph shows an
air/fluid level within the distended gallbladder and gas
within the gallbladder wall. This is diagnostic of emphysematous
of Emphysematous Cholecystitis
Abdominal radiography is the test
of choice to diagnose emphysematous cholecystitis. It shows air
within a distended gallbladder or gallbladder wall (Figure 10).
Gas in the gallbladder wall is pathognomonic for emphysematous cholecystitis.
Air in the biliary system also occurs with an enteric-biliary
fistula (gallstone ileus or surgical anastomosis). However in these
cases, the gallbladder is collapsed rather than distended, and a branching
air pattern is seen within the biliary ducts.
Suspected emphysematous cholecystitis could be considered a third
indication for radiography in patients with abdominal pain (in addition
to pneumoperitoneum and bowel obstruction). However, emphysematous
cholecystitis is difficult to suspect based on clinical findings
alone. Emphysematous cholecystitis occurs predominantly in elderly
patients and if radiographs were obtained liberally in elderly patients
with abdominal pain, emphysematous cholecystitis could potentially be
diagnosed. Nonetheless, detecting emphysematous cholecystitis would
be a relatively rare occurrence.
Not all cases of emphysematous cholecystitis have diagnostic
radiographs. In one series of eight patients, only one had a diagnostic
radiographs (Gill 1997). Reasons for nondiagnostic radiographs include
insufficient gas in the gallbladder or gas that cannot be distinguished
from bowel gas. In addition, gas collections in the right upper
quadrant may also be due to an intrahepatic or subdiaphragmatic
abscess infected with gas-forming bacteria (the gas collection usually
has a stippled appearance), or a loop of bowel that has migrated
anterior to the liver. CT can accurately localize a gas collection
when the diagnosis is uncertain (Figure 11).
Emphysematous Cholecystitis—CT diagnosis.
The upright abdominal radiograph of a 67-year-old man with diabetes
shows a gas collection with an air/fluid level in the right
upper quadrant superior to the liver edge (black
arrow). CT confirmed that the gas was within the gallbladder
lumen representing emphysematous cholecystitis (white
Because ultrasonography is the
best initial test for biliary colic/cholecystitis, the
sonographer must be able to recognize the sonographic signs
of emphysematous cholecystitis—a bright echogenic crescent
in the gallbladder fossa with dirty shadowing and ring-down artifacts.
Confirmation with abdominal radiography or CT is necessary because
other disorders can have similar sonographic appearances. These
include a contracted stone-filled gallbladder (WES sign) (Figure
5) or a “porcelain gallbladder” with a calcified
wall due to chronic cholecystitis (Figure 12).
Calcification of the gallbladder wall is due to chronic cholecystitis
(arrows). This may be an incidental
finding or be seen in a patient having pain from an exacerbation
A porcelain gallbladder has a similar sonographic appearance
to emphysematous cholecystitis. There is an increased incidence
of gallbladder carcinoma in these patients.
of Acute Cholecystitis
In the past, abdominal radiography was
used in patients suspected of having cholecystitis because 15% of
gallstones are sufficiently calcified to be detected. However, ultrasonography
is a considerably more sensitive test and has supplanted abdominal
radiography for suspected biliary colic/cholecystitis.
Ultrasonography can detect nearly
all gallstones as well as signs of acute cholecystitis (Figures 2 and 3). However, mere detection of gallstones does not prove that
they are responsible for the patient’s symptoms. Asymptomatic
gallstones are common (up to 80% of patients with gallstones)
and gallstones may therefore be an incidental finding unrelated
to a patient’s pain. Other disorders such as acute coronary
syndrome, pneumonia or bowel obstruction, can have clinical presentations
similar to biliary colic or cholecystitis—upper abdominal
pain, nausea and vomiting. These diagnoses could potentially be
missed if incidentally found gallstones are mistakenly considered
the cause of a patient’s symptoms (Figure 13).
Identification of gallstones does not mean they are
responsible for a patient’s illness.
A 75-year-old man presented with fever, altered mental status,
and right-sided chest and abdominal pain. The abdominal radiograph
showed multiple gallstones (arrow)
and adynamic ileus. Although the initial impression was that he
had acute cholecystitis, the chest radiograph showed pneumonia.
A subsequent HIDA scan was normal. The gallstones were asymptomatic
and an incidental finding.
The diagnosis of acute cholecystitis is
based on the patient's clinical presentation, signs of gallbladder
inflammation on sonography or CT, or in some cases, an hepatobiliary
Sonographic signs of acute cholecystitis include gallbladder
wall thickening (>3 to 5 mm), pericholecystic fluid, and the sonographic
Murphy’s sign (Figure 3). With the sonographic Murphy’s sign,
the site of maximal tenderness is localized with the sonographic
transducer overlying the gallbladder. The sensitivity of sonography
for acute cholecystitis has been reported to be from 70% to
90%. This depends on the experience of the sonographer
and the criteria used. The sonographic Murphy’s sign is
the most sensitive but least specific sign of acute cholecystitis
(specificity as low as 66%) (Ralls 1985, Liang 1992, Shea
1994, Rosen 2001).
Because of the limited sensitivity of ultrasonography for acute
cholecystitis, a sonogram that does not show signs of acute cholecystitis
does not exclude that diagnosis when a patient has a convincing
clinical presentation (persistent pain, focal tenderness or Murphy’s
sign, leukocytosis or fever). Occasionally additional imaging studies
are needed to determine whether a patient’s symptoms are
due to cholecystitis, biliary colic or another cause, i.e., an hepatobiliary
scan or CT.
An hepatobiliary scan (also referred
to as a “HIDA scan”) can directly determine whether
gallstones are causing cholecystitis by detecting cystic duct obstruction.
An injected radiolabeled tracer (99mtechnetium-hydroxy-iminodiacetic
acid [HIDA] or others) is taken up by the liver
and excreted into the bile (Figure 14). In a normal study, (patent
cystic duct), tracer fills the gallbladder and common bile duct,
and then enters the duodenum and small bowel. In such a patient,
gallstones are either not responsible for the patient’s
abdominal pain or transient obstruction has caused biliary colic
without cholecystitis. With acute cholecystitis, the cystic duct
is obstructed and the gallbladder is not visualized. However, other
intra-abdominal conditions and systemic illnesses cause delayed
uptake of tracer in the gallbladder. When the gallbladder is not
seen, additional images at 2–4 hours are necessary to differentiate nonvisualization of the gallbladder
(cholecystitis) from delayed visualization of
the gallbladder due to a non biliary condition. Alternatively, if
the gallbladder is not visualized after one hour, an injection of
low-dose morphine will induce spasm of the sphincter of Oddi, increase
intraluminal biliary pressure, and promote filling of the gallbladder
if the cystic duct is patent (Grossman and Joyce 1991, Kalimi et
al. 2001, Chatziioannou et al. 2000).
Hepatobiliary scan (HIDA scan).
(A) In a normal study (patent cystic
duct), the radiolabeled tracer fills the gallbladder (GB) and proximal
small bowel (SB). (B) With acute cholecystitis,
the cystic duct is obstructed and the gallbladder is not visualized
(D = duodenum).
Computed tomography is often the
initial diagnostic test in patients with abdominal pain when biliary
tract disease is not the primary disorder suspected. Although CT
is not the usual test of choice for acute cholecystitis, it is able
to detect signs of acute cholecystitis (gallbladder wall thickening,
pericholecystic fluid and fat stranding). The sensitivity of CT
for acute cholecystitis has not been studied in a controlled fashion.
One recent study found that CT had a sensitivity of 92% for
acute cholecystitis, equivalent to ultrasonography (Bennett et al.
2002). Two earlier studies found a lower sensitivity of 40% to
50%. (Harvey 1999, Fidler 1996). However, gallstones are
sufficiently calcified to be detected by CT in only 50% to
75% of cases. Noncalcified gallstones have the same attenuation on
CT as bile and are not visible (Figure 15).
CT diagnosis of cholelithiasis and cholecystitis.
(A) Multiple calcified gallstones
incidentally discovered on CT.
(B) In a patient with acute cholecystitis,
CT shows stranding of the pericholecystic fat surrounding the gallbladder
(GB). Gallstones are not visible because they were not calcified.
This should not be misinterpreted as “acalculous cholecystitis.” Sonography
(C) A gallstone is visible due to
its calcified rim (arrow). Gangrenous
cholecystitis causes sloughing of the anterior gallbladder wall
forming an intraluminal membrane (arrowhead).
Gangrenous cholecystitis is present
in 10% (range: 2% to 30%) of patients
with acute cholecystitis and can be detected by CT or ultrasonography.
Ischemia and necrosis of the gallbladder wall necessitates emergency cholecystectomy.
CT signs of gangrenous cholecystitis are: an irregular or absent
gallbladder wall (irregular contour or enhancement), intraluminal
membranes (also seen on sonography) (Figure 15C), pericholecystic
abscess (encapsulated fluid collection adjacent to gallbladder),
and gas in gallbladder wall or lumen (emphysematous cholecystitis
is only seen in a third of patients with gangrenous cholecystitis).
The sensitivity of CT for gangrenous cholecystitis is only 30% and
the diagnosis depends largely on clinical findings. However, the
specificity of CT for gangrenous cholecystitis is high (96%)
and when present, these CT signs should be considered highly suggestive
of the diagnosis (Bennett 2002).