Gallbladder and Bile Ducts



CHAPTER 13 Gallbladder and Bile Ducts




CLINICAL CONSIDERATIONS


Disorders of the biliary tree and gallbladder commonly require imaging evaluation for diagnosis and management, although imaging findings are often nonspecific when patients present with suspected biliary disease. For example, gallbladder wall thickening may be entirely incidental and benign (adenomyomatosis), life-threatening (gallbladder cancer), or unrelated to primary gallbladder disease (congestive heart failure). When interpreting nonspecific imaging findings, it is helpful to interpret them within the appropriate clinical context. For this reason, we briefly discuss clinical presentations of common diseases of the gallbladder and biliary tract.





Cholangitis








ANATOMY OF THE BILIARY SYSTEM






NORMAL IMAGING APPEARANCE OF THE BILIARY SYSTEM


The normal intrahepatic bile ducts course along with the portal veins and appear as thin structures that may be visible on either side of the accompanying vein on imaging studies. The right and left hepatic ducts are usually less than 3 mm in diameter. With improvements in imaging techniques, intrahepatic ducts are routinely visible with a variety of imaging modalities. The common bile duct usually measures less than 6 mm in diameter, although larger ducts are occasionally visible in patients without bile duct obstruction. There is significant variability in the literature regarding the location from which extrahepatic bile-duct measurements are taken. With ultrasound (US), the bile duct is typically measured at the level of the right hepatic artery, although the maximum duct diameter may be a more useful measurement. Advanced age has been associated in some studies with increased common duct diameter, although most elderly patients have a common bile duct diameter less than 6 mm. Likewise, several studies have linked prior cholecystectomy with increased bile duct diameter, although several other studies have failed to confirm a clinically relevant effect of cholecystectomy on common bile duct diameter. When the common bile duct is dilated from choledocholithiasis, the bile duct will return to normal in three fourths of patients after choledochostomy.


For better or worse, many radiologists have adopted 6 mm as the common bile duct diameter threshold beyond which further evaluation is indicated, particularly when clinical evidence for pancreaticobiliary disease exists. Many radiologists will also accept a common bile duct diameter of 8 or even 10 mm in an elderly patient in the absence of other radiological or clinical evidence for pancreaticobiliary disease. Be aware that the effect of age and prior cholecystectomy on bile duct diameter is a subject of controversy


With US, intrahepatic bile ducts appear as thin, anechoic tubes, although usually only the first- (right and left hepatic ducts) and second-order bile ducts are readily visible with US. The ducts usually appear on the ventral side of the portal vein with US, although color Doppler or spectral analysis may be necessary to distinguish between blood vessels and bile ducts. Harmonic imaging may improve conspicuity of the bile duct and its contents.


With computed tomography (CT), the intrahepatic ducts are thin structures with nearly imperceptible walls that parallel the portal veins. The bile within the ducts normally measures fluid attenuation and appears homogeneous. On magnetic resonance (MR) images, the normal bile ducts parallel simple fluid on all pulse sequences. The signal intensity of the gallbladder contents varies on T1-weighted images but usually appears relatively bright on T2-weighted images.


The normal gallbladder wall measures less than 3 mm in thickness. A small fundal gallbladder division (Phrygian cap) may be present as an incidental finding.



GALLSTONES AND SLUDGE



Gallstones


Gallstones are common, particularly in the Western hemisphere. They can be associated with biliary colic, acute or chronic cholecystitis, bile duct obstruction, cholecystenteric fistula formation, and gallbladder carcinoma.


Abdominal radiographs show gallstones as rounded densities in the right upper quadrant. However, radiographs may miss up to 85% of gallstones. Occasionally, gas within gallstones is visible on radiographs. Signs of rare complications of cholelithiasis such as pneumobilia, gallstone ileus, or emphysematous cholecystitis can occasionally be detected with abdominal radiographs.


Transabdominal US is highly sensitive for the diagnosis of uncomplicated cholelithiasis. Larger (>5 mm) stones appear as echogenic foci with strong posterior acoustic shadowing (Fig. 13-4). The color comet tail artifact (“twinkle artifact”) can be helpful for confirming the presence of a stone, and the intensity of this artifact is related to the surface characteristics of the stone. Isolated stones smaller than 5 mm may not demonstrate acoustic shadowing but can be differentiated from polyps by their mobility. In general, when echogenic foci are detected with sonography, one should image in several different patient positions to confirm mobility of the abnormality (Fig. 13-5). Large stones or collections of smaller stones may completely fill the gallbladder, making the gallbladder difficult to identify. The WES (wall-echo-shadow) complex has been described as a means of differentiating gallstones filling the gallbladder from other abnormalities such as emphysematous cholecystitis or porcelain gallbladder, or structures such as the colon (Fig. 13-6). Before invoking the WES complex, one must ensure that the wall is seen as a distinct entity. When air or calcium is present in the gallbladder wall, a normal wall is not visualized. Instead, only an echogenic line and posterior shadow are seen. Stones impacted within the neck or cystic duct of the gallbladder may not be outlined by anechoic bile and can be missed. Therefore, it is always important to examine the neck region and cystic duct to detect evidence of posterior acoustic shadowing and color comet tail artifact. Cystic duct stones, in particular, are a relatively frequent cause of false-negative US results for cholelithiasis.







CT is considerably less sensitive than US but more sensitive than radiographs for detection of gallstones. CT is primarily used in patients with abdominal pain when acute cholecystitis is not the prime consideration and should not be relied on to exclude the presence of gallstones. On CT, gallstones range from hypodense (pure cholesterol stones) to hyperdense and may occasionally contain gas. It is not unusual for even large stones to be missed on technically excellent CT images (Fig. 13-7).





Although MRI is rarely performed as the primary means of diagnosing cholelithiasis, MRI is highly sensitive for the detection of gallstones, particularly when motion-insensitive T2-weighted images are performed. Gallstones are typically very low signal intensity on T2-weighted images and variable (very dark to very bright) signal intensity on T1-weighted images.


Occasionally, dropped gallstones will be encountered on imaging examinations after cholecystectomy (usually laparoscopic cholecystectomy). These most often accumulate in the subhepatic space but may be found as far away as the pelvis (Fig. 13-8). The most frequent complication of dropped gallstones is abscess formation. Abscesses related to dropped gallstones can present months to years after cholecystectomy.




Other Substances That Fill the Gallbladder







THICK-WALLED GALLBLADDER


Diffuse thickening of the gallbladder wall is one of the most common abnormalities seen on an US examination of the right upper quadrant. This is due to the myriad of processes that result in gallbladder wall thickening. Many of the causes of gallbladder wall thickening are listed in Table 13-1.


Table 13-1 Causes of Gallbladder Wall Thickening










































Cause Diagnostic Clues (Variably Present)
Adenomyomatosis (Fig. 13-11)

Adjacent inflammation
Carcinoma (Fig. 13-12)



Cholangiopathy/cholangitis (e.g., acquired immune deficiency syndrome cholangiopathy, sclerosing cholangitis)
Cholecystitis (Fig. 13-13)







Cirrhosis




Congestive heart failure (Fig. 13-14)





Gallbladder torsion (Fig. 13-15)



Hepatitis (Fig. 13-16)



Hypoalbuminemia
Nondistention

Varices (Fig. 13-17)






CHOLECYSTITIS


Cholecystitis refers to inflammation of the gallbladder. A number of variants of cholecystitis warrant discussion.



Mar 6, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Gallbladder and Bile Ducts
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