Imaging features of gallbladder and biliary tract disease

Biliary hamartoma

Bile duct hamartomas are common benign tumors composed of disorganized bile ducts and ductules surrounded by a fibrocollagenous stroma. The tumors are generally multiple, range from 1 to 15 mm, rarely communicate with the biliary tree, and are scattered throughout the liver. They are most often confused with cysts, although they may also be mistaken for metastases or microabscesses with delayed marginal enhancement on CT or MRI. ^, On MRI, they appear cystic ( Fig. 16.5 ), and may show a small internal mural nodule related to the fibrocollagenous component (see Chapters 47 , 48 , and 88 ).

Bile duct adenoma

Bile duct adenomas are rare benign epithelial neoplasms, usually incidentally detected and asymptomatic. They are usually solitary and without specific imaging findings, although hyperenhancement has been reported in some series. Definitive diagnosis can be made only at histologic analysis. Internal heterogeneous enhancement has been reported on MRI, with reported hypointensity on delayed hepatobiliary phase imaging using hepatocyte contrast agents (see Chapters 47 and 48 ).

Cholelithiasis

Ultrasound is usually the modality of choice in the evaluation of uncomplicated cholelithiasis or cholecystitis due to its high sensitivity and lower cost, but choledocholithiasis is more effectively imaged by MRI. On MRI, gallstones are well-circumscribed, low-signal filling defects within a fluid-filled gallbladder or common duct on T2-weighted and MRCP images ( Fig. 16.6 ). Coronal T2-weighted imaging, performed routinely with MRI, readily identifies common duct stones. MRCP can also be obtained to evaluate whether retained stones are present after cholecystectomy, although surgical clip artifact may limit visualization of the adjacent portion of the main bile duct. A negative MRCP may obviate the need for ERCP (see Chapters 33 and 34 ).

Given that most gallstones in Western countries are mixed cholesterol stones and noncalcified, CT is limited for stone detection, but it may reveal unsuspected gallstones during studies performed for other reasons. On CT, gallstones are visible when either calcified or containing material of substantially lower attenuation than the surrounding bile (such as trapped nitrogen gas or high cholesterol content). Newer dual energy CT techniques have shown promise in detecting noncalcified cholesterol stones and acute cholecystitis. ^,

On ultrasound, gallstones are echogenic, mobile, and demonstrate posterior acoustic shadowing when imaging is optimized and when 3 mm or greater in size ( Fig. 16.7 ). Ultrasound is technique and operator dependent. It is important to scan patients in different positions to differentiate gallstones from polyps, as polyps are fixed. Optimized Doppler analysis may also help, as stones are associated with “twinkle artifact” and polyps often show vascularity. Stones fixed in the gallbladder neck are frequently associated with cholecystitis. If large stones or multiple stones fill the entire gallbladder lumen, there may be little surrounding bile, limiting ultrasound evaluation due to acoustic shadowing. Identification of the “wall echo shadow” sign, produced by echoes from the anterior gallbladder wall, echogenic stones, and posterior acoustic shadowing produced by the stones, is helpful ( Fig. 16.8 ). A porcelain gallbladder has echogenic calcification in the gallbladder wall. Gallbladder sludge is echogenic nonshadowing bile that sometimes takes on a rounded shape called “tumefactive” sludge. Sludge can obscure the interfaces of small stones. Sludge is avascular, and it usually changes with positional variation, albeit slowly.

Choledocholithiasis and biliary obstruction

MRCP is the imaging modality of choice for choledocholithiasis, as it has the highest sensitivity and specificity. ^, Intrahepatic calculi are rare in Western countries, but they most frequently occur in association with iatrogenic bile duct strictures ( Fig. 16.9 ). Bile duct calculi appear as intraluminal filling defects on MRCP, or echogenic foci on ultrasound ( Fig. 16.10 ). Calculi may form or reflux into intrahepatic ducts, and small calculi can be mistaken for air. Because there is little bile surrounding intraductal calculi, and because the stones may be small, acoustic shadowing may not always be elicited on ultrasound, which has poor sensitivity in detecting choledocholithiasis. On CT, dense intraluminal calcification or a target sign representing a halo of bile surrounding a higher attenuation stone are reliable indicators of intraductal calculi. Cholesterol stones may blend imperceptibly with surrounding bile, although dual-energy CT may have value in demonstrating these stones as either hypoattenuating or hyperattenuating relative to bile, depending on the energy level. In addition, a minority of patients with choledocholithiasis have no biliary ductal dilatation (see Chapter 37, Chapter 38, Chapter 39 and 44 ).

On ultrasound, debris or thick bile within the ducts may cause internal echoes within ducts or fluid levels, but they do not shadow and will shift with positional variation. Adherent intraductal clot as well as intraductal tumors often show no associated acoustic shadowing and will not shift with position. ^, Obstruction as a result of biliary ascariasis is associated with tubular structures within the bile duct, and movement of the worms is pathognomonic (see Chapter 45 ). MRI and MRCP are superior to CT, which is of less value when stones are small and noncalcified and bile duct dilation is minimal or absent. Intrahepatic choledocholithiasis may have an unusual appearance, with segmental or subsegmental biliary radicles filled with calculi (see Chapter 44 ). In Asian patients with recurrent pyogenic cholangitis who subsequently form bile pigment stones, the debris filling the biliary system generally has higher attenuation than normal bile on CT. Marked bile duct dilation is present, and often the larger intrahepatic ducts are dilated without side-branch dilation. Eccentric and diffuse extrahepatic bile duct wall thickening is usually seen.

In biliary obstruction, both the pattern of obstruction and appearance of the duct wall are useful for diagnosis. A spectrum of chronic progressive, cholestatic disorders exists, with etiologies varying from recurrent infection to autoimmune disorders, and unknown. Recurrent pyogenic cholangitis is related to repeated bacterial infections and is evidenced by dilated ducts with intraductal calculi and segmental dilatation ( Fig. 16.11 ) (see Chapter 44 ). Lobar atrophy may also be present. Of note, intraductal papillary mucinous tumor of the bile ducts may be confused with recurrent pyogenic cholangitis on imaging, because both diseases involve repeated episodes of incomplete biliary obstruction and evident intraluminal masses or filling defects (see Chapters 47 and 51 ).

Sclerosing cholangitis may be primary and of unknown etiology, or secondary and due, for example, to autoimmune disorders, infection, or ischemia (see Chapter 41 ). It causes a beaded appearance of the ducts with wall thickening and enhancement, strictures, and discontinuous areas of dilatation ( Fig. 16.12 ); dilated ducts contain debris such as pus, sludge, or sloughed epithelium. Primary sclerosing cholangitis (PSC) carries an increased risk of cholangiocarcinoma, and MRCP remains the most sensitive and specific noninvasive imaging modality to assess these patients and to detect concomitant cholangiocarcinoma. ^, Mural thickening of the ducts also is seen with HIV-associated cholangiopathy; however, HIV-associated cholangiopathy often shows added papillary stenosis, a finding not typically seen in PSC.

In patients with biliary obstruction who are being considered for surgical resection or palliative biliary drainage, the distribution of ductal dilatation should be carefully evaluated to determine management. Any isolated biliary ductal segments that do not communicate with the main ducts should be noted, because isolated segments may alter surgical approach, and biliary drainage may require placement of multiple catheters.