Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

LIVER

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Technique

Cholangiography

Intravenous

Although intravenous (IV) cholangiography currently is rarely performed in the United States, in some European countries and Japan it is part of a preoperative workup for suspected stones. Computed tomography (CT) drip infusion cholangiography, however, has supplanted this procedure considerably.

Endoscopic Retrograde Cholangiography

Endoscopic retrograde cholangiography (ERC) has evolved into a major diagnostic and therapeutic modality. It is a common method for performing diagnostic cholangiography, followed if necessary by sphincterotomy, biliary stone extraction, stent insertion for biliary decompression, or biopsy. Disadvantages are that it is very operator dependent, the cholangiographic image quality is often poor, and more complex therapeutic modalities are available only in certain centers. Although the final diagnosis is image based, in many institutions quality control is limited and little radiologist involvement is evident.

Cholangiography establishes the presence of a biliary obstruction. A subtle partial obstruction, however, can be missed with cholangiography, regardless of how it is performed.

Computed Tomography

Development of multislice CT has led to a major improvement in bile duct visualization. Twoand three-dimensional (3D) bile duct reconstructions can be obtained without the use of a cholangiographic agent in a majority of patients, although a biliary contrast agent does improve bile duct visualization and is often employed. Either a cholecystographic or a cholangiographic contrast agent is used, resulting in cystic duct visualization in a majority of patients. The study is performed during a single breath hold, and CT data are then reconstructed to obtain a variety of images: multiplanar reconstruction and volume rendering images. These images can then rotated and viewed from a number of directions.

Bile CT attenuation is 0 to 30 Hounsfield units (HU). Water absorption by gallbladder mucosa changes bile concentration (specific gravity) considerably. Likewise, the presence of sludge, calcium, or contrast material increases bile attenuation.

Computed tomography 10 to 12 hours after oral administration of a cholecystographic contrast agent (iopanoic acid) visualizes extrahepatic and major intrahepatic bile ducts in most individuals even after a cholecystectomy (1,2); 3D volume rendering reconstructions aid in evaluating the bile ducts, including assessing position of surgical clips in patients with postcholecystectomy syndrome.

Another approach is slow infusion of intravenous cholangiographic contrast and CT per-

formed 30 to 60 minutes after infusion. Resultant biliary images are superior to those obtained with conventional intravenous cholangiography. Such studies yield good-quality studies even in children.

Indications for CT cholangiography are still evolving. Preliminary evidence suggests that CT cholangiography is somewhat superior to MRCP in visualizing small biliary stones. The test is not useful in jaundiced patients because insufficient contrast is excreted into the bile ducts.

Magnetic Resonance

Magnetic resonance (MR) is discussed in more detail in Chapter 7. A fast spin echo (FSE) sequence separates solid from fluid-containing structures. Nonflowing fluid–filled structures such as bile ducts are hyperintense on FSE T2weighted images, while a solid structure is hypointense. A fat-suppression technique helps accentuate bile ducts and related structures and is often employed.

Bile ranges from hypoto hyperintense on T1-weighted sequences, approaching water intensity when dilute and becoming hyperintense when concentrated. At times a fluid–fluid layer is apparent, with the more concentrated hyperintense bile being dependent.

A potentially useful modality is intrabiliary MRI. An internal receiver coil, commercially available and used for esophageal and other internal structure MR imaging,is passed through a biliary tube into the region in question.

Magnetic resonance cholangiopancreatography is a noninvasive imaging technique of visualizing the biliary and pancreatic ducts. Resultant images are similar to those obtained with endoscopic retrograde cholangiopancreatography (ERCP), and MRCP is evolving as an alternative both to diagnostic ERCP and diagnostic percutaneous cholangiography (Fig. 8.1). An obvious sequela after introduction of MRCP has been obviation of much of diagnostic ERCP. Magnetic resonance cholangiopancreatography is feasible after a choledochojejunostomy.

Two MR approaches are feasible to visualize bile ducts: an intravenous contrast-assisted technique and a technique without contrast by using heavily T2-weighted images to make nonflowing fluid (i.e., bile) hyperintense to surrounding structures.

Figure 8.1. Normal magnetic resonance (MR) cholangiography in a patient with suspected pancreatitis. (Courtesy of Patrick Fultz, M.D., University of Rochester.)

A contrast-assisted technique is feasible because such IV contrast agents as manganese (II) N,N’-dipyridoxylethylenediamine-N,N’- diacetate-5,5’-bis phosphate (Mn-DPDP) and gadolinium ethoxybenzyl diethylene-triamine- pentaacetate (Gd-EOB-DTPA) are taken up by hepatocytes and excreted (secreted) into bile ducts. Using T1-weighted sequences, resultant contrast-containing bile is hyperintense, and the gallbladder and bile ducts are then visualized using 2D or 3D image manipulation techniques. The major limitation of this procedure, similar to previously popular intravenous cholangiography, is that reasonable hepatocyte function is required to accumulate enough biliary contrast to be visualized. Thus in a setting of jaundice due to bile duct obstruction and impaired hepatic function—a common sce- nario—this technique is unsatisfactory. On the other hand, in a postoperative patient, contrastMRCP appears superior to noncontrast-MRCP in detecting subtle biliary leaks.

Bile ducts can be opacified if an indwelling biliary tube is in place. A simple technique is injection of dilute Gd-DTPA and the use of T1weighted sequences. Or bile ducts can be distended with saline rather than gadolinium.

A contrast-less technique has attracted most attention and is commonly called MR cholangiopancreatography (MRCP). This technique is possible because by using heavily T2-weighted

GALLBLADDER AND BILE DUCTS

pulse sequences, stationary fluid has a high signal intensity while surrounding soft tissues are suppressed and appear hypointense. Bile has a prolonged T2 relaxation time, compared to surrounding soft tissues, and thus on heavily T2-weighted sequences bile and pancreatic secretions are bright, surrounded by dark background tissue. Also, with heavily T2weighted pulse sequences flowing blood has low to no signal, similar to solid structures. Thus blood vessels are not visualized with this technique.

Initial work with MRCP involved a heavily T2-weighted gradient echo sequence providing a steady state of free precession (SSFP) signals, and this technique appeared promising in the early 1990s. A limitation of this technique is the inability to visualize small fluid collections, to the point that normal bile ducts and pancreatic ducts are not seen. Another limitation is the relatively long scanning time required and the resultant motion artifacts.

A non–breath-hold heavily T2-weighted 2D FSE sequence was refined in the mid-1990s. The FSE sequences are less sensitive to motion and even nondilated ducts are routinely visualized. The 3D FSE uses thinner slices than is possible with 2D sequences, improving image quality. Other improvements include use of surface coils, a fat saturation technique, and image acquisition during quiet respiration.

Comparing 3D SSFP and 2D FSE imaging in patients with bile duct obstruction, FSE visualizes more extrahepatic and intrahepatic bile ducts than SSFP; likewise, the pancreatic duct is better seen with FSE.

The use of rapid acquisition with relaxation enhancement (RARE) and half-Fourier acquisition single-shot turbo spin echo (HASTE) techniques allows imaging during a single breath-hold. The HASTE sequence with MRCP has been refined to the point that one slice can be obtained in 2 seconds. A limitation of this technique is that slice thickness is greater than achievable with the FSE sequence. Whether better definition of selective respiratory triggered 3D turbo spin echo (TSE) outweighs the simplicity of breath-hold single-shot MRCP is not clear, but both techniques define bile ducts.

Magnetic resonance cholangiopancreatography is feasible even with a 0.2-tesla (T) unit. Fewer stones are detected with such a low field

unit compared to a 1.5-T unit, but obstructive sites can be identified. A 0.5-T magnet can identify biliary obstructions and detect over 90% of stones.Acquisition times with MRCP performed using a midfield unit range up to 5 minutes or longer, depending on the resolution desired.

Some radiologists employ tap water as an oral contrast agent, but removing gastrointestinal tract fluid signals from MRCP images by using a negative oral contrast agent is preferred. Barium sulfate, iron gluconate, iron oxide agents, ferric ammonium citrate, and similar agents improve bile duct visualization.

Magnetic resonance cholangiopancreatography is technically difficult in children. However, IV sedation often suffices for MRCP, while ERCP in children requires general anesthesia. The MRCP success rates in children average about 80% to 90%, similar to those for ERCP. Non–breath-hold one-shot MRCP appears advantageous in children.

Initially, MRCP was advocated for patients with a failed ERCP. Some enthusiastic researchers have touted MRCP accuracies of 100% in identifying pancreaticobiliary disease, but in the process causing more harm than good for this procedure. Yet even more critical analyses have achieved high sensitivities and specificities. A 3D FSE technique can provide diagnos- tic-quality images in over 90% of patients, detecting bile duct dilation, strictures, and intraductal abnormalities. Both coronal images and 3D images rotated at different angles are useful. An axial plane tends to better outline the most distal common bile duct and pancreatic duct segments. Both dilated and nondilated extraand main intrahepatic bile ducts are routinely imaged with MRCP. More peripheral normal side branches usually are not visualized.

In patients with suspected bile duct obstruction, MRCP initially achieved about 75% sensitivity and 75% specificity in diagnosing obstruction but did not detect stones <3mm in diameter if the bile ducts were not dilation (3). These rather pessimistic results were later balanced by a number of more positive studies: which achieved sensitivities approaching 100% and specificities >90% in detecting biliary or pancreatic duct obstruction. In a typical study, MRCP detects all obstructions due to stones >3 mm in patients after failed or inadequate ERCP, yielding an overall sensitivity and specificity of over 95% (4).

Small calculi tend to be masked by partial volume averaging, a condition partly overcome by thinner collimation. Thin sections are also useful in evaluating strictures and bilioenteric anastomoses. The relevance of periportal edema in bile duct wall visualization is not clear.

Current indications for MRCP are an unsuccessful ERCP, a contraindication for ERCP, pregnancy, or prior anastomosis precluding ERCP access. More indications continue to be added, however, and some investigators predict that MRCP will replace ERCP for most diagnostic questions and ERCP will be relegated to a therapeutic role. The primary advantages of MR cholangiography over ERCP are its noninvasive nature, more reproducible results (i.e., less operator dependent), no sedation required, and the ability to outline ducts proximal to an obstruction.With suspected biliary obstruction, MRCP often not only confirms an obstruction but also identifies the level and suggests an etiology.

Current limitations to better MRCP duct visualization are motion artifacts, surgical clips close to the ducts, and artifacts such as air bubbles within ducts. Currently, spatial resolution of MRCP is inferior to that obtained with technically excellent ERCP, although it is probably on a par with or better than some of the suboptimally calibrated digital ERCP systems in use in a number of institutions. Spatial resolution of some MR units limits its role in characterizing bile duct stenoses and in visualizing papilla of Vater tumors. Subtle changes of sclerosing cholangitis or primary biliary cirrhosis are not evident. False-positive results, including an appearance resembling a stricture, are produced by imaging artifacts, and to minimize them optimal image manipulation is crucial.

Percutaneous Transhepatic Cholangiography

For a percutaneous approach to right lobe ducts, most radiologists use fluoroscopic guidance. If access to a left lobe duct is desired, a combination of US and fluoroscopy is convenient. Prior liver CT, especially with contrast enhancement, provides useful guidance for needle placement and subsequent biliary drainage.

Overlap of some intrahepatic ducts tends to limit their visualization. Generally by turning the patient into an oblique position duct overlap can be minimized; it is often more convenient to

ADVANCED IMAGING OF THE ABDOMEN

rotate the x-ray tube-image intensifier assembly (if feasible) rather than the patient to achieve similar results.

Operative Cholangiography

Operative cholangiography is discussed under the respective therapies in the Acute Cholecystitis section. During laparoscopic cholecystectomy, cholangiography is typically performed by cannulating the cystic duct. Gallbladder puncture and contrast injection into the gallbladder instead of cystic duct cholangiography results in worse bile duct visualization and common duct stone detection.

T-Tube Cholangiography

Placement of an intraoperative T-tube allows routine postoperative T-tube cholangiography. Commonly these patients are placed on prophylactic antibiotic therapy. Is such therapy necessary? The literature provides limited guidance. In the United States a hand-held syringe injection technique is generally used to perform T-tube cholangiography. In Europe an infusion technique is preferred.

Other Techniques

In some patients ERCP is technically not feasible. One proposed approach in such a situation is the use of endoscopic ultrasonography (US) to locate the common bile duct and guide a transduodenal aspiration needle into the duct.

Ultrasonography

Prior to laparoscopic cholecystectomy, conventional US readily detects gallstones, a gangrenous gallbladder, and abnormalities in adjacent structures. It not only determines the caliber of intraand extrahepatic bile ducts, but also occasionally detects anomalous ducts and their insertions. Prior to initiating oral bile acid therapy, cystic duct patency can be inferred from US performed before and after a fatty meal.

In endoscopic US a transducer placed in the distal portion of the descending duodenum outlines periampullary structures and portions of the pancreatic head. The distal common bile duct and main pancreatic duct can be visual-

GALLBLADDER AND BILE DUCTS

ized. The more superior portion of the pancreatic head, gallbladder, cystic duct, and common bile duct are studied through the duodenal bulb. Compared to conventional US, the higher frequencies used in endoscopic US provide good resolution of near objects, but more distal porta hepatis structures are poorly visualized. Also, an enlarged or calcified pancreatic head limits common bile duct visualization.

Miniaturized probes are available for endoluminal bile duct US. A probe is introduced transhepatically if a percutaneous biliary drain is in place, through a surgically inserted T-tube, or through a duodenoscope into the bile ducts. Probes 2.0mm in diameter use a frequency of about 20MHz, achieving a spatial resolution of <1mm. Intraluminal, intramural, and adjacent structures are readily visualized. Such intraluminal US has limited depth penetration and should be combined with other imaging techniques for nodal and metastatic staging.

Preliminary results of 3D intraductal US are promising.

An US probe has been introduced into the gallbladder using an endoscopic transpapillary approach; a guidewire through the papilla aids probe insertion. An intraductal sonographic probe can be inserted into the gallbladder fundus in a majority of patients, but indications for this study are not clear.

Oral Cholecystography

Oral cholecystography (OCG) was the traditional imaging study for detecting gallbladder abnormalities, but with the introduction of US and scintigraphy it has almost disappeared. A nonopacified gallbladder after two consecutive contrast doses was considered evidence of gallbladder disease, assuming that extrabiliary causes for nonopacification are excluded. The earlier literature suggesting an accuracy rate for OCG of almost 100% was overoptimistic; nevertheless, OCG is accurate when stones are clearly visible in a contrast opacified gallbladder. Overall, the accuracy of OCG is probably close to 80% to 90%.

Currently the primary role for OCG is as a supplement to cholecystosonography or cholescintigraphy in those patients for whom the latter studies do not show a gallbladder abnormality in the face of strong clinical suspicion of

underlying disease. In addition, in patients considered for extracorporeal gallstone lithotripsy, OCG has a role in counting the number of stones present, determining their size, and establishing cystic duct patency.

Cholangioscopy

While an endoscopic approach to the bile ducts is preferable, occasionally this technique is not feasible. Percutaneous transhepatic cholangioscopy provides biliary access and allows direct duct visualization, cholangiography, and biopsy.

Cholangioscopy can be performed for lithotripsy of biliary stones.

Scintigraphy

Magnetic resonance cholangiopancreatography and ERCP provide bile duct morphology while cholescintigraphy primarily evaluates function. These tests are thus complementary and, depending on clinical indications, at least 2 of these are often necessary for adequate biliary evaluation.

A number of iminodiacetic acid (IDA) tracer compounds are in use worldwide for cholescintigraphy, but in the United States technetium-99m-DISIDA and Tc-99m–BrIDA are available. Intravascularly, these compounds bind to albumin, are taken up by hepatocytes, are excreted into bile canaliculi, and appear in extrahepatic bile ducts 30 minutes or so after injection. In most patients small bowel activity is evident within 60 minutes.

The relative amounts of tracer flowing into the duodenum and gallbladder depend on sphincter of Oddi pressure and gallbladder tonicity. Pharmacologic agents modify flow to either the gallbladder or the duodenum. For instance, morphine sulfate induces sphincter of Oddi contraction and thus tracer flow is preferentially into the gallbladder. Likewise, IV cholecystokinin (CCK) is indicated to contract the gallbladder prior to cholescintigraphy in fasting patients and as an aid for suspected sphincter of Oddi dysfunction or acalculous cholecystitis. In a setting of a dilated duct, CCK aids in excluding an obstruction.

Cholecystokinin also provides data for measuring gallbladder ejection fraction. An ejection