103 Hepatobiliary Imaging
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103 Hepatobiliary Imaging Abdominal Plain Films doned as other modalities such as ultrasound, computed tomography (CT), and magnetic resonance imaging Abdominal plain films are usually used in the initial eval- (MRI) have improved. uation of disease. Specific information concerning hepa- tobiliary disease cannot generally be gained from plain films. This being said, important information can be gained from plain films. This information includes calci- Radionuclide Imaging fications and gas shadowing. Calcifications can be asso- ciated with hemangiomas, granulomatous disease, and Liver scintigraphy is used to evaluate a variety of metastatic neoplasms. Chronic cholecystitis can be asso- hepatic diseases, ranging from malignancy to cholecysti- ciated with gallbladder wall calcification, called porcelain tis. The technique is limited by its lack of resolution gallbladder. The risk of gallbladder cancer is greater with and its higher cost. Other modalities, such as CT, MRI, a porcelain gallbladder. and ultrasound, are less time consuming and less expen- 99m Overlying gas shadowing can indicate a number of sive. Technetium-labeled sulfur colloid can be used to hepatic conditions. Portal venous gas can indicate an evaluate for hepatic metastases. Hemangiomas may be 99m ischemic process such as bowel infarction, necrotizing visualized with tagged red cell scans ( Tc-labeled red enterocolitis, hemorrhagic pancreatitis, or ulcerative blood cell). Hepatobiliary iminodiacetic acid (HIDA) colitis. The portal venous system is generally demon- scans can provide some important information concern- strated in the periphery of the liver as branching linear ing gallbladder function when evaluating patients for gas shadows. Abscesses can appear as large collections of suspected biliary colic or cholecystitis. These scans overlying gas. Pneumobilia can be the result of an incom- involve the administration of iminodiacetic acid labeled 99m petent sphincter of Oddi or a gastroduodenal ulcer or can with Tc (such as diisopropyl IDA or DISIDA). This occur after enteric anastomosis. radionucleotide is taken up in the liver and excreted within the biliary system. The test is commonly used to evaluate the contractibility of the gallbladder, the gall- bladder ejection fraction (normal is 55% to 75%), and Oral Cholecystography the patency of the biliary system. Failure to opacify the gallbladder signifies biliary obstruction, and decreased Oral cholecystography is an older imaging technique ejection fractions (<55%) suggest biliary dyskinesia. that requires the patient to drink oral contrast. Once There are many factors that can cause false-positive ingested, the contrast material is absorbed through the results, including hepatitis, cirrhosis, pancreatitis, pro- intestines and excreted into bile. The gallbladder con- longed fasting, and hyperalimentation. The study is also centrates the contrast material contained in the bile. This helpful for evaluating for postoperative bile leaks and method allows for visualization of stones or wall abnor- congenital disorders such as biliary atresia. Radionu- malities that appear as filling defects. Gallbladder dys- cleotide imaging is also helpful in the workup of malig- function is evident through the failure to opacify the nant diseases. Neuroendocrine tumors can be located gallbladder and failure to contract. The accuracy of using radioactive somatostatin analogues (111-indium oral cholecystography for diagnosing gallbladder disease octreotide). This is especially useful for localizing gastri- approaches 100%. The technique has largely been aban- nomas preoperatively. 240 103. Hepatobiliary Imaging 241 Ultrasound systolic velocity, and finally to the entire loss of arterial waveforms. Ultrasound is the most common noninvasive test of the liver. Ultrasound is close to 100% accurate for evaluat- Computed Tomography ing gallbladder and biliary disease. Gallstones are intra- luminal in ultrasound images. Characteristics commonly Parenchymal imaging of the liver is better with CT than associated with acute cholecystitis include a thickened with ultrasound. The specifics and background of CT are gallbladder wall, pericholecystic fluid representing discussed in Chapter 16. Computed tomography scans of edema, distention, gallstones, sludge, and sonographic the liver can be done as noncontrasted or as single-, dual- Murphy’s sign. In addition, the biliary tree (intra- and , or triple-phase intravenous contrast studies. The type of extrahepatic) can be evaluated for signs of obstruction study needed is determined by the specific disease and dilation. Common bile duct dilatation (<6mm) can process being evaluated. Diffuse hepatic diseases may be be seen with ultrasound, although the cause may not better detected with noncontrast imaging. Intravenous always be identified (i.e., common duct stones or malig- contrast can obscure diseases such as hemochromatosis nancy). Ultrasound is quite good for evaluating gallblad- and choledocholithiasis. der and biliary disease but begins to lose accuracy when Single-phase studies are done to visualize focal hepatic examining hepatic masses, especially with cirrhosis. lesions (e.g., trauma, malignancy). Single-phase studies Hepatic metastasis can be overlooked in 50% of cases. acquire images approximately 1 minute after contrast is Therefore, ultrasound of the liver is a poor tool for administered (peak enhancement of the liver). Intrahep- excluding liver metastasis. When a focal mass is seen in a atic masses enhance at a lower intensity than does the cirrhotic liver, the chance of malignancy is about 98%. normal surrounding liver tissue. Single-phase studies are Doppler ultrasound is a useful tool for evaluating the used for demonstrating biliary dilatation. hepatic vascular system. Its use is indicated for conditions In dual-phase studies, images are acquired before and such as portal vein thrombosis, evaluation after liver after intravenous contrast enhancement. This allows for transplant, and follow up after transjugular intrahepatic imaging of the liver during hepatic artery enhancement portal-systemic shunt (TIPS) placement. Portal vein and portal venous enhancement.This technique is helpful thrombosis is diagnosed by the absence of flow through for staging malignancies and determining the resectabil- the portal vein seen on Doppler ultrasound. Other asso- ity of hepatic lesions. The evaluation of hepatocellular ciated findings include dilation of the vein or the pres- carcinoma is easier than more laborious methods such as ence of echogenic material within the lumen. In some iodized oil CT (poppy seed oil uptake by hepatocellular instances the portal vein is replaced by venous collater- carcinoma) and CT arterial portography. Triple-phase als when thrombosis is long standing. Pulsatile blood flow studies involve capturing images before and during arte- from within the venous thrombosis can be a sign of neo- rial and portal venous phases of enhancement. plastic invasion. Computed tomography can also be useful for detect- Doppler ultrasound helps exclude extrahepatic portal ing abnormalities of the biliary system. Dilated bile ducts vein obstruction and determine baseline flow velocities can be clearly seen with single-phase contrast imaging. before TIPS placement is attempted. The function of the Primary sclerosing cholangitis is associated with beaded TIPS is evaluated at regular intervals (every 3 months for dilations of the intrahepatic ducts. Cholangiocarcinoma 1 year) with Doppler ultrasound to monitor for hepatic often appears with central duct obstruction along with vein stenosis. Velocities less than 60cm/sec suggest steno- intrahepatic ductal dilation. sis. Many institutions implement routine Doppler ultra- sound evaluation of hepatic vasculature after liver transplantation. In particular, the hepatic artery is exam- ined for signs of stenosis caused by the associated mor- Magnetic Resonance Imaging bidity and mortality. The most common indication of stenosis is an intrahepatic tardus parvus waveform (i.e., Magnetic resonance imaging is used primarily as an prolonged systolic acceleration time [>0.1 second] and adjunct to other imaging modalities. The basic concepts low resistive index [>0.5 second]). Thrombosis of the behind MRI are discussed in greater detail in Chapter 16. hepatic artery is indicated by the absence of arterial Magnetic resonance imaging relies on the variations in waveforms in the hepatic artery and liver. Postoperative water content among tissue types to differentiate them. edema can produce false-positive results. Thrombosis Hepatic malignancies tend to have higher water content may be predicted when a series of Doppler ultrasound than the surrounding liver parenchyma, which produces examinations show normal hepatic artery flow initially a relatively low signal on T1-weighted images. Unfortu- that progresses to loss of diastolic flow, then to decreased nately, the liver parenchyma also tends to produce a low 242 Part XII. Gastrointestinal Disorders signal on T1 images, making the distinction between examined in detail. Additional information concerning normal and neoplastic tissue difficult to resolve. T2- the spread of the tumor can be obtained, which is useful weighted scans are generally done following T1 to for staging the malignancy. provide better contrast between the two tissues (i.e., bright signal in tumor, low signal in normal liver) and in conditions such as fatty infiltration of the liver, which can Invasive Imaging Procedures be confused with a neoplasm.