LIVER, BILIARY TRACT, AND PANCREAS: INTRODUCTION

The diagnosis of diseases of the liver, biliary tract, and pancreas optimally depends on using both clinical and radiographic data. Understanding the proper use of these data and ordering radiographic studies in the optimal sequence are helpful for making the diagnosis most efficiently. Frequently, the clinical presentation and associated laboratory work provide most of the clues for diagnosis. Physical examination, history, and pertinent laboratory values are often helpful in making the diagnosis or at least in providing clues for selecting the optimal radiographic studies. If clinical information is insufficient or if radiographic confirmation is necessary, plain films and contrast studies may be performed. Upright and supine plain radiographs are helpful for the detection of free air, calcifications, and other abnormalities. Contrast studies such as endoscopic retrograde cholangiopancreatography (ERCP) and percutaneous transhepatic cholangiography (PTC) are often helpful in analyzing diseases of the liver, biliary tree, and pancreas. For instance, pancreatic or biliary ductal systems, fistulae from these ductal systems, and associated abnormalities such as encasing tumors can be diagnosed by cholangiography.

Digital cross-sectional imaging, nuclear medicine (NM) and an important form of NM called positron emission tomography (PET), and angiography have provided considerable information in analyzing diseases of these organs, which cannot be directly visualized with plain radiography, even using traditional contrast material, i.e., barium. Cross-sectional techniques consist of ultrasound (US), computed tomography (CT), and magnetic resonance (MR) imaging. This chapter reviews the use of cross-sectional imaging and, where pertinent, nuclear medicine and angiography to evaluate abnormalities of the liver, biliary tract, and pancreas.

TECHNIQUES AND NORMAL ANATOMY

Ultrasonography

Ultrasonography utilizes a high-frequency sound wave transmission through the body. A transducer is used both to emit and to receive a very high-frequency sound (3.5–12 MHz). The technique employs a radar-like detection of objects within the beam, in which the high-frequency sound waves are bounced off the objects and detected by the transducer. These signals are relayed to a computer, which displays a two-dimensional image in whatever plane is defined by the orientation of the transducer. The term for the different shades of gray seen within an US image isechogenicity.

Vascular flow can be visualized by US with Doppler imaging, which consists of three types: color, spectral, or power Doppler imaging. In color Doppler imaging, a color-coded display creates color maps of the vessels. Color shade and color intensity reflect blood flow direction and velocity, respectively. Spectral Doppler imaging is portrayed as a sine-wave form in which peaks represent increasing velocity and valleys represent decreasing velocity of flow. This is often combined with color Doppler imaging and together is called duplex imaging. An alternate form of Doppler imaging employing color mapping is power Doppler imaging. Power Doppler imaging is more sensitive than color Doppler to flow velocity, but unlike color Doppler, cannot indicate flow direction.

Nuclear Medicine

Nuclear medicine techniques utilize the administration of radioactively labeled substances chemically bonded to physiologic agents. These combined substances are administered to the patient and travel to the organs that concentrate the physiologic agents. The radioactivity within the labeled substances is then detected with a camera sensitive to the presence of radioactive emissions. A more recent application of NM is PET, in which positrons emitted, often from radioactive sugar-containing compounds, are detected and imaged. The sugar-containing compounds are metabolized more actively by growing malignant tissue as compared to surrounding normal tissue and are, therefore, visible as areas of greater emissions. The term for different shades within a nuclear medicine image is activity.

Computed Tomography

Computed tomography utilizes x-rays and a ring-shaped structure called a gantry. The gantry contains an x-ray tube, which is directed toward a row of detectors on the other side of the gantry. The patient is placed on a table that is incrementally (axial CT) or continuously (spiral CT) shifted through the opening of the gantry. The x-ray tube rotates around the patient, emitting a focused beam that passes through the patient. The attenuated beam is received by the detectors. These signals are transmitted to a computer, which reconstructs a series of two-dimensional images in a transverse plane through the body, much like cutting a loaf of bread.

Blood vessels can be demonstrated by using intravenously injected iodinated contrast material, and the bowel can be demonstrated by means of an orally administered contrast agent. These images can be reconstructed in other planes, termed multiplanar reconstruction, or MPR, or in a three-dimensional, or 3-D, image. Most existing CT scanners have a single detector system that permits the acquisition of a single image at a time with each gantry rotation. Newer CT scanners, called multidetector CT scanners, or MDCT scanners, employ 4 to 16 adjacent detectors, which allow the simultaneous acquisition of multiple images with one gantry rotation. MDCT scanners scan extremely quickly and provide markedly improved image quality by reducing motion artifacts and by permitting the acquisition of much thinner sections. These newer scanners also greatly facilitate the performance of MPR and 3-D scanning. The term for different shades within a CT image is attenuation or density.

Angiography

In angiography, contrast agents are administered intravascularly, and complex radiographic machines trace the injected contrast material through the blood vessels by a rapid sequence of x-ray films or with digital imaging techniques. The term for different shades within an angiographic image is density.

Magnetic Resonance Imaging

Magnetic resonance imaging is a very complex technique that evaluates magnetism within the patient. The device is outwardly similar to a CT unit. The patient is placed on table that carries the patient into a cylinder, which contains a magnet. The magnet emits a radio-frequency pulse that causes the protons of the atoms within the body to line up together. The magnet then emits another radio-frequency pulse, which perturbs the orientation of the protons so that as a group they are flipped to the side. The protons then return to their original orientation, and their return is accompanied by a release of energy. The rate at which the protons return to normal orientation and release energy is defined by the characteristics of the tissue, which in turn are defined by the relaxation times, T₁ and T₂, of the protons. Blood flow and proton density also affect the image. Signals from this process are sent to a computer, which reconstructs either a two-dimensional image in any plane or a 3-D image.

MR imaging has superb contrast resolution, but historically required long data acquisition times. More recently, faster MR imaging techniques have brought imaging times toward alignment with those of CT and are fast enough to reduce or eliminate motion artifacts. Generalized parenchymal lesion detection and characterization, together with MR angiography, or MRA, have been further improved greatly with the use of MR imaging-specific intravenous contrast agents, especially gadolinium, Gd. The term for different shades within an MR image is signal intensity. Flow is identified by signal intensity changes in the blood vessels.

Normal Anatomy

With US, normal organs are displayed as structures of different echogenicity. In general, fluid is anechoic (i.e., has no echoes). Soft tissue has echoes of mild to moderate intensity. Bone has extremely strong echoes. Abnormal organs are displayed as areas of diffuse inhomogeneity or as focal regions of decreased or increased echogenicity within the organ. The normal appearances of the liver, biliary system, and pancreas have been well established. The liver is second to the pancreas in echogenicity among organs in the upper abdomen. The liver typically has homogeneous parenchymal detail (Fig. 11–1). Numerous intrahepatic vessels including portal veins and hepatic veins are easily seen within the liver. The gallbladder appears as an anechoic pear-shaped structure along the inferior aspect of the liver (Fig. 11–2). It normally has a thin, homogeneous wall less than 3 mm thick. The degree of distention of the gallbladder varies with postprandial intervals. The biliary ducts are thin tubes, the walls of which are 1.5 mm or less. The ducts increase in caliber as they extend from the liver to the sphincter of Oddi (Fig. 11–3). The upper limit in caliber of the extrahepatic biliary ducts increases with age. The pancreas is the most echogenic organ in the abdomen (Fig. 11–4). It is homogeneous, comma shaped, and parallel to splenic vein and extends from the left upper quadrant caudally and to the right. In anteroposterior dimension the pancreatic head is 3 cm, the body 2.5 cm, and the tail 2 cm.

With NM techniques, normal organs are displayed as regions of homogeneous activity conforming to the general shape of the organ. Abnormal organs are displayed as diffuse inhomogeneity or as focal areas of reduced or increased activity. In the past, the liver was most commonly studied with NM with technetium-labeled sulfur colloid. However, this technique has largely been replaced by CT, US, and MR imaging. The most common NM study of the liver today utilizes technetium-labeled red blood cells to evaluate for cavernous hemangioma. Evaluation of the biliary system is a common application for NM studies. Technetium-labeled hepatobiliary imaging iminodiacetic acid derivatives, especially disophenin and mebrophenin, are taken up by the liver, excreted into the bile, carried to the biliary tree and gallbladder, and from there travel to the bowel through the extrahepatic ducts (Fig. 11–5). Depending on the exact agent used, these are termed hepatic iminodiacetic acid, (HIDA) scans. Currently no practical imaging of the pancreas is done by means of NM techniques.

With CT, normal organs are displayed as regions of differing attenuation. Abnormal organs are displayed as diffuse inhomogeneity or as focal areas of decreased or increased attenuation. The liver, biliary system, and pancreas are well demonstrated by CT (Fig. 11–6). The liver is the most dense organ in the abdomen. The normal liver parenchyma appears homogeneous, just as in an US image. The portal and hepatic vessels and the biliary ductal system are likewise easy to identify. Overall measurements of wall thickness and biliary duct caliber are the same as for US. The pancreas is easily identified on CT, and the pancreatic duct is frequently well seen.

At angiography, normal organs enhance to variable extents. Abnormal organs either inhomogeneously enhance or have focal areas of decreased or increased enhancement. Although the parenchyma of the normal organs is rarely demonstrated, the blood vessels of these organs are seen in exquisite detail (Fig. 11–7). In the liver, both the hepatic artery and all of its branches can be seen. Delayed studies through the liver in the venous phase demonstrate the portal vein. The cystic artery and any collateral vessels can be angiographically demonstrated. Angiographic studies of the pancreas can demonstrate major pancreatic branches, as well as encasement, displacement, stenosis, or occlusion.

On MR imaging, normal organs have homogeneous signal intensity or well-recognized variations in signal intensity. Abnormal organs have inhomogeneous signal intensity or areas of increased or decreased signal intensity. The normal liver, biliary system, and pancreas are well demonstrated on MR imaging (Fig. 11–8). The liver has a homogeneous signal intensity that is usually higher than that of muscle and lower than that of the spleen. The biliary system is normally demonstrated as an area of low signal intensity on T_l-weighted images and high signal intensity on T₂-weighted images. This appearance reflects the fluid bile within the gallbladder and biliary tree. Magnetic resonance cholangiopancreatography, or MRCP, demonstrates the biliary system as very high signal intensity structures against a very low signal intensity background of surrounding solid tissues (Fig. 11–9). The pancreas is of intermediate signal on both T₁- and T₂-weighted images and may be hard to differentiate from bowel if no oral contrast agent is administered to the patient. As in CT and US, the normal fatty change within the pancreas that occurs with age is visible.

TECHNIQUE SELECTION

Diseases of the liver, biliary system, and pancreas can be conveniently, if arbitrarily, separated into the following categories to help illustrate the optimal sequences of imaging techniques: diffuse hepatocellular disease, focal hepatic diseases, abdominal trauma, inflammatory disease of the biliary tract, and pancreatic inflammation or neoplasm.

Diffuse Hepatocellular Disease

In diffuse hepatocellular disease, CT is probably the first study used to survey the liver because it is moderately sensitive to liver lesions and is also helpful for evaluating surrounding organs. Ultrasound may have application unless fatty liver is present, because fat attenuates the US beam. NM has only infrequent applications. MR imaging may be the most sensitive modality for detecting and characterizing diffuse diseases of the liver, including cirrhosis and hemochromatosis, especially when combined with contrast agents. Angiography may be used to study collateral formation in cirrhosis.

Focal Hepatic Diseases

In focal diseases of the liver, US is often used first, because it is inexpensive, widely available, and moderately sensitive to localized lesions in the absence of preexisting diffuse diseases, such as cirrhosis. It is, however, of limited value in obese patients and whenever air is present, for example, when air-filled bowel obscures the liver. CT is a pivotal examination, often employed after US. It is used as a survey of the entire body, is easy to compare in serial studies, and is sensitive to disease. Air and bone do not interfere with CT examinations. Contrast-enhanced MDCT scanners can be used to perform CT angiography, or CTA, which is a noninvasive means of producing images depicting vessels much like conventional angiography. NM techniques can be used to analyze a focal lesion within the liver for possible cavernous hemangioma. MR imaging is used frequently to characterize focal lesions within the liver, especially those discovered during survey techniques like US or CT.

NM and MR imaging are considered the optimal means for evaluating the liver for cavernous hemangioma, and both are highly accurate (approximately 95%) in evaluating the liver for cavernous hemangioma. In the opinion of some authorities, MR imaging is the optimal means for both detection and characterization of focal liver lesions of all types. Newer MR pulse sequences, contrast agents, and fast scanning techniques arguably make MR imaging the optimal means for both detection and characterization of focal liver lesions of all types. Angiography is primarily used to provide a vascular road map in planning surgery for focal liver lesions.

Abdominal Trauma

CT is the only commonly accepted means for analyzing abdominal trauma, particularly of the liver. CT is reasonably accurate in the detection of trauma-related abnormalities of the liver, biliary system, and pancreas. US may be useful if CT is not available or to quickly identify intraperitoneal hemorrhage in patients who are in the emergency department and are going directly to the operating room. Angiography may be useful to embolize persistently bleeding arteries in the liver or spleen when surgery is not possible. Currently, NM and MR imaging have no application in studying the liver, biliary tract, or pancreas in trauma.

Pancreatic Inflammation or Neoplasm

Ultrasound is often the initial means to study pancreatic inflammation or neoplasm. It is effective in evaluating the pancreas if not interrupted by surrounding bowel gas. If ileus is present, or if a lesion has already been detected by US and additional confirmation is required, CT is the method of choice. NM has no major current application in studying the pancreas. MR imaging may be useful to study endocrine tumors of the pancreas. Recent advances in MR imaging, especially MRCP, have brought MR imaging further to the forefront of pancreatic and biliary duct evaluation. This latter technique highlights fluid-containing structures such as biliary or pancreatic ducts, and voids nearly all signal intensity from background solid structures. Angiography is useful to identify bleeding arteries as a source of hemorrhagic pancreatitis but is occasionally used to identify encasement of arteries in a pancreatic neoplasm.

Patient Preparation for Radiographic Techniques

Generally, these radiographic techniques require little patient preparation. This is convenient, especially in evaluation of trauma. Ideally, a patient should fast after midnight before an US examination. As a minimum, the patient should fast for 6 hours. Patients ideally should fast before CT examinations as well, but this requirement is not crucial. Dilute oral contrast medium for CT is administered at least 2 hours in advance and again just before the examination begins. Intravenous contrast material is often given as a bolus by a power injector immediately prior to the study. Proper laboratory evaluation of renal function, including serum creatinine below 1.5 mg/dL, is usually required before administering iodinated intravenous contrast material since it can be nephrotoxic. Ideally, NM is also performed after fasting. Preparation for angiography again requires fasting and laboratory evaluation of renal function and possible coagulopathy. Proper preparation of patients for MR imaging is controversial. However, some authorities advise administering an iron-containing oral contrast agent and an agent to relax the bowel, such as glucagon, before scanning. No assessment of renal function is necessary because MR contrast agents are not nephrotoxic.

Conflicts among Examinations

These examinations may interfere with each another. No barium should be administered before US or CT. Oral contrast agents may generate bowel gas, decompress the gallbladder, and hinder US. The oral contrast agent administered prior to a CT examination interferes with angiography by obscuring the abdomen. Intravenous contrast material interferes with any subsequent NM tests studying iodine metabolism such as those involving the thyroid gland because intravenous contrast agents contain iodine. Previous angiography usually requires that a CT examination be postponed for a day or two so that residual contrast material within the kidneys can be excreted. Usually, there are no conflicts between these examinations and NM or MR imaging.

EXERCISE 11-1: DIFFUSE LIVER DISEASE

Clinical Histories:

Case 11-1. A 55-year-old American patient presents with abdominal swelling (Fig. 11–10).

Case 11-2. A 33-year-old long-time diabetic patient presents with right upper quadrant "mass" (Fig. 11–11).

Case 11-3. A 65-year-old patient presents with fever and increased liver function tests (Fig. 11–12).

Case 11-4. An 80-year-old patient presents without symptoms referable to the abdomen (Fig. 11–13).

Questions:

Radiologic Findings:

11-1. In this case (Fig. 11–10), the overall liver size is small, especially the right lobe, with disproportionate enlargement of the left and caudate lobes; multiple collaterals are present around the stomach and in the central upper abdomen; and ascites is present—all findings of cirrhosis. (A is the correct answer to Question 11-1.)

11-2. In this case (Fig. 11–11), the overall liver size is large, the predominant finding is marked low density throughout entire liver, and no mass effect is present on any vessel—all findings of fatty liver. (B is the correct answer to Question 11-2.)

11-3. In this case (Fig. 11–12), the overall liver size is enlarged and attenuation inhomogeneous and mildly reduced. Also, no focal mass is present. These are findings of hepatitis. (C is the correct answer to Question 11-3.)

11-4. In this case (Fig. 11–13), multiple small, highly attenuating, punctate lesions are scattered throughout liver and spleen, characteristic of calcifications from old granulornatous disease, without any other predominant finding. (B is the correct answer to Question 11-4.)

Discussion:

Differentiation of liver disease into diffuse or focal disease is an artificial but convenient way to analyze liver disorders radiographically. Diffuse hepatocellular diseases are a common diagnostic problem. Although historical, physical, and laboratory testing are the first means for identifying these diseases, imaging may be required as a part of the overall assessment of the patient.

Cirrhosis is a chronic disease of the liver. It is characterized by injury and regeneration of hepatic parenchymal cells and is accompanied by formation of connective tissue within the liver. In the United States, the most common cause of cirrhosis is alcoholism, whereas in Asia, the most common cause is viral hepatitis. Cirrhosis results in disproportionate diminution of the right lobe compared to the left lobe and caudate lobe of the liver (Fig. 11–14). Nodular regeneration of the liver results in a nodular edge of the liver and inhomogeneity of the parenchyma. The process is accompanied by, first, increased resistance to normal hepatopetal (toward the liver) flow and, finally, the development of hepatofugal (away from the liver) flow. The increased resistance in the portal vein secondarily enlarges the spleen. This process also creates enlarged collateral venous channels to reroute blood around the liver (Fig. 11–10). These portosystemic collaterals are visible frequently on cross-sectional imaging studies, most commonly in paraumbilical veins, coronary veins, and even spontaneous splenorenal shunts. Ascites is nearly always present. Most authorities are increasingly convinced that MR imaging is the most sensitive imaging modality for examination of the liver in cirrhosis and other diffuse diseases of the liver. MR imaging can demonstrate not only the contour changes and collateral formation visible with CT, but also the more subtle intraparenchymal nodular changes consequent to formation of regenerative and dysplastic nodules characteristic of cirrhosis within the complex fibrotic and inflamed host hepatic tissue (Fig. 11–15). Importantly, MR imaging is considered to be a sensitive imaging means in the diagnosis of tumor such as hepatoma superimposed on a background of cirrhosis (Fig. 11–16).

Diffuse tumor in the liver can occur in patients with certain primary malignancies (Fig. 11–17), particularly breast carcinoma. It is usually distributed randomly throughout the left and caudate lobes. Collateral veins normally are not found. Portal venous or intrahepatic biliary radicles may be compromised or displaced, although portal vein thrombosis is uncommon.

Budd-Chiari syndrome is a condition involving obstruction of the hepatic veins or the intrahepatic inferior vena cava (IVC). It is due to hypercoagulable states that produce thrombosis; tumors of the liver, kidneys, adrenal glands, or IVC; trauma (the "three T's"; i.e., thrombosis, tumors, trauma); pregnancy; and even webs or membranes in the lumen of the IVC. This syndrome produces a marked congestion of the liver resulting from resistance to flow out of the liver, which consequently enlarges and becomes edematous. The liver has a mottled appearance on CT that is due to the interstitial edema, especially after administration of intravenous contrast material (Fig. 11–18).

Schistosomiasis is one of the world's most common parasitic diseases and is rarely seen in persons living outside the endemic areas of China, Japan, the Middle East, and Africa; it does occur, however, in immigrants to the United States. The larvae are hosts that enter the human gastrointestinal system, pass into lymphatic channels, migrate into mesenteric veins and portal veins, and, as adult worms, deposit ova that embolize to the portal system. This process leads to a granulomatous inflammation, periportal fibrosis, portal vein occlusion, varices, and splenomegaly. Imaging studies demonstrate periportal fibrosis. The fibrosis enhances on CT after contrast material administration and appears on US as increased echogenicity of the periportal sheath surrounding the portal veins.

Fatty liver, or steatosis, is a common disorder. It is found in up to 50% of patients who are diabetic or alcoholic, and has been found in up to 25% of nonalcoholic, healthy adults who die accidentally. The many causes of fatty liver, besides diabetes and alcoholism, include (1) obesity, (2) chronic illness, (3) corticosteroid excess, (4) parenteral nutrition, and (5) hepatotoxins, including chemotherapy. Fatty liver may be distributed evenly or focally. When distributed uniformly, fatty liver is recognizable as a pattern of homogeneous increased echogenicity on US, decreased attenuation on CT (Fig. 11–11), or increased signal intensity on T_l-weighted MR images. When distributed nonuniformly, it resembles focal disease of the liver in that normal islands of liver tissue are seen against the background of lower density fatty liver (Fig. 11–19). Specialized MR imaging scans, NM studies, or biopsy may be required to differentiate among the possibilities.

Hepatic iron overload can be due to deposition in hepatocytes or reticuloendothelial cells. Parenchymal iron deposition occurs in primary idiopathic hemochromatosis, secondary hemochromatosis, cirrhosis, or intravascular hemolysis; the iron overload in these conditions is generally referred to as hemochromatosis. Reticuloendothelial iron deposition occurs in transfusional iron overload or rhabdomyolysis; the iron overload in these conditions is referred to ashemosiderosis. The liver, including the right lobe, is enlarged greatly unless cirrhosis is present. On CT, the density of the liver is very high (Fig. 11–20), and on MR imaging the liver has extremely low signal on both T₁- and T₂-weighted images (Fig. 11–21). Patients with hepatic iron overload may develop hepatocellular carcinoma.

Old granulomatous disease is a disorder in which prior granulomatous inflammation, usually caused by Histoplasma capsulatum, involves the liver. Other granulomatous inflammatory conditions that could be involved include sarcoidosis, Wegener's granulomatosis, and certain toxins. The granuloma tends to undergo necrosis, and dystrophic calcification forms within the lesion. This gives the lesion its most characteristic form, multiple punctate calcifications. The granuloma is visible on US as focal, extremely hyperechoic, shadowing lesions, and on CT as extremely high-density punctate lesions (Fig. 11–13).

Thorotrast, a thorium-containing contrast agent, was used in the early 20th century for angiography and other purposes. Unfortunately, Thorotrast emits alpha and beta radiation, has a biologic half-life of 400 years since it is not excreted, and therefore has been responsible for the development of several malignancies of the liver and spleen, including angiosarcoma and hepatoma. The particles are taken up by liver, spleen, lymphatics, and bone marrow. They appear on CT studies as large, dense particles in the liver, spleen, and peripancreatic and periportal lymph nodes (Fig. 11–22). US shows typical calcifications.

Hepatitis is a diffuse inflammation of the liver, occurring as either acute or chronic disease. Patients with acute hepatitis have hepatocellular necrosis. In chronic cases, periportal inflammation and even fibrosis may occur. In acute hepatitis, the echogenicity of the parenchyma is decreased as a result of the edema, and the portal radicles are more evident; this has been termed the "starry sky" appearance. In chronic hepatitis, the texture of the liver is coarsened as a result of the fibrotic change in the periportal space, and this may decrease the visibility of the portal vein radicles. Findings on CT include hepatomegaly and decreased density (Fig. 11–12). Most commonly, no important findings except hepatomegaly occur on CT in hepatitis. On MR imaging, the liver has low signal intensity on T_l-weighted images and high signal intensity on T₂-weighted images because of the edema and inflammation.

Osler-Weber-Rendu disease, or hereditary hemorrhagic telangiectasia, affects many organs and is seen predominantly, but not exclusively, in skin and the gastrointestinal tract. In the liver, it produces either telangiectasias, cirrhosis, or both. Multiple small aneurysms may be present, and hematomas may occur if the aneurysms bleed. These aneurysms and any consequent hematomas from aneurysmal rupture can be visible on both US and CT. Angiography can demonstrate enlarged hepatic arteries and early but not immediate hepatic vein opacification.

EXERCISE 11-2: FOCAL LIVER DISEASES

Clinical Histories:

Case 11-5.A 44-year-old American patient presents with right upper quadrant pain and fever (Fig. 11–23).

Case 11-6. A 45-year-old female presents with incidentally discovered liver lesion (Fig. 11–24A,B).

Case 11-7. A 65-year-old female presents with a long history of a pancreatic mass (Fig. 11–25).

Case 11-8. A 61-year-old male presents with upper abdominal pain (Fig. 11–26).

Questions:

Radiologic Findings:

11-5. In this case, Fig. 11–23 shows an inhomogeneous liver lesion with central necrosis and a peripheral rim of edema. Although this could conceivably represent an echinococcal or amoebic abscess in this patient with fever, the patient is from the United States rather than a foreign country, thus, the most likely diagnosis is a pyogenic abscess. (A is the correct answer to Question 11-5.) As more individuals from other countries, especially Third World nations, immigrate to the United States, more echinococcal or amoebic liver abscesses will be seen.

11-6. In this case, Fig. 11–24 shows a focal lesion in the caudate lobe of the liver, which enhances early and fills in later with contrast material. This early peripheral and nodular-appearing distribution of intravenous contrast within the lesion, and eventual centripetal accumulation of contrast material to fill in the lesion is characteristic of cavernous hemangioma. (A is the correct answer to Question 11-6.)

11-7. In this case, Fig. 11–25 shows a focal lesion occupying the left lobe of the liver (M), and there is a focal enhancing mass in the pancreatic tail (T), representing a pancreatic neoplasm metastatic to the liver. (C is the correct answer to Question 11-7.)

11-8. In this case, Fig. 11–26 shows a focal lesion within the right lobe of the liver, which is associated with a clot entering the hepatic vein and even the inferior vena cava, findings typical for hepatocellular carcinoma. (B is the correct answer to Question 11-8.)

Discussion:

Recognition of the focal or diffuse nature of liver disease is helpful for sorting out the possible causes. The two can overlap, especially since one may lead to another, e.g., cirrhosis can cause hepatoma.

Pyogenic liver abscesses are relatively common focal inflammatory lesions of the liver caused by bacteria. These lesions have high morbidity and mortality, if undiscovered. They are multiple in many cases, involving both hepatic lobes. These abscesses create a severe leukocytosis. Pyogenic abscesses occur when collections of leukocytes undergo necrosis and become walled off. The imaging studies, while not definitive, provide helpful findings. On US, these lesions often are well demarcated, may be multiloculated, and have fluid centers and irregular walls. Gas within an abscess creates an echogenic structure with shadowing. On CT, the abscess appears as a low-density lesion. Intra-abscess gas occurs in approximately 50% of abscesses (Fig. 11–27), and enhancement of the border of the lesion after intravenous contrast infusion also occurs in approximately 50% of abscesses. Low-density edema may surround the abscess (Fig. 11–23). Rapid enhancement of the edge of an abscess after bolus injection of contrast material may be helpful. On ^99mTc-sulfur colloid scans, the abscess appears as a defect within the liver. MR imaging demonstrates signs of an irregular, fluid-containing lesion, i.e., low signal intensity on T_l-weighted examinations and high signal intensity on T₂-weighted examinations. Edema may be visible surrounding the lesion on T₂-weighted images.

Echinococcal disease is a parasitic infestation that involves multiple organs, most commonly the liver. It is endemic in several regions around the world. The most common form is due to Echinococcus granulosis, which, after being ingested by humans, is carried into the gut, transmitted to the portal circulation, and eventually deposited in the liver, where it develops into large, occasionally multiloculated, cysts, which may calcify. On US, these lesions appear as well-defined cysts with regular borders, which may contain swirling debris and multiple septae. Smaller, "daughter" cysts often surround them. Small calcifications are present. CT shows similar morphologic findings, as well as enhancement of the wall after intravenous contrast material infusion. Calcifications are crescentic, corresponding to the membranes. MR imaging shows a cystic mass with a rim-like periphery of low signal intensity on both T₁- and T₂-weighted images and with a central matrix of high signal intensity.

Candidiasis is a fungal disease. It affects the liver primarily in renal transplant patients and patients who have been immunocompromised by malignancy or chemotherapy for the malignancy. The organism forms multiple microabscesses, which create the characteristic appearance on imaging studies. US shows several patterns, the most common being multiple small, hypoechoic structures containing a hyperechoic central spot, the "bull's-eye" lesion. Other patterns may occur. CT shows similar multiple small abscesses (Fig. 11–28), including the bull's-eye lesion.

Amoebic abscesses are caused by a parasite, Entamoeba histolytica, and the liver is the most commonly involved organ. The leukocytosis is much less severe than with a pyogenic abscess. Unlike pyogenic abscesses, which require drainage, amoebic abscesses can often be cured by medical treatment. Like echinococcal abscesses, amoebic abscesses start when organisms reach the liver through the portal circulation from the bowel. The abscesses may rupture into the peritoneal cavity or even into the thorax. Imaging studies, including NM, US, and CT, are usually nonspecific and demonstrate focal defects within the liver (Fig. 11–29). The lesions can resemble echinococcal abscesses. One helpful finding is intraperitoneal or intrathoracic fluid, if rupture has occurred.

Hemangioma is the most common benign tumor of the liver and is second only to metastases as the most common tumor overall within the liver. Symptomatic tumors are more often found in women, probably because of bleeding. Hemangiomas are often peripherally located in the liver, less than 2 cm in diameter, and not associated with abnormalities in liver function tests. They are most commonly single. On US they are usually homogeneous and hyperechoic (Fig. 11–30A ), but an important variant is the isoechoic mass with hyperechoic periphery. They are often peripheral, with posterior acoustic enhancement. Some large lesions have central scars. CT shows homogeneous, low-attenuation lesions, which enhance after intravenous contrast material administration, have nodular peripheral enhancement (called "puddling" of contrast material), and accumulate contrast material centripetally over a period of several minutes (Fig. 11–24). This finding is most useful when the patient has no known primary tumor; otherwise, this pattern is more likely due to a metastasis. Technetion-99m-labeled red blood cell scans are diagnostic for hemangioma when early vascular-phase images show decreased activity and delayed blood-pool scans demonstrate increased activity at the lesion site (Fig. 11–30B ). MR imaging demonstrates lesions with low signal intensity on T_l-weighted scans, which is typical for most lesions. However, T₂-weighted MR imaging demonstrates high signal intensity similar to that of fluid, which is considered diagnostic of hemangioma or cyst. Intravenous Gd "puddles" in cavernous hemangioma and gradually migrates centripetally toward the center of the lesion (Fig. 11–31), analogous to the distribution of iodinated contrast material in cavernous hemangioma on CT, and likewise is considered diagnostic of cavernous hemangioma. This puddling in cavernous hemangioma is different from the more curvilinear or heterogeneous distribution of contrast accumulation seen in malignant tumor. Angiography can be very helpful, because it shows punctate collections of contrast material shortly after injection, analogous to puddling seen in CT or MR imaging (Fig. 11–32A ). These collections become denser, usually within a minute, because contrast puddles in the vascular spaces of the tumor (Fig. 11–32B ).

Although typically diffuse, steatosis of the liver can present as focal deposits of fat. Furthermore, sometimes steatosis can present as the reverse, namely, residual focal islands of hepatic tissue unaffected by fatty deposition. Both of these conditions can be confusing since they may resemble focal solid masses including tumor on CT or US. MR imaging is the most accurate means to identify sites of focal fat or focal fatty sparing. In particular, a pulse sequence called out-of-phase T₁-weighted imaging, which emphasizes the presence of fat intermixed with any host water-containing tissue, is very sensitive in the detection of the presence or absence of fat within focal fat or focal fatty sparing, respectively. Wherever fat is intermixed with water-containing parenchyma, there is loss of signal intensity on out-of-phase images. Therefore, focal fatty infiltration appears as sites of relative signal loss, whereas focal fatty sparing appears as sites of relative signal gain (Fig. 11–33). This imaging technique is the most sensitive and specific cross-sectional modality for characterizing focal fatty distribution, a very common condition.

The liver is a common and important site for metastatic disease. As many as 25% to 50% of cancer patients have liver metastases at autopsy. Most tumors metastasize to the liver, and metastases to the liver strongly affects the stage of the tumor and prognosis of the patient. Most metastases are multiple, diffusely distributed, variable in size, and solid. They may be necrotic and appear more cystic. Liver metastases may be present even when both general and specific serum markers for tumor, e.g., liver function tests and carcinoembryonic antigen are normal. Metastases may be poorly vascularized or highly vascular, a difference that affects their appearance after intravenous contrast administration. Mucin-producing carcinomas, e.g., breast and colon carcinoma, frequently produce calcification, which can be detected with imaging studies. Metastases are almost always evaluated with cross-sectional imaging studies. However, more recently, PET has been used to detect certain malignancies, including liver metastases, with great success. Although US can evaluate for liver metastases when used by skilled operators, it is limited by ileus and relative insensitivity to subtle lesions, especially against the background of preexisting liver disease. On US, metastases are usually hypoechoic, poorly defined, and hypovascular, and may have a peripheral halo (Fig. 11–34). Some types, such as those of breast cancer, may be diffusely distributed in minute form. In most institutions, CT is used to survey and monitor patients for liver metastases, because CT can detect metastases and is probably the most useful technique for evaluating extrahepatic disease. On CT, metastases are usually multifocal, of low attenuation, and often better shown after administration of intravenous contrast material when compared to preinfusion scans (Fig. 11–35). Again, some forms present as diffuse inhomogeneity. Because of its sensitivity and potential for characterizing some lesions specifically, MR imaging may become the preferred technique for detecting and characterizing liver metastases. Lesions have low signal intensity on T_l-weighted images and higher signal intensity (but never as high as in cavernous hemangioma) on T₂-weighted studies (Fig. 11–36). Certain lesions, e.g., melanoma, carcinoid, and endocrine tumors of the pancreas, have very high signal with strongly T₂-weighted scans.

Angiosarcoma is a rare, highly vascular tumor of the liver. It is seen in patients who have had an occupational exposure to certain chemicals, particularly polyvinyl chloride or Thorotrast. If lesions rupture, they may produce serious hemorrhagic sequelae. On US, angiosarcoma is usually hypoechoic. Sometimes the attendant fibrosis so obscures the tumor that it is impossible to identify. On CT, the lesions have low attenuation (Fig. 11–37), may enhance markedly, and, if arising in the presence of Thorotrast, can displace and distort the Thorotrast collections.

Focal nodular hyperplasia (FNH) and liver cell adenoma are easily confused. Both are histologically benign liver disorders that produce single or multiple lesions. Both processes can occur in young adults. On imaging studies, they can resemble primary or metastatic liver tumors. However, some important differences pertain. FNH is probably a hamartoma of the liver, i.e., a localized overgrowth of mature cells that are identical to the types constituting the liver and contain fibrous tissue, blood vessels, bile ducts, and occasional well-differentiated hepatocytes. Adenoma is a true benign tumor composed of one tissue element of the liver, the hepatocyte. FNH often contains a central fibrotic scar. Adenoma is associated with the use of oral contraceptives, whereas FNH probably is not. Adenoma, unlike FNH, tends to undergo hemorrhage, and thus to present as acute abdominal pain. US is nonspecific in studying FNH. Adenoma is usually hyperechoic but heterogeneous. On CT, FNH is transiently but markedly and uniformly hypervascular, and the central scar may be seen (Fig. 11–38). Adenoma usually shows low density, may hemorrhage as high-density collections on preinfusion scans, and enhances variably. On Tc-sulfur colloid NM scans, FNH can show either increased, decreased, or normal activity compared to that of liver. Adenoma usually shows no increased uptake in NM studies, but this varies. FNH has low signal intensity on T₁-weighted MR imaging and slightly high signal intensity on T₂-weighted images (Fig. 11–39). If the central scar is present, it may exhibit high signal intensity on T₂-weighted images. Adenoma, like many lesions, has a nonspecific appearance of low signal intensity on T_l-weighted examinations and slightly high signal intensity on T₂-weighted examinations. Hemorrhage is recognizable as high signal intensity on T_l-weighted images. Angiographically, FNH is hypervascular with radiating branches that produce a "spoke-wheel" appearance. Adenoma has a variable angiographic appearance but is generally less vascular than FNH.

Hepatocellular carcinoma, or hepatoma, is a primary malignancy of the liver. It is found in older cirrhotic patients in the United States and in younger patients in areas of the Far East and Africa, where it is endemic. Chronic hepatitis B and C infection and exposure to aflatoxin predispose to formation of hepatoma. On imaging studies, hepatoma appears as (1) a single predominant lesion (most common form), (2) a predominant lesion with multiple, smaller, surrounding daughter lesions, or (3) diffuse tumor. Portal vein invasion by the tumor in any form is relatively common and can aid in distinguishing hepatoma from other lesions. On US, hepatoma is most commonly a discrete lesion with increased, similar, or decreased echogenicity in comparison to that of liver. On CT, lesions are most commonly of low density and may enhance if fast scans in the arterial phase are performed after contrast material administration (Fig. 11–40). Portal venous thrombosis can be seen, and preexisting cirrhosis or Thorotrast can be demonstrated. MR imaging findings are similar to those of CT, but as with CT, the lesion is inhomogeneous.

EXERCISE 11-3: UPPER ABDOMINAL TRAUMA

Clinical Histories:

Case 11-9. A 45-year-old motor vehicle accident victim presents with upper abdominal pain (Fig. 11–41).

Case 11-10. A 57-year-old man presents who was beaten in the abdomen with a baseball bat (Fig. 11–42).

Questions:

Radiologic Findings:

11-9. In this case (Fig. 11–41), the liver has an irregularly linear lesion in its central aspect, representing a liver laceration. (B is the correct answer to Question 11-9.)

11-10. In this case (Fig. 11–42), there is a low-density bulbous enlargement of the pancreatic tail, representing a pancreatic injury. (A is the correct answer to Question 11-10.)

Discussion:

Hepatic injury is common after blunt trauma. Hepatic injuries may be life threatening as a result of bleeding and shock, but more often surgery is not required. Observation and systemic support may be the only treatment necessary. Like trauma to any other organ, injury to the liver varies from mild to severe. A mild injury of the liver produces a localized collection of traumatized liver tissue and an interstitial hematoma, like a bruise, which is termed acontusion. More severe injuries that involve complete disruption of the tissue into fracture planes, perhaps involving the hepatic veins, inferior vena cava, or portal veins, and are called lacerations.

Most blunt abdominal trauma in the United States is radiographically evaluated with CT. Angiography is used to a lesser extent. US, NM, and MR imaging are of little or no value in a general survey of abdominal trauma. On CT, hepatic contusion is seen as a low-attenuation lesion, perhaps with mass effect on surrounding hepatic vessels. Associated hemoperitoneum is not usually seen. On CT, hepatic laceration appears as an irregular, stellate, or linear lesion through the liver parenchyma (Fig. 11–41), sometimes extending to the porta, liver capsule, or IVC (Fig. 11–43). A hallmark of severe trauma to upper abdominal organs is accompanying hemoperitoneum, which appears as a collection of high-density material at the site of bleeding and is termed the sentinel clot. Acute blood that has migrated away from the site of active bleeding, or old hemoperitoneum at any site, often has the attenuation of simple or near-simple fluid and can resemble intraperitoneal fluid, or ascites, from a number of causes.

Ascites is a nonspecific reaction of the peritoneal space to a variety of causes, including tumor, inflammation, trauma, increased systemic venous resistance (e.g., congestive heart failure), renal or hepatic insufficiency, and many other conditions. It is characterized by the production of intraperitoneal fluid. This fluid can be simple, a transudate, in which case it has fluid density (Fig. 11–44) and is free to move to the dependent portion of the abdominal or pelvic cavity with patient movement. Alternatively, it can be complex, an exudate, in which case it is denser than simple fluid, is accompanied by solid tissue (e.g., tumor deposits in peritoneal metastases) or layered material (e.g., blood from trauma or inflammatory cellular debris in peritonitis), and often is loculated, or unable to move freely throughout the intraperitoneal cavity (e.g., abscess).

Pancreatic injury is uncommon, but potentially serious. Mortality from pancreatic injuries is nearly 20%. Being crushed against the spine probably accounts for the frequency of injury to the body of the pancreas. Pancreatic trauma may or may not be associated with increased amylase. Usually caused by blunt trauma, pancreatic trauma is often associated with injuries to other organs, such as liver and bowel. These injuries produce intraperitoneal blood and fluid, and interstitial mesenteric edema, which can be confusing. As with hepatic trauma, CT is usually the modality of choice to evaluate pancreatic trauma, but even on CT, the diagnosis can be difficult. On CT, the pancreas may be ill defined, enlarged, or even disrupted, i.e., fractured.

Bowel and mesenteric injuries are found in approximately 5% of all patients undergoing laparotomy after motor vehicle accidents. Injuries of the bowel and mesentery frequently accompany injury to the liver or pancreas. These injuries can result in massive intraperitoneal bleeding from disruption of mesenteric vessels or peritonitis from bowel perforation. As elsewhere, CT is the modality of choice to evaluate patients for possible bowel or mesenteric injuries, but these injuries, like those to the pancreas, can be difficult to detect. On CT, injuries of the bowel and mesentery include free air with the intraperitoneal or retroperitoneal spaces (Fig. 11–45), free intra-abdominal fluid, circumferential or eccentric bowel wall thickening, enhancement of the bowel wall, streaky soft-tissue infiltration of the mesenteric fat, free mesenteric hematoma, and especially sentinel clot. Angiography may demonstrate free extravasation of contrast material in injuries of the mesenteric vessels, and percutaneous embolization may stop bleeding when surgery is not possible.

EXERCISE 11-4: BILIARY INFLAMMATION

Clinical Histories:

Case 11-11. A 53-year-old male presents with acute right upper quadrant pain, fever, pain on palpation over the gallbladder, and elevated liver function tests (Fig. 11–46).

Case 11-12. A 22-year-old HIV-positive female presents with debiliatating and chronic illness with vague right upper quadrant pain, but no tenderness on palpation over the gallbladder (Fig. 11–47).

Case 11-13. An 84-year-old male presents with right upper quadrant pain, marked fever, and suspicion of sepsis (Fig. 11–48).

Case 11-14. A 53-year-old male presents with history of cholecystectomy, upper abdominal pain, jaundice, and fever (Fig. 11–49).

Questions:

Radiologic Findings:

11-11. In this case, Fig. 11–46 shows that the gallbladder is distended, and the wall is thickened, measuring more than 5 mm, and has multiple lamina, indicating gallbladder wall inflammation from acute cholecystitis. (A is the correct answer to Question 11-11.)

11-12. In this case (Fig. 11–47), the gallbladder wall is markedly thickened, measuring over 1 cm., with multiple lamina, but was not tender to palpation, findings seen often with AIDS cholangiopathy. (B is the correct answer to Question 11-12.)

11-13. In this case (Fig. 11–48), gas within the gallbladder wall and lumen is the primary abnormality, indicating emphysematous cholecystitis. (B is the correct answer to Question 11-13.)

11-14. In this case, Fig. 11–48 shows that the biliary ducts are distended and irregular, which in the clinical presentation of fever and jaundice most strongly suggests cholangitis. (B is the correct answer to Question 11-14.)

Discussion:

Calculi are a common problem in the gallbladder and biliary ducts. Cholelithiasis is one of the most common abdominal disorders overall and is the most common cause of cholecystitis, as well as the most common indication for abdominal surgery. Gallstones develop when the composition of bile, which includes bile salts, lecithin, and cholesterol, varies from normal and creates supersaturation of cholesterol, which then precipitates. Historically, patients thought to be harboring gallstones on the basis of clinical criteria were examined by oral cholecystography, which shows filling defects in the gallbladder lumen opacified by orally ingested iodinated contrast material. However, this examination has been largely replaced by sonography, occasionally supported by other imaging information. On US, gallstones usually appear as mobile, intraluminal, echogenic foci that cast a well-defined acoustic shadow (Fig. 11–50). Two other possible appearances are echogenic foci in the gallbladder fossa without visible surrounding bile when the gallbladder is contracted, and small, mobile, echogenic foci that do not cast a shadow. On CT, gallstones appear as dense, well-defined, intraluminal structures (Fig. 11–51), but their density can vary from fat density to near bone density, depending on the relative concentration of calcium and cholesterol. MR imaging of the biliary system, especially MRCP, has become more important in biliary imaging, including the detection of calculi of the gallbladder and biliary tree. Although US remains the primary and initial means of identifying biliary calculi, MRCP can be used as a supplementary technique, especially in ductal calculi, because imaging of the biliary tree by US may be suboptimal when obscured by bowel gas. MRCP can depict the biliary system, filling defects within the biliary tree (Fig. 11–52), and congenital variants of the biliary ducts, and is about as accurate as ERCP in displaying a biliary "road map." It can also be used to evaluate the biliary ducts when ERCP is impossible to perform such as when the patient has undergone a Billroth procedure, interrupting the continuity of the upper gastrointestinal tract. NM and angiography have no major role at this time in assessment of gallstones.

Choledocholithiasis occurs when calculi pass from the gallbladder into the biliary ducts or when calculi develop originally within the ductal system. Regardless of origin, they may obstruct the biliary ducts, cause biliary colic, and lead to cholangitis. Common duct stones are usually evaluated with US and CT and by direct visualization with ERCP. On US, choledocholithiasis appears as echogenic foci within the lumen of the biliary duct. Sonographically, common duct stones are detected less readily than gallbladder stones, and meticulous technique is required. Choledocholithiasis can cause acoustic shadows, but for technical reasons they are detected less frequently than cholelithiasis (Fig. 11–53). On CT, choledocholithiasis appears as intraluminal biliary ductal foci, which, like gallbladder stones, may vary in density from hypodense to isodense to hyperdense to bile, depending on their composition (Fig. 11–54).

Cholecystitis is inflammation of the gallbladder that is almost always caused by obstruction of the cystic duct, usually by an impacted calculus. The inflammation may be acute or chronic, uncomplicated or complicated, calculous or acalculous. As the gallbladder continues to accumulate bile, intraluminal pressure increases and vascular insufficiency of the wall occurs, causing ischemia, necrosis, and often supervening inflammation. The gallbladder distends, the gallbladder wall thickens from edema, and the patient is tender to palpation over the gallbladder (positive Murphy's sign).

Both ultrasound and hepatobiliary NM studies are the modalities of choice to evaluate possible cholecystitis. Sonographic signs of acute cholecystitis include cholelithiasis, gallbladder wall thickening (greater than 3 mm), irregular or linear hypoechoic structures within the gallbladder wall, a positive Murphy's sign, and marked gallbladder distention (Fig. 11–55). A combination of these signs is a good positive predictor of acute cholecystitis. In marked chronic cholecystitis, US shows persistent gallbladder wall thickening or sludge, stones, and contraction of the gallbladder. However, in the presence of cholelithiasis, the gallbladder almost always shows signs of chronic inflammation histologically, even without symptoms or sonographic findings.

Hepatobiliary NM HIDA scans depict acute cholecystitis as an absence of filling of the gallbladder with the radionuclide once it is excreted by the liver into the biliary ducts; this absence of filling is due to the obstruction of the cystic duct lumen by inflammatory edema of the cystic duct wall (Fig. 11–56). Sufficient time must be given to fill the gallbladder. This time interval depends on whether or not morphine is administered. Morphine increases the tone of the sphincter of Oddi and increases intraluminal common bile duct pressure to overcome the resistance to bile flow into the gallbladder in chronic cholecystitis, but not in acute cholecystitis when a stone obstructs the duct. Acute cholecystitis is diagnosed when absence of activity is noted either 45 minutes after morphine augmentation or after 4 hours without morphine augmentation. Delayed gallbladder visualization after 1 hour usually reflects chronic cholecystitis. On CT, the morphologic findings in patients with acute cholecystitis are similar to the US findings, including gallstones and thickened and inhomogeneous gallbladder wall. However, CT is not as sensitive as US or NM to the presence of either gallstones or to acute cholecystitis. MR imaging can depict the presence of gallbladder wall inflammation in the absence of wall thickening by demonstrating wall enhancement following Gd infusion (Fig. 11–57). The exact role of MR imaging in cholecystitis has not yet been completely evaluated. Angiography has no role in the diagnosis of cholecystitis.

Many potential complications and conditions are associated with cholecystitis. These include hydrops, porcelain gallbladder, milk-of-calcium bile, and emphysematous cholecystitis.

Hydrops refers to the marked distention of the gallbladder by clear, sterile mucus, usually under conditions of chronic, complete cystic duct obstruction. On imaging studies, the primary finding is enlargement of the gallbladder (Fig. 11–58).

Porcelain gallbladder refers to calcification of the gallbladder wall, as a result of chronic inflammation causing dystrophic calcification and often associated with recurrent acute cholecystitis. Gallbladder stones are usually present, and there is a higher incidence (approximately 10%–20%) of gallbladder carcinoma. On imaging studies, complete or incomplete circular wall calcification is present and is seen as a curvilinear, highly echogenic wall on US or as a curvilinear, high-attenuation wall on CT (Fig. 11–59).

Milk-of-calcium bile refers to a precipitation of calcified material within the lumen of the gallbladder, usually associated with chronic cholecystitis. US shows echogenic sludge-like material, possibly with gallstones. CT demonstrates the distinctive appearance of a horizontal bile-calcium level.

Emphysematous cholecystitis is a distinctive condition. Like acute cholecystitis, it is marked by intense gallbladder wall inflammation, but unlike acute cholecystitis, it is not necessarily associated with gallstones. It may be related to ischemia of the gallbladder wall from small-vessel disease, and it affects an older age group than does acute cholecystitis. Gas is released by bacterial invasion and accumulates in the gallbladder wall, lumen, or both. On US, gas is seen as an echogenic focus producing poorly defined or "dirty" shadowing behind it. The wall is thickened, perhaps focally, with gas. On CT, air-density gas is seen within the lumen or wall (Fig. 11–48). MR imaging and angiography have no current role in evaluation of these complications.

Like inflammation of the gallbladder, inflammation of the biliary ducts, or cholangitis, is an important clinical condition. It is less common than cholecystitis. AIDS-associated cholangiopathy, ascending cholangitis, and oriental cholangiohepatitis are three important forms of cholangitis.

AIDS-associated cholangiopathy is marked by the frequent isolation of opportunistic organisms, includingCryptosporidium and cytomegalovirus from the bile, and by considerable inflammation of the bile duct wall. On US or CT, the gallbladder or biliary duct walls may be markedly thickened (greater than 4 mm) (Fig. 11–47) and may contain irregular lamina. Inflammation is present, but stones may or may not be present. Cholangiography shows irregular strictures, papillary stenosis, or both.

Ascending cholangitis is a bacterial inflammation of both walls and lumina of the biliary system, including the gallbladder. It is almost always due to obstruction of the biliary tract, especially when caused by choledocholithiasis and distal bile duct stenosis. The presence of grossly purulent material within the duct indicates suppurative cholangitis. Cross-sectional imaging studies are used to define the level and cause of obstruction. Cholangiography can show the abnormal biliary ducts directly. The purulent material of suppurative cholangitis may be seen as echogenic material on US, high-density material on CT, or filling defects on cholangiography.

Oriental cholangiohepatitis is a common illness in endemic areas of Asia and can be seen in Asian immigrants in this country. It may be caused by bile duct wall injury from the parasitic infestation. Ductal stones commonly form, and the ducts are dilated. A characteristic finding is the presence of intraductal (especially intrahepatic ductal) calculi. These findings are readily demonstrated with US, CT, and cholangiography.

EXERCISE 11-5: PANCREATIC INFLAMMATION

Clinical Histories:

Case 11-15. A 54-year-old alcoholic male presents with marked epigastric pain and increased amylase (Fig. 11–60).

Case 11-16. A 45-year-old male presents with marked epigastric pain and a falling hematocrit; he is "crashing" (Fig. 11–61).

Case 11-17. A 65-year-old male presents with chronic epigastric pain (Fig. 11–62).

Questions:

Radiologic Findings:

11-15. In Fig. 11–60, the overall size of the pancreas is enlarged, the tissue around the pancreas is edematous, and the fat planes between the pancreas and the stomach are blurred, findings of acute edematous pancreatitis. (A is the correct answer to Question 11-15.)

11-16. In Fig. 11–61, peripancreatic inflammatory changes and a high-density collection are seen adjacent to the pancreatic head, representing a collection of blood created by hemorrhagic pancreatitis. (B is the correct answer to Question 11-16.)

11-17. In Fig. 11–62, multiple high-density calcifications are distributed throughout the pancreatic head, and the pancreatic head is mildly enlarged, findings of chronic calcific pancreatitis. (B is the correct answer to Question 11-17.)

Discussion:

Pancreatitis, an inflammatory condition of the pancreas, has a number of causes including alcohol abuse, trauma, cholelithiasis, peptic ulcer, hyperlipoproteinemia, hypercalcemia, and infection. Pancreatic inflammation may be acute or chronic. Acute pancreatitis and chronic pancreatitis may not represent different stages of the same disease.

Acute pancreatitis can occur once or repetitively and usually has the potential for healing. It can be associated with mild to severe inflammatory edema (edematous or interstitial pancreatitis) or with hemorrhage (hemorrhagic or necrotizing pancreatitis). These two forms of acute pancreatitis may be distinguishable only by the severity and time course of the disease. Edematous pancreatitis resolves within 2 to 3 days with appropriate therapy, whereas hemorrhagic pancreatitis requires much longer to resolve. The diagnosis of simple pancreatitis is usually based on medical history, physical examination, and laboratory results. With this information imaging studies are usually unnecessary, and scans show the pancreas to be normal or only slightly enlarged. The surrounding fat is edematous. The pancreas appears hypoechoic on US (Fig. 11–63). On CT the surrounding fat appears as areas of streaky interstitial soft-tissue density in the transverse mesocolon around the pancreas (Fig. 11–64).

Clinical criteria to predict the severity or likelihood of complications of pancreatitis correlate well with the presence and extent of extrapancreatic abnormalities on imaging studies. Imaging is useful in acute pancreatitis when assessing potential complications. These complications include hemorrhagic pancreatitis, vascular complications, phlegmon, and abscess.

Hemorrhagic pancreatitis is usually due to erosion of small vessels, is often a serious problem, and indicates an acutely and critically ill patient. It appears as a collection of echogenic material on US. On CT it appears as a collection of high-density material, and can be extremely extensive because it is an aggressive process (Fig. 11–65). This material represents the blood.

Large vessels are at risk for developing pseudoaneurysms when the histiolytic enzymes released by the inflamed pancreas erode their walls, leading to a focal, highly vascular structure within the region of the pancreas. The splenic, gastroduodenal, and hepatic arteries are particularly vulnerable. On US and CT, flow within an enlarged rounded vessel can be seen. Angiography establishes the diagnosis by showing a focally enlarged vessel, sometimes with extravasation. However, CTA is also effective for detecting pseudoaneuryms related to pancreatitis.

Phlegmon is an inflammatory, boggy, edematous, soft-tissue mass, distinct from fluid, arising from the pancreas and diffusely spreading away from it. Phlegmon appears as a diffuse soft-tissue echogenicity or density process surrounding the pancreas and contains neither the blood of hemorrhagic pancreatitis nor the fluid of an abscess (Fig. 11–66).

Abscesses are a potentially life-threatening complication of pancreatitis. Infection associated with pancreatitis can be thought of as representing infected necrosis (diffuse infection without pus collection) or pancreatic abscess (collection of pus surrounded by a capsule). Infected necrosis is harder to identify on imaging studies than is pancreatic abscess, since it is less distinct and blends into the surrounding edema. On US, an abscess appears as a poorly defined anechoic or hypoechoic lesion. It enhances sound posteriorly and may contain debris. Gas appears as a poorly defined echogenic focus within the nondependent aspect of the lesion and casts a "dirty" shadow. On CT, the lesion is poorly defined and may contain gas collections (Fig. 11–67). After contrast material infusion, the border enhances. If gas is absent, abscess cannot be differentiated from phlegmon or pseudocyst. In general, NM and angiography do not have a major role in evaluation of acute pancreatitis.

Unlike acute pancreatitis, chronic pancreatitis is considered to indicate permanent pancreatic damage. Chronic pancreatitis may or may not be preceded by prior attacks of acute pancreatitis. The pancreas will develop calcifications within the ductal system (Fig. 11–62). Mass-like enlargement of the pancreas can periodically occur, but often the gland eventually atrophies. The pancreatic duct may dilate. These findings are visible on both US and CT. NM and angiography do not have a current major role in evaluation of chronic pancreatitis.

EXERCISE 11-6: PANCREATIC NEOPLASM

Clinical Histories:

Case 11-18. A 62-year-old female presents with vague, deep, and persistent abdominal pain (Fig. 11–68).

Case 11-19. A 65-year-old male presents with midepigastric pain over a long period of time (Fig. 11–69).

Case 11-20. Consider a 32-year-old healthy, asymptomatic woman (Fig. 11–70).

Questions:

Radiologic Findings:

11-18. In this case (Fig. 11–68), there is a low, but not fluid, density lesion in the pancreatic body, expanding the contour of the pancreas, and not associated with any inflammatory changes in the peripancreatic fat, findings most consistent with a ductal adenocarcinoma. (B is the correct answer to Question 11-18.)

11-19. Figure 11–69 shows a fluid density lesion within the pancreatic head and uncinate process of the pancreas, not associated with inflammatory changes in the peripancreatic fat, findings most compatible with a cystic neoplasm of the pancreas. (B is the correct answer to Question 11-19.)

11-20. In Fig. 11–70, several small, simple, unilocular cysts are seen in the pancreatic parenchyma, without inflammatory changes or signs of peripancreatic extension of any disease process. (C is the correct answer to Question 11-20.)

Discussion:

Pancreatic masses include tumors, tumor-like masses such as cysts and developmental anomalies, and inflammatory lesions. These can overlap in appearance, such as when an inflammatory mass simulates a neoplastic mass on imaging studies. They can be causally related, such as when a neoplastic mass secondarily causes an inflammatory mass. Therefore, differentiation among them is not entirely possible, either clinically or radiographically. However, the prognostic and management implications of the lesions that create pancreatic masses differ considerably and therefore require extensive and often invasive investigation. Although contrast studies of the gastrointestinal tract can be used to infer the presence of a mass, cross-sectional imaging studies are usually employed to establish the diagnosis.

Tumors of the pancreas are important clinical entities; some have an extremely poor prognosis and some produce serious clinical symptoms. They can be classified according to origin as epithelial tumors, endocrine tumors, and miscellaneous lesions. Epithelial tumors can be solid or cystic. Solid ductal adenocarcinoma is the most common overall and carries the worst prognosis (mean survival of 4 months). Cystic tumors can be divided into cystic lesions arising from the pancreatic parenchymal cells, such as cystadenoma or cystadenocarcinoma, and those arising from the pancreatic ductal cells, such as intraductal papillary mucinous tumors. Compared to adenocarcinoma, these tumors have a less serious prognosis. Endocrine, or islet cell, tumors elaborate hormonal substances and can create serious symptoms. The two most common of these are insulinoma, which releases insulin and produces hypoglycemia, and gastrinoma, which releases gastrin and produces Zollinger-Ellison syndrome. There are many other important kinds of hormonally active pancreatic endocrine tumors, and each is designated by the hormone it secretes (e.g., glucagonoma, somatostatinoma). Miscellaneous lesions arise from pancreatic parenchymal tissue (e.g., metastases, especially from melanoma, and lung or breast cancer) or from tissue other than pancreas (e.g., intrapancreatic cholangiocarcinoma or peripancreatic lymph node). These miscellaneous lesions are important because they sometimes strongly simulate true pancreatic neoplasms on imaging studies.

Ductal adenocarcinoma has a variety of appearances on imaging studies. On US, it usually is seen as a focal, hypoechoic, irregular, solid mass. Rarely, it is isoechoic or involves the entire gland. In some pancreatic head masses, the only finding may be that the uncinate process is rounded. The pancreatic or biliary duct may be dilated by the obstructing tumor. Pseudocysts, cystic collections in or around the pancreas, may form because of pancreatic duct dilatation and perforation. On CT, the tumor presents as a solid, low-density, irregular mass, perhaps with ductal dilatation, pseudocyst formation, or both (Fig. 11–68). Occasionally, the tumors will enhance brightly (Fig. 11–71). Angiography may be used to demonstrate the vascular anatomy and establish definitively whether certain key vessels (e.g., the superior mesenteric artery or vein) are encased. If so, the lesion is unresectable. NM currently has no established role in evaluation of pancreatic tumors. Associated metastases in the liver establish the fact that a pancreatic mass cannot be simply inflammatory (Fig. 11–72). General pertinent negatives on cross-sectional imaging may help to differentiate adenocarcinoma from other nontumorous masses. Calcification is rarely, if ever, seen in ductal adenocarcinoma, and it is almost never hypervascular.

Ductal adenocarcinoma is simulated by a number of other entities. These include peripancreatic lymphadenopathy, intrapancreatic cholangiocarcinoma, and pancreatic metastases.

Peripancreatic lymphadenopathy from lymphoma, leukemia, or any other primary malignancy can closely resemble a pancreatic mass. On imaging studies it may appear as solid soft tissue in the pancreatic region (Fig. 11–73). Keys to differentiating lymphadenopathy from a primary solid mass include smooth lobulation and pseudoseptations caused by incomplete coalescence of the lymph nodes. Also, peripancreatic lymphadenopathy is much less likely to obstruct the pancreatic duct, although suprapancreatic lymph nodes obstruct the biliary duct as it passes through the porta hepatis.

Two uncommon neoplastic processes that occur in the pancreas are cholangiocarcinoma and metastases. Cholangiocarcinoma usually does not occur within the pancreas, but when it does, it can exactly mimic a pancreatic head mass to the extent of producing biductal dilatation. Metastases appear as solid intrapancreatic lesions, but with necrosis, they appear as fluid masses. Because they may be completely indistinguishable from primary tumors, the diagnosis may be inferred only from the clinical history. Pancreatic metastases are quite uncommon, usually arise from melanoma or lung primary lesions, and mimic a focal mass lesion of any neoplastic origin (Fig. 11–74).

Pancreatic endocrine tumors may also simulate ductal adenocarcinoma and, in fact, no specific features consistently distinguish the two. Occasionally, however, certain imaging features can be helpful, especially when combined with the history. Many islet cell tumors appear simply as solid masses within the pancreas. However, some (especially in insulinoma) may appear hypervascular when studied with fast bolus, or dynamic, CT, and they may appear as extremely dense lesions immediately after enhancement with intravenous contrast material. Calcifications, which sometimes are very dense, are more commonly seen with islet cell tumors. MR imaging may have a role in the evaluation of islet cell tumors, because these tumors have a characteristic appearance on MR studies. Islet cell tumors and their metastases have extremely high signal intensity on T₂-weighted MR imaging, which can be used to characterize the origin of the lesion.

Primary cystic pancreatic malignancies and pancreatic cysts are not readily confused with typical ductal adenocarcinoma. Currently, cystic pancreatic masses are classified according to whether they arise from parenchymal cells or ductal cells. Cystic malignancies arising from pancreatic parenchyma are classified as either microcystic adenomas or mucinous cystic neoplasms. This classification is helpful, because the two lesions are distinguishable from each other and from solid lesions on imaging studies. Microcystic adenomas are composed of innumerable very small cysts (1 mm to 2 cm). Sometimes they contain highly vascularized fibrous septa and a central stellate fibrotic scar, which may calcify. They are not thought to be malignant or premalignant. Mucinous cystic neoplasms are composed of unilocular or multilocular cysts larger than 5 cm and may have large papillary excrescences. They are considered malignant or premalignant lesions. Both microcystic adenomas and mucinous cystic neoplasms are cystic, but differences in the sizes of the cysts can be recognized on US or CT. Cystic malignancies arising from pancreatic ductal epithelial cells are called intraductal papillary mucinous tumors. These tumors contain considerable mucus and therefore exhibit complex appearances on MR imaging. Pancreatic cysts can occur as isolated congenital cysts or as part of a more generalized multiorgan process that includes adult polycystic disease or von Hippel-Lindau disease (Fig. 11–75). Regardless, their appearance is similar to that of a cyst in any other organ (Fig. 11–70). US and CT depict a uniloculated or multiloculated cyst. A pancreatic cyst may be difficult to differentiate from a mucinous cystic neoplasm.

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