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Biliary Ultrasound

Quick Image Reference

Illustration 1a:  Anatomical overview of upper abdomen.

Illustration 1b: Overview of porta hepatis.

flash video iconVideo clip 1: Subcostal sweep video.

flash video iconVideo clip 2:  Video clip showing the “X-minus 7” approach.

flash video iconVideo clip 3:  Video clip showing the “Flattening the probe” technique.

flash video iconVideo clip 4:  Gallbladder long axis: The gallbladder in the long-axis.

flash video iconVideo clip 5:  Rotating the probe.

flash video iconVideo clip 6:  Gallbladder short axis.

flash video iconVideo clip 7: Measuring the gallbladder wall.

flash video iconVideo clip 8:  Contracted gallbladder wall.

flash video iconVideo clip 9:  Mickey Mouse sign:  The video shows an enlarged view of the portal triad in the short-axis.

flash video iconVideo clip 10:  Measuring the CBD.

flash video iconVideo clip 11:  Normal CDB.

flash video iconVideo clip 12:  Large gallstone.

flash video iconVideo clip 13:  Multiple gallstones.

flash video iconVideo clip 14:  Multiple gallstones; Three or more gallstones.

flash video iconVideo clip 15:  Full of stones;  This gallbladder is full of stones. 

flash video iconVideo clip 16:  Layer of stones.

flash video iconVideo clip 17:  Very small stone.

flash video iconVideo clip 18:  SIN sign:  This gallstone lies in the neck of the gallbladder.

flash video iconVideo clip 19:  Polyp:  This patient has multiple gallbladder polyps adhering to the wall.

flash video iconVideo clip 20:  Sludge:  This gallbladder is full of sludge.

flash video iconVideo clip 21:  Thick wall:  This patient’s gallbladder wall is thickened to 7.1 mm. 

flash video iconVideo clip 22:  Pericholecystic fluid (PCCF).

flash video iconVideo clip 23:  Dilated CBD:  This patient’s CBD is enlarged to over 1 cm.

flash video iconVideo clip 24:  Large CBD:  A massively dilated CBD may be very disorienting.

flash video iconVideo clip 25:  Air in the GB wall.

flash video iconVideo clip 26:  Duodenum:  The duodenum abuts the posterior aspect of the gallbladder.

flash video iconVideo clip 27:  Gallstone vs. duodenum:  The duodenum sits very close to the gallbladder.

flash video iconVideo clip 28:  WES sign.

J. Christian Fox, M.D., RDMS, William P. Scruggs, M.D., RDMS

I.  Introduction and Indications

Gallbladder disease affects 8 % of men and 17 % of women in the United States resulting in over 600,000 surgeries each year.(1,2)  Cholecystitis, the most common emergent surgical condition of the gallbladder, is diagnosed in up to 10 % of total patients and 21 % of elderly patients presenting to the emergency department with acute abdominal pain.(3-5)  Overall, biliary tract disease is the 3rd most common cause of acute abdominal pain presenting to the emergency department.  The associated annual costs total $5.8 billion and result in more than 5000 deaths a year.(6,7)
In the acute presentation of biliary tract disease, the priority is to discriminate between biliary pain (also called biliary colic) and more serious diseases such as acute cholecystitis that require hospital admission, intravenous antibiotic therapy, and possibly emergent surgery.  Unfortunately, no combination of clinical symptoms, signs, and laboratory values can reliably make that differentiation. Classically, patients with symptomatic biliary disease present with severe, steady pain in the epigastrium or right upper quadrant and may radiate to the right scapular region.(8,9)  Dyspeptic symptoms such as nausea, vomiting, bloating, belching, acid regurgitation, and heartburn are not uncommon but do not add to the diagnostic certainty.(10,11 Self-resolving biliary pain generally lasts 15-minutes to 5-hours, but may persist up to 24 hours.  Right upper quadrant tenderness is typically present on physical examination in both biliary pain and cholecystitis.  Murphy’s sign, the arrest of inspiration on deep palpation of the right upper quadrant, is present in 90% of patients with acute cholecystitis.  Fever and leukocytosis are common, but the combination of both does not occur in one-third of patients with cholecystitis.(12) 
Due to the difficulty in confirming the appropriate diagnosis on clinical grounds, imaging is used in most cases.  Ultrasound is the initial imaging study for the diagnosis of acute cholecystitis because it is performed relatively quickly and does not expose the patient to radiation.(13)  Adjusting for verification bias, sensitivity is 88 % and specificity is 80 %.(14)  Ultrasound does not expose the patient to radiation and is much more accurate than plain film radiographs or computed tomography.  Furthermore, the modality is faster and more generally more readily available than cholescintigraphy or MRI.
It is important to note, however, that ultrasound is not the most accurate imaging modality for the diagnosis of acute cholecystitis.  Cholescintigraphy, usually in the form of the HIDA scan, is 97 % sensitive and 90 % specific (after adjusting for verification bias).(14)  Radionucleotide material is injected intravenously and is excreted through the bile.  A nuclear study is then performed to evaluate the filling of the gallbladder.  If the cystic duct is obstructed, as in the case of acute cholecystitis, the gallbladder will not fill and the test is considered positive.  Cholescintigraphy should be considered in patients with a high clinical suspicion and a negative ultrasound.

II.  Anatomy

The gallbladder is an elongated, pear shaped organ that functions to concentrate and store bile.  Along with the liver, it is an embryologic derivative of the foregut.  It normally lies on the visceral surface of the liver between the quadrate and right hepatic lobes covered by a continuation of the hepatic peritoneum. 
The gallbladder consists of the fundus, body, and neck.  The fundus is the most caudal portion of the gallbladder and often will protrude from under the inferior margin of the liver and contact the anterior wall of the abdomen.  The body of the gallbladder tapers cranially, eventually becoming the neck of the gallbladder, which continues to narrow to the junction with the cystic duct.  At its neck, the gallbladder makes a sharp turn away from the parenchyma of the liver into the peritoneal cavity where it becomes the cystic duct.
The cystic duct carries bile to and from the gallbladder and common bile duct into the duodenum.  It is up to 5 cm long, and it joins the common hepatic duct to create the common bile duct (CBD).  The CBD courses along the free border of the lesser omentum along with the hepatic artery and the portal vein.  It passes behind the duodenum and into the head of the pancreas where it meets the ampulla of Vater at the duodenal papilla.  The sphincter of Oddi functions at the ampulla to inhibit the flow of biliary and pancreatic secretions into the duodenum, until it is relaxed by cholecystokinin in response to the ingestion of food.



Illustration 1a:
Anatomical overview of upper abdomen.



Illustration
1b: Overview of porta hepatis.

Bile is produced by hepatocytes and passes down through the biliary ducts of the liver.  When the ampulla of Vater is open, bile flows freely into the duodenum.  When closed, bile backs up into the cystic duct and into the gallbladder.  The smooth muscle of the gallbladder aids flow toward the duodenum by contracting with stimulation by cholecystokinin.
Within the abdominal cavity, the gallbladder most commonly lies anterior to the first and second portion of the duodenum, hepatic flexure, and stomach. Variations in gallbladder shape are common.  Folds within the body of the gallbladder occur frequently.  Septations and duplication of the gallbladder are rare and make diagnoses in pathologic conditions difficult. 

III.  Scanning Technique and Normal Findings

Gallbladder: Curvilinear abdominal probes with a frequency ranging from 2-5 MHz are ideal for examination of the gallbladder.  The lower end of this range may be necessary for sufficient penetration in larger patients.  However, sonographers should increase the frequency whenever possible as the evaluation of wall thickness, pericholecystic fluid, and gallstones is significantly improved with better resolution.
The gallbladder is identified with three basic approaches: The “subscostal sweep” , the “X minus 7” and the "flattening the probe" approach.  The subcostal sweep is generally the most effective window and is usually attempted first.  Start the scan with the probe in longitudinal orientation and the probe-indicator oriented toward the patient’s head and instruct the patient to take a deep breath.  Sweep the probe inferiorly and laterally along the subcostal margin (Video 1).

Video 1

Video clip 1:  Subcostal sweep video (includes audio).

The X-Minus 7 approach is an intercostal window.  Find the xiphoid process and move laterally to the right approximately 7 centimeter.  Place the probe perpendicular to the skin between the ribs.  In most cases, the gallbladder will be found posterior to the liver parenchyma immediately beneath the probe.  In the few instances where the gallbladder is not identified, move the probe laterally, sweeping through the liver (Video 2).

Video 2

Video clip 2:  Video clip showing the “X-minus 7” approach (includes audio).

In many young patients in a supine position, the gallbladder will be very anterior and cranial.  In these cases, it is often helpful to point the indicator toward the patient’s right and flatten the probe against the abdomen while aiming the beam toward the right shoulder.  Fan the beam anterior to posterior to identify the gallbladder (Video 3).

Video 3


Video clip 3:  Video clip showing the “Flattening the probe” technique (includes audio).

With any of the three views, once the gallbladder is identified, stop moving the probe and make small adjustments to create the best long-axis view.  In the long-axis, the gallbladder will usually appear as a pear-shaped, hypoechoic structure with a hyperechoic wall.

Video 4

Video clip 4:  Gallbladder long axis: Note the main hepatic fissure extending from the tapered end of the gallbladder to the cross-section of the portal vein.  The entire complex resembles an exclamation point.

Because the gallbladder sits in the fossa created by the right and left main lobes of the liver, the main hepatic fissure appears as an echogenic line that extends from the neck of the gallbladder to the portal vein and serves as a landmark.  The complex of the gallbladder, main hepatic fissure, and portal vein (in the short-axis) has the appearance of an exclamation point (Video 4).  Fan the ultrasound beam through the entirety of the gallbladder to identify any pathology.
As with any organ of the body, the gallbladder should be viewed in two planes.  After the long-axis is thoroughly examined, pivot the probe 90 degrees and demonstrate the short-axis (Video 5).  In the short-axis, the gallbladder will appear spherical.  Again, scan through the entire organ in the short axis (Video 6).



Video clip 5:  Rotating the probe (includes audio).

Video 6

Video clip 6:  Gallbladder short axis:  The video demonstrates fanning through the short axis of the gallbladder from the neck to the fundus.

The gold standard to evaluate the g allbladder wall thickness is evaluated from the short-axis.  Measure the anterior gallbladder wall at its most narrow point.  It is important to obtain a view while the probe and ultrasound beam are perpendicular to the gallbladder wall.  If an oblique section of the wall is measured, the reading will be falsely elevated (Video 7).

Video 7

Video clip 7:  Measuring the gallbladder wall:  To properly measure the gallbladder wall, view the organ in the short-axis and freeze the image on the narrowest portion of the wall.

Importantly, the normal gallbladder may be contracted.  The lumen of the gallbladder will appear very narrow in these cases and the wall will be thickened with a characteristic three layer appearance.  The inner and outer walls are echogenic while the middle layer is relatively hypoechoic (Video 8).



Video clip 8:  Contracted gallbladder wall:  This image demonstrated the classic appearance of the contracted gallbladder wall.  Note the three distinct layers of the wall.  The wall is thickened, but pathologic thickening will not demonstrate the three layers of the wall.

Common Bile Duct (CBD): The CBD is most easily identified through its association with the portal vein and the portal vein is most easily identified in the long axis of the gallbladder.  It is the ‘point’ of the exclamation point that is created with the gallbladder in the long-axis.  Follow the main lobar fissure from the neck of the gallbladder to the porta hepatis.  The portal vein will appear as a large, hypoechoic circle with echogenic walls.  The CBD and hepatic artery will appear as two smaller circles anterior to the portal vein.  Often times, it gives the appearance of a face with two ears – also called a ‘Mickey Mouse’ sign (Video 9).

Video 9

Video clip 9:  Mickey Mouse sign:  The video shows an enlarged view of the portal triad in the short-axis.  The CBD and hepatic artery form the ears of the Mickey Mouse sign.  When power flow Doppler is added, the portal vein (large circle) and the hepatic artery (Mickey’s left ear) demonstrate flow.

With the indicator directed toward the patient’s right, the right ear will be the common bile duct and the left ear the hepatic artery.
The best way to evaluate the CBD is in the long axis as gallstones, strictures, or external compression may occur at any point in the tubular structure.  Maintain the Mickey Mouse sign in the center of the screen and rotate the probe 90 degrees without changing the location of the probe on the patient’s skin.  In the long axis, three echogenic lines will stretch across the screen.  The line closest to the probe will be the anterior wall of the common bile duct.  The second line is the shared anterior wall of the CBD and portal vein.  Finally, the third line is the posterior wall of the portal vein.  If there is confusion differentiating between the CBD and portal vein, color flow or power Doppler can be used.  The portal vein will demonstrate flow and the CBD will not (Video 10).

Video 10

Video clip 10:  Measuring the CBD:  This video demonstrates the porta hepatis in the long-axis with power flow Doppler.  The CBD lies in the long-axis just anterior to the portal vein.  The Doppler is not absolutely necessary, but it is helpful as the hepatic artery is sometimes visualized instead.  This CBD is not measured in the video, but is of normal caliber.

Video 11

Video clip 11:  This video shows a normal CBD. The clip has three sections: first, three important structures are demonstrated in regular B-Mode: The long, black structure closest to the bottom of the image is the inferior vena cava (IVC) coursing beneath the liver.  Above that is the portal vein. The portal vein is tortuous in this video and the black circular structure at the end of the portal vein to the left is the continuance of the portal vein.  Finally, above the portal vein is the narrow CBD.  In the second part flow is demonstrated using power Doppler. Note the flow through the IVC and several sections of the portal vein. In the third part of the clip, magnification (Zoom function) is applied in B-Mode and the CBD is measured.

Follow the CBD as far distally as possible by moving the probe medial in relation to the patient.  It is very difficult to follow the CBD, as it passes posterior to the second portion of the duodenum.  It is often identified within the head of the pancreas medially.  Keep in mind that pathology other than choledocholithiasis can result in a dilated CBD and remain suspicious for tumors in the head of the pancreas.
The CBD should be measured at its largest diameter in the long-axis.  Calipers should be used to measure from the interior margin of the anterior wall to the interior margin of the posterior wall.

IV. Pathology

There are five major pathologic findings in the ultrasonographic diagnosis of acute cholecystitis:

1.  Gallstones/Sludge:  Gallstones are evident in 90-95 % of acute cholecystitis and likely play a role in the development of gallbladder cancers as well.  They are demonstrated on ultrasound with a thin, echogenic rim with pronounced shadowing obscuring the tissues behind.  Small gallstones may not shadow.  In such cases, increasing the frequency will improve resolution and shadowing may become apparent.  Most often, gallstones are mobile and will “roll” to the most dependant portion of the gallbladder.  This phenomenon may be demonstrated on ultrasound by maintaining a view of the gallbladder while a patient is rolled to a new position such as left lateral decubitus.  Note the location of the gallstones in relation to the neck of the gallbladder.  Stones in the neck of the gallbladder may be more likely to cause cholecystitis.
Gallstones come in many shapes and sizes.  Some will be only a couple of millimeters in diameter while others will grow to larger than 2 centimeters.  Sometimes only a single gallstone will be present, while other patients will have multiple stones (Video 12-18).

             
  Video 12     Video 14  
 
Video clip 12
 
Video clip 13
 
Video clip 14
 

Video clip 12:  Large gallstone: The video shows a large gallstone within the lumen of the gallbladder.  The gallstone reflects virtually all of the sound waves.  Therefore, the edge of the stone is very bright on the screen and the tissues behind the stone are obscured by shadows because no sound waves make it past the stone. Video clip 13:  Multiple gallstones:  Multiple echogenic foci within the gallbladder lumen with shadowing behind. Video clip 14:  Multiple gallstones:  Three or more gallstones lie within the gallbladder with shadowing.

 

       
  Video 15   Video 16  
 
Video clip 15
Video clip 16
 

Video clip 15:  Full of stones:  This gallbladder is full of stones.  Many echogenic foci with shadowing are seen within the lumen of the gallbladder. Video clip 16:  Layer of stones:  Notice the very bright line of stones at the lower left aspect of the gallbladder with shadowing behind. 

 

       
  Video 17   Video 18  
 
Video clip 17
Video clip 18
 

Video clip 17:  Very small stone:  This video shows a very small stone that we missed in our department on first pass.  Can you find it? Video clip 18:  SIN sign:  This gallstone lies in the neck of the gallbladder. The so-called SIN (Stone In the Neck) sign may be more indicative of cholecystitis in the correct clinical scenario.

 

Gallbladder wall polyps may be confused with gallstones.  They are soft tissue masses attached to the wall of the gallbladder and differentiated from gallstones by their lack of mobility and shadowing (Video 19).

Video 19

Video clip 19:  Polyp:  This patient has multiple gallbladder polyps adhering to the wall.

 The position of polyps in the gallbladder is important as they may cause acute cholecystitis if they lodge in the neck of the gallbladder. 
Gallbladder sludge is likely made up of very small stones making the bile viscous and giving the bile echogenicity.  Sludge is identified on ultrasound as slightly hyperechoic material forming a meniscus within the gallbladder lumen.  Sludge may be a precursor to gallstones and has been related to pathology such as acute cholecystitis and acute pancreatitis (Video 20).

Video 20


Video clip 20:  Sludge:  This gallbladder is full of sludge.  Several small gallstones are also found toward the neck of the organ.

2. Sonographic Murphy’s Sign:  The sonographic Murphy’s sign differs from Murphy’s sign identified on physical exam (arrest of inspiration on deep palpation of the right upper quadrant).  Sonographic Murphy’s sign is positive when the point of maximal tenderness is identified in the right upper quadrant while the gallbladder is identified on the ultrasound monitor.  Multiple points in the epigastrium and right upper quadrant must be tested with the ultrasound probe when the gallbladder is not demonstrated in order to properly evaluate this sign.  The sensitivity of the sonographic Murphy’s sign is reported from 75-86 % with a positive predictive value of 92 % when combined with the finding of gallstones.(15,17)

3.  Gallbladder Wall Thickness:  The normal gallbladder wall measures less than 4 mm.  As detailed above, the gallbladder wall is measured at the most narrow point of the anterior wall in the short-axis.  Care must be taken to not measure the wall at an oblique angle (Video 21).
The gallbladder wall may be thickened in many disease states.  Acute cholecystis is the most common of these. Ascites and congestive heart failure are the second and third most common cause of gallbladder wall thickening.  Hepatitis may also cause gallbladder wall edema. Gallbladder wall cancers may show a thickened and/or calcified gallbladder wall.

Video 21


Video clip 21:  Thick wall:  This patient’s gallbladder wall is thickened to 7.1 mm.  Note the difference between the gallbladder wall
edema andthe contracted wall we looked at earlier.  There is also a small wedge of pericholecystic fluid on the left.

4.  Pericholecystic Fluid:  Pericholecystic fluid (PCCF) is generally found in wedges around the acutely inflamed gallbladder wall.  It is most often seen posterior to the gallbladder at the around the neck, but may also be seen layering on the anterior wall.  Ascites makes evaluation of pericholecystic fluid due to gallbladder inflammation impossible, as the patient will have free fluid throughout their abdomen, including around their gallbladder (Video 22).

Video 22


Video clip 22:  Pericholecystic Fluid (PCCF):  This patient has a thick gallbladder wall and a small
wedge of free fluid to the left of that thickened wall.  It is subtle, but note the dark area next to the gallbladder.

5.  Dilated Common Bile Duct:  The CBD may dilate when obstructed by a stone, a mass, or a stricture.  The normal width of the CBD is 4 mm.  Older patients may have a normally dilated duct up to 1mm for every decade past the age of 40.  The CBD may be dilated up to 1cm normally after cholecystectomy (Video 23 & 24). 

 

       
  Video 23    
 
Video clip 23
Video clip 24
 

Video clip 23:  Dilated CBD:  This patient’s CBD is enlarged to over 1 cm.  The IVC demonstrates flow
near the bottom of the screen.  Above that is the portal vein.  The enlarged CBD is next. Video clip 24:  Large CBD:  A massively dilated CBD may be very disorienting.  Watch how these sonographers use power Doppler ultrasound to differentiate the biliary tract from the vasculature in the liver.

In the setting of acute cholecystitis, the common hepatic duct may be dilated due to inflammation of the gallbladder wall neck and cystic duct causing external pressure. This is a rare complication termed Mirizzi syndrome and may lead to hyperbilirubinemia.
Gangrenous and emphysematous cholecystitis are serious complications of acute cholecystitis that may be identified with ultrasound by the presence of air within the gallbladder wall or lumen (Video 25).  Air on ultrasound is represented by “comet-tail” artifacts. Gallbladder perforation may also be diagnosed by ultrasound.  Findings of perforation include significant amounts of pericholecystic fluid that may contain echogenic material which may be walled off from the rest of the abdomen.


Video clip 25:  Air in the GB wall:  There are comet-tail artifacts extending down
from this gallbladder wall indicating air due to emphysematous cholecystitis.

 

 

V.  Pearls and Pitfalls

1.  Duodenal Air: Because the second portion of the duodenum lies just behind the posterior wall of the gallbladder, air in the duodenum can inhibit the ability to successfully image the gallbladder.  Furthermore, this “slice” of sound actually has a thickness of 1.5 mm.  As it is directed towards the posterior wall of the gallbladder, some of this sound is reflected off the duodenum and the image produced is a combination of the two organs.  Because air can cause shadowing, it can appear that there is a gallstone lying on the posterior wall when in reality this is air in the second portion of the duodenum (Video clip 26 and 27).

2.  Renal Cyst: The kidney is closely related to the gallbladder and cysts in the superior pole of the kidney may be confused for the gallbladder.  This is one reason why it is important to view the gallbladder in both a short and long axis.  The renal cyst will appear the same in both axes, while the gallbladder elongates in a long axis.

3.  Ascites: Free intraperitoneal fluid, regardless of the etiology, will appear anechoic.  Be careful not to mistake ascitic fluid for pericholecystic fluid.  Ascitic fluid is typically located throughout the peritoneum, including Morrison’s pouch, whereas pericholecystic fluid is localized to the anterior side of the gallbladder.  Patients with ascites also have markedly thickened gallbladder walls not associated with an inflammatory process.  Keep in mind that fluid that is found posterior to the gallbladder can be the result of a ruptured gallbladder.

4.  WES Sign- Wall-Echo-Shadow:  When a gallbladder is contracted around a gallstone, sometimes the only visualization of this is a shadow coming out of the liver.  This is due to the stone’s reflection that obscures the rest of the gallbladder.  The three layers of the gallbladder wall of the anterior gallbladder are generally seen, followed by the echogenic stone which is followed by the shadow caused by the stone.

5.  Patient Positioning: Typically the images are best obtained with the patient in a supine position, although rolling the patient into a left lateral decubitus position may be helpful.  This is thought to move the second portion of the duodenum away from the posterior wall of the gallbladder reducing the negative effect of duodenal air.

6.  Obtain Multiple Windows: The “X minus 7” approach refers to using an intracostal window in probes that have smaller footprints. This approach is especially useful in the morbidly obese patient.  Furthermore, it is important to view the gallbladder in both the short and long axes.  This can help in not mistaking the gallbladder for the IVC, renal cyst, or portal vein.  In general the gallbladder will appear in a perpendicular axis to the portal vein.  The “exclamation point sign” helps to draw this concept home.  When the gallbladder is in the long axis (the top of the exclamation point), the portal vein is in the short axis (the point of the exclamation point).

7.  Probe Orientation: With the indicator aimed towards the patient’s head the gallbladder is generally shown in the long axis.  As anatomical variation is common with the biliary system, this orientation may need to be adjusted in order to obtain this window.  Keep in mind that with biliary scanning the orientation of the gallbladder is with respect to how the organ appears on the screen, not necessarily the position of the probe on the patient.  The short axis of the gallbladder is typically found with the indicator towards the patient’s right

 

 

Video 26


Video clip 26:  Duodenum mimicking gallbladder filled with stones:  The duodenum adjoins the posterior aspect of the liver.  At times, it may appear as the gallbladder filled with stones.  The air in the duodenum appears as “dirty shadowing” .
The shadows are very indistinct, rather than the clean shadows behind stones.

Video 27

Video clip 27:  Gallstone vs. Duodenum:  The duodenum sits very close to the gallbladder.  So close
that the thickness of the ultrasound beam may pick up parts of both, superimposing the duodenum
onto the gallbladder in the image.  This can cause problems because the duodenum may shadow due to air
leading the sonographer to believe there is a gallstone within the gallbladder.

Video 28


Video clip 28:  WES sign:  The Wall-Echo-Shadow sign can make it very difficult to identify and evaluate the gallbladder.  There is a very strong echo at the rim of the stone and wall of the gallbladder.  The shadow is very thick and clean and extends through the liver.

 

 

VI.  References

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Gallstones. In: Digestive diseases in the United States: Epidemiology and impact US Department of  Health and Human Service. Public Health Service, National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases,1994.NIH Publication no. 94-1447.

3  Powers RD, Guertler AT.
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4  de Dombal FT.
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7  Diehl AK.
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8  Berger MY, Olde Hartman TC, Bohnen AM.
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Festi D, Sottili S, Colecchia A, Attili A, Mazzella G, Roda E, Romano F. 
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10  Thijs C, Knipschild P.
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11  Kraag N, Thijs C, Knipschild P.
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12  Singer AJ,  McCracken G, Henry MC, Thode HC Jr, Cabahug CJ.
Correlation among clinical, laboratory, and hepatobiliary scanning findings in patients with suspected acute cholecystitis. Ann Emerg Med.1996;28(3):267-72.

13  Bree RL, Ralls PW, Balfe DM, DiSantis DJ, Glick SN, Levine MS, Megibow AJ, Saini S, Shuman WP, Greene FL, Laine LA, Lillemoe K.
Evaluation of patients with acute right upper quadrant pain. American College of Radiology. ACR Appropriateness Criteria. Radiology.2000;215(Suppl):153-7.

14  Shea JA, Berlin JA, Escarce JJ, Clarke JR, Kinosian BP, Cabana MD, Tsai WW, Horangic N, Malet PF, Schwartz JS, et al.
Revised estimates of diagnostic test sensitivity and specificity in suspected biliary tract disease. Arch Intern Med.1994;154(22):2573-81.

15  Ralls PW, Colletti PM, Lapin SA, Chandrasoma P, Boswell WD Jr, Ngo C, Radin DR, Halls JM.
Real-time sonography in suspected acute cholecystitis. Prospective evaluation of primary and secondary signs. Radiology.1985;155(3):767-71.

16  Bree RL.
Further observations on the usefulness of the sonographic Murphy sign in the evaluation of suspected acute cholecystitis. J Clin Ultrasound.1995;23(3):169-72.

17 Kendall JL, Shimp RJ.
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18 Menakuru SR, Kaman L, Behera A, Singh R, Katariya RN.
Current management of gall bladder perforations. ANZ J Surg.2004;74(10):843-6.

19 Fagan SP, Awad SS, Rahwan K, Hira K, Aoki N, Itani KM, Berger DH.
Prognostic factors for the development of gangrenous cholecystitis. Am J Surg.2003;186(5):481-5.

20  Contini S, Corradi D, Busi N, Alessandri L, Pezzarossa A, Scarpignato C.
Can gangrenous cholecystitis be prevented?: a plea against a "wait and see" attitude. J Clin Gastroenterol.2004;38(8):710-6.

21 Ryu JK, Ryu KH, Kim KH.
Clinical features of acute acalculous cholecystitis. J Clin Gastroenterol.2003;36(2):166-9.

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