Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

GALLBLADDER AND BILE DUCTS

risk for gallstones. Gallbladder emptying is reduced in patients with cholesterol gallstones.

Although an occasional patient has only bile duct stones and no gallbladder stones, most stones are of gallbladder origin. Primary bile duct calculi develop in a setting of strictures, cholangitis, choledochal cyst, or a congenital condition such as Caroli’s disease.

A foreign body in the extrahepatic bile ducts acts as a nidus for stone formation. Case reports describe suture material and surgical clips acting as a nidus. Clip migration from its original position is occasionally diagnosed from serial conventional abdominal radiographs. Computed tomography or cholangiography confirms the diagnosis. Sphincterotomy and clip or stone extraction should be curative.

An association among initial gallstones, cholecystectomy, and subsequent development of colorectal cancer has been raised. Currently no definitive conclusions can be drawn; some studies suggest no association, other studies suggest a relationship, while still other studies suggest such an association only in women.

Gallbladder sludge probably is a factor in subsequent gallstone formation. Still, the presence of sludge is generally not believed to represent underlying gallbladder disease.

Cystic duct anatomy has been implicated in stone formation. Comparing ERC in patients with and without gallstones, stone formers have significantly longer and narrower cystic ducts than those without stones (81); also, the angle between the gallbladder and cystic duct is more acute in those with gallstones than those without.

Stone Composition

Calculi range from single to multiple, large to small, round, oval, or faceted. They contain primarily cholesterol, pigment, or a mixture of cholesterol and pigment. Pigmented stones contain a high proportion of bilirubin and tend to be considerably smaller than cholesterol stones. If a stone contains sufficient calcium to be visible with conventional radiography, it contains a predominance of pigment rather than cholesterol. Central calcifications also point toward pigment composition. In general, the composition of multiple gallstones within one gallbladder is similar, but exceptions occur.

An inverse correlation exists between CT attenuation and cholesterol content and a direct correlation among pigment contents, inorganic calcium salts, and total calcium content (82); most cholesterol stones are hypodense, but in practice many stones are mixtures. Stones containing more than about 3% calcium are hyperdense.

Oral cholecystography suggests cholesterol composition if stones are radiolucent; if multiple stones without calcification are present, they are large or have a calcified rim. Similar signs are also useful with CT to determine gallstone composition.

Only a rare stone has a specific gravity less than that of bile and thus floats in bile. Ultrasonography thus rarely detects a floating stone. On the other hand, with iodinated contrast within the gallbladder, stones composed primarily of cholesterol are buoyant. Presumably iodine increases bile-specific gravity sufficiently so that cholesterol stones float. Thus the most reliable sign of cholesterol stones with an oral cholecystogram is buoyancy. This effect of iodine upon buoyancy can also be shown with US if the examination is done after administering an appropriate oral contrast agent; with sufficient contrast in the gallbladder, cholesterol stones float.

Primary bile duct stones (i.e., stones that form in bile ducts) are pigmented.

A rare gallstone contains a drug. Specific drugs include ceftriaxone, glaphenine, and dipyridamole.

Cholecystolithiasis

Imaging

Most radiologists believe that CT is inferior to US in visualizing stones in the gallbladder. The CT appearance of a gallstone depends on its cholesterol and calcium content and ranges from that of a hypodense defect surrounded by bile, to isodense to bile and thus not visible, to an obvious calcification.

Ultrasonography should detect almost all gallbladder calculi, with detection depending on stone size rather than composition. An intraluminal, mobile, echogenic mass detected by US and associated with posterior acoustic shadowing is virtually pathognomonic for a calculus. A gallbladder filled with stones results in a hyper-

echoic tumor superficially mimicking gas-filled bowel; a porcelain gallbladder has a similar appearance. A diagnosis of stones should be made with caution in the absence of acoustic shadowing, although shadowing is absent with small calculi. Shadowing is not dependent on presence of calcifications. Sludge is mobile but generally does not have posterior shadowing. Sludge can be inhomogeneous in appearance.

Intravenous cholangiography also detects most gallbladder stones, but less than with US.

One study detected 96% of stones with 3-D fast SGE and single-shot SE MR sequences (83); a prevalent view among some MR enthusiasts is that MRI is superior to US in detecting subtle gallstones, yet the relationship between stone composition and its MR signal intensity is poorly understood. Magnetic resonance imaging reveals most gallbladder stones as round or faceted signal void (hypointense) structures on both T1and T2 spin-echo (SE) images surrounded by the higher signal intensity bile. Some stones contain hyperintense foci on both T1and T2-weighted sequences. An occasional stone is hyperintense on T1-weighted sequences. Stones do not enhance postcontrast, allowing differentiation from polyps.

Cholescintigraphy is insensitive in detecting gallbladder stones. Adjacent bowel compressing the gallbladder may mimic a gallstone.

Therapy

A percutaneous cholecystostomy tract can be used for gallbladder access for stone dissolution using methyl-tert-butyl ether, stone fragmentation, or stone extraction. Most of these procedures have been supplanted by laparoscopic cholecystectomy, although they still have a place in select, poor surgical risk patients.

Most medical therapies for gallstones are effective with cholesterol stones only. Therefore, prior to such therapy cholesterol stones must be distinguished from pigmented gallstones, keeping in mind that overall over 80% of radiolucent stones are composed primarily of cholesterol.

ADVANCED IMAGING OF THE ABDOMEN

stones. In general, fragments smaller than 3mm in diameter pass spontaneously. Extracorporeal shock-wave lithotripsy achieves 65% to 85% clearance at 3 years, with success rate being inverse to stone volume and stone size. Patients with a single stone have significantly higher clearance rates and lower recurrence rates than those with multiple stones.

For stones with a CT density of <50HU, stone clearance can be achieved in most patients, but with a density of >100HU only a minority of patients are completely cleared. About a third of patients have posttherapy side effects such as biliary colic. Fragment impaction at the papilla of Vater can result in pancreatitis and require sphincterotomy. Complication rate increases with larger fragments.

In some centers ESWL is combined with bile acid dissolution therapy (oral ursodeoxycholic acid), although the value of adjuvant bile acid therapy is not clear.

Cholescintigraphy shows better fragment clearance associated with a higher gallbladder ejection fraction and a larger gallbladder discharge volume than in patients who retain stone fragments.

Cholecystolithotomy

High-risk patients with acute calculus cholecystitis can be treated by percutaneous cholecystostomy followed by cholecystolithotomy. After initial percutaneous gallbladder puncture and drainage, the tract is dilated and stones are extracted. Usually an interval of 4 to 6 weeks is allowed between initial gallbladder puncture and eventual dilation and stone removal to allow tract maturation. Large stones are crushed using a wire basket technique.

Cholecystolithotomy success rate of clearing gallstones is over 80%. These patients are left with an intact gallbladder and initially without gallstones. After such percutaneous stone removal, recurrent gallstones and biliary sludge are common.

Dissolution

Lithotripsy

Extracorporeal shock-wave lithotripsy (ESWL) is used in select centers to shatter gallbladder

Gallstone dissolution is feasible through direct contact with methyl-tert-butyl ether or similar substances instilled via an appropriate catheter,

GALLBLADDER AND BILE DUCTS

and is performed in a number of European centers. Usually US is used for catheter guidance. In a multihospital European study of 803 patients, puncture was successful in 95% and in those with successful puncture stones were dissolved in 95% (84); residual sludge remained in 44% of gallbladders. The most severe complication was bile leakage in 1.6% of patients, treated with a cholecystectomy.

Once gallstones are dissolved with methyl- tert-butyl ether, patients generally undergo long-term bile acid therapy. Cessation of adjuvant bile acid therapy leads to a recurrence of stones. The 5-year stone recurrence rate is about 40% for solitary stones and 70% for multiple stones (116).

Ethyl propionate, a C5 ester, has also been used to dissolve cholesterol gallstones; clinical experience with this agent is limited.

At times an opacified rim appears in a gallstone during chemolitholysis; it consists of calcium carbonate.

Bile acid therapy is also used to dissolve gallstones. Not all gallstones, however, dissolve. Most hypodense or isodense stones respond to bile acid therapy, while those with laminated or rimmed calcifications rarely do. Thus in vivo CT analysis of gallstones can predict the response to bile acid therapy.

Choledocholithiasis

Biliary Colic

Biliary colic is a clinical description of the severe pain experienced by patients with biliary obstruction by stones. Sphincter of Oddi spasm also plays a role in this condition because manometry reveals significantly higher pressures during a biliary colic attack.

Gallbladder volume during biliary colic is several times greater than during the postcolic state.

Preoperative Detection

Early published sensitivities for detecting choledocholithiases with CT and transabdominal gray-scale US have been up to 90%, although most later studies report considerably lower sensitivities. Computed tomography with thin

collimation and data acquisition in a single breath-hold should prevent misregistration. However, a number of bile duct stones are isodense to bile and thus not detected, a situation differing from urinary stones where even radiolucent stones are visible with CT. Specificity, however, is >90% for both CT and US. In a study of patients with suspected choledocholithiasis, the sensitivities for detecting choledocholithiasis were 65% for unenhanced CT, 92% for oral contrast-enhanced CT cholangiography (using iopanoic acid), and 96% for MRCP (85); the specificities did not differ significantly. It is because of this low CT sensitivity that unenhanced CT has never achieved the popularity it has reached with ureteral stone detection, having been supplanted initially by ERC and now by MRCP, which has a stone detection accuracy similar to that of ERC.

Although US detects only some bile duct stones, it does have a role prior to laparoscopic cholecystectomy. In a study of open cholecystectomy with routine intraoperative cholangiography for cholelithiasis, the most important preoperative predictor of bile duct stones was US detection of a dilated common bile duct (86); the authors defined three levels of risk:

1.Low: common bile duct not dilated; prevalence of bile duct stones was 1.5%

2.Moderate: dilated common bile duct but normal liver function tests; prevalence of stones was 49%

3.High: both dilated common bile duct and abnormal liver function tests; prevalence of stones was 67%

One recommendation is that ERCP should be performed if the common bile duct diameter is more than 6 to 7mm for possible stone extraction. Yet dissenting voices are also heard. Thus among consecutive patients undergoing laparoscopic cholecystectomy in a single surgical unit, 10% had stones and almost half of patients with stones had small ducts (<6mm as measured by preoperative US) (87); the author cautions that a preoperative US finding of a nondilated common bile duct does not imply the absence of stones.

In an occasional patient noninvasive studies are inconclusive; ERCP similarly is inconclusive or contraindicated and percutaneous cholan-

Figure 8.39. Common bile duct stones (arrows) in a jaundiced patient with dilated ducts.

giography is necessary if the diagnosis is in doubt (Fig. 8.39).

Computed Tomography

Computed tomography shows most extrahepatic bile duct calculi to be of soft tissue density; most are sharply marginated and most have a higher density than adjacent bile. An occasional obstructing stone is isodense to bile, with CT showing an abrupt termination to the column of bile; the appearance mimics that of malignancy. At times, thin collimation reveals crescentappearing bile adjacent to a stone.

Ultrasonography

Conventional US can identify the entire common bile duct course in about 80% of patients, but detection of bile duct stones achieves only about a 60% to 70% sensitivity.

Endoscopic US achieves >90% sensitivity and almost 100% specificity in detecting bile duct stones regardless of size or bile duct caliber, and is at least as sensitive as ERC, but the need for endoscopy introduces its own complexities. Nevertheless, the complication rate for endo-

scopic US is significantly lower than with diagnostic ERCP.

Endoscopic US reveals bile duct stones as intraductal hyperechoic foci with, in most, acoustic shadowing. Residual debris can also be hyperechoic but typically little or no shadowing is evident.

Intraductal US has had limited application. It can detect extrahepatic bile duct stones, but its role is yet to be established.

Magnetic Resonance

Can MRI detect bile duct stones? Making study comparisons difficult is that different imaging techniques achieve different results. T2weighted MR sequences identified stones in 78% (41), results better than with US.

Magnetic resonance cholangiopancreatography reveals choledocholithiasis better than conventional MRI, with a number of publications suggesting that detection rates are comparable to ERCP. In fact, in some studies MRCP achieves a higher sensitivity in detecting intrahepatic stones than ERCP (88). In general, MRCP sensitivities are 80% to 100% and specificities are 95% to 100% for detecting extrahepatic bile duct stones, although some studies have achieved a sensitivity of only about 60% (89). As one example, MRCP using non–breath-hold 3D fast spin echo (FSE), breath-hold single section half-Fourier RARE and breath-hold multisection half-Fourier rapid acquisition with relaxation enhancement (RARE) sequences all achieved similar sensitivities and specificities exceeding 90% in detecting bile duct stones (90). Evidence suggests that volume-rendered MRCP is superior to maximum intensity projection or thick-section MRCP for stone detection (89). Magnetic resonance cholangiopancreatography tends to be less optimal if the bile ducts are not dilated or patients cannot hold their breath during scanning. Stones not surrounded by bile, such as those close to the papilla, tend to blend into surrounding structures and even mimic a stenosis.

On T1-weighted images most stones are hyperintense to bile, while on T2-weighted images most are hypointense. Magnetic resonance cholangiopancreatography, relying on heavily T2-weighted images, reveals bile duct stones as hypointense foci within high signal intensity bile; stones as small as 2mm in diam-