The liver is bordered superiorly by the undersurface of the diaphragm; medially by the stomach, duodenum, right adrenal gland, and transverse colon; inferiorly by the hepatic flexure of the colon; and posteriorly by the right kidney. With the exception of the surfaces adjacent to the inferior vena cava (IVC), the gallbladder fossa and the posterosuperior aspect of the diaphragm, the liver is covered by peritoneum.

Three fissures delineate the margins of the hepatic lobes and segments (Figs. 6.1 and 6.2). The interlobar fissure, which lies on the inferior surface of the liver, is oriented along a line that passes through the middle hepatic vein superiorly and the gallbladder fossa inferiorly. It divides the liver into right and left lobes. The left intersegmental fissure (also known as the fissure for the ligamentum teres) lies along the caudal aspect of the left hepatic lobe, dividing the lobe into medial and lateral segments. The fissure for the ligament venosum, which contains the remnant of the ductus venosus, is oriented in a coronal or oblique plane, separating the posterior margin of the left lateral hepatic segment from the anterior margin of the caudate lobe.

The caudate lobe is considered anatomically distinct because it has its own arterial supply and venous drainage. It extends medially from the right hepatic lobe, insinuating itself between the portal vein and the IVC (4). The isthmus between the portal vein and vena cava is termed the caudate process. The most medial extension of the caudate lobe is termed the papillary process. The arterial supply of the caudate lobe is from both the right and left hepatic arteries, whereas the venous drainage is directly into the IVC.

Knowledge of the segmental anatomy of the liver is important for localization and surgical management of hepatic neoplasms. The traditional segmental nomenclature described by Goldsmith and Woodburne divided the liver into right and left lobes, with each lobe divided into two segments (5). In this system the main hepatic veins are used to define the major hepatic segments. Thus, the right and left lobes are defined by a plane passing through the middle hepatic vein and gallbladder fossa. The anterior and posterior segments of the right lobe are divided by a plane drawn through the right hepatic vein, and the medial and lateral segments of the left lobe are divided by a plane drawn through the left hepatic vein. In this traditional system, no distinction is made between superior and inferior subsegments within each major segment. However, knowledge of these hepatic subsegments is important since surgical techniques have been developed that allow resection of such subdivisions. Consequently, the system proposed by Couinaud and later modified by Bismuth has largely replaced the traditional nomenclature (6) (Table 6.1).

In the nomenclature system of Bismuth and Couinaud, the hepatic segments, except for the caudate lobe and medial segment of the left lobe, are divided not only by three vertical planes along the hepatic veins, but also by a transverse plane defined by the right and left portal vein (7,8,9,10). Thus, eight segments are defined by this system. The caudate lobe is segment I, and the other segments, II through VII, are numbered in a clockwise direction when the liver is viewed from its ventral aspect (Figs. 6.1 and 6.2; Table 6.1). Each segment has separate afferent and efferent vessels and biliary channels.

The portal veins and hepatic arteries are the afferent vessels of the liver (Figs. 6.1 and 6.2). The portal vein arises ventral to the neck of the pancreas at the junction of the splenic and superior mesenteric veins. It courses cephalad and to the right, posterior to the bile ducts and hepatic artery within the hepatoduodenal ligament, to enter the porta hepatis. Within the porta hepatis, the portal vein divides into right and left branches that run parallel to the bile ducts and right and left hepatic arteries. After giving off several branches that supply the porta hepatis and caudate lobe, the right portal vein courses to the right and cranially. Within the parenchyma of the right lobe, the right portal vein divides into anterior and posterior branches that supply their respective segments. Each of these branches then divides into superior and inferior branches that supply their corresponding hepatic segments. The first portion of the left portal vein courses horizontally, giving off a branch to the lateral segment (segments II and III). The left portal vein then turns medially to join the obliterated umbilical vein within the fissure for the ligamentum teres. This intrafissural portion (umbilical segment) of the left portal vein divides into ascending and descending branches that supply the superior and inferior divisions of segment IV.

The common hepatic artery usually arises from the celiac axis and courses anteriorly and to the right, giving off right gastric and gastroduodenal branches. It then continues as the proper hepatic artery. At the level of the porta hepatis, the hepatic artery lies anterior to the portal vein and medial to the bile duct. Within the porta hepatis, the hepatic artery divides into right and left branches. This usual pattern of hepatic arterial anatomy is present in slightly more than 50% of individuals; the remaining population has varying arterial anatomy (11). The common variations are origin of the left hepatic artery from the left gastric artery and origin of the right hepatic artery branches from either the celiac axis or the superior mesenteric artery. Within the liver, the right and left hepatic artery branches divide in a pattern similar to that of the portal vein branches to supply their respective segments.

The hepatic veins are the efferent vessels of the liver (12). The three major hepatic veins drain into the IVC just below the diaphragm (Figs. 6.1 and 6.2). The right hepatic vein, running between the anterior and posterior segments of the right lobe, drains segments V, VI, and VII. The middle hepatic vein, running in the interlobar fissure, drains segments IV, V, and VIII. The left hepatic vein, lying in the fissure between the medial and lateral segments of the left lobe, drains segments II and III. The middle and left hepatic veins often form a common trunk before emptying into the IVC.

The attenuation value of unenhanced hepatic parenchyma usually is in the range of 40 to 65 Hounsfield units (HU) and is slightly higher than that of the spleen (mean difference of 8 HU) (13). After administration of intravenous contrast medium, the attenuation value of the liver is

usually less than that of the spleen (Fig. 6.2). On precontrast scans, the portal and hepatic veins appear as low-density, branching, linear, or rounded structures within the hepatic parenchyma. The inherent attenuation difference between blood and hepatic parenchyma is accentuated in the presence of anemia or iron overload. Reversal of the attenuation difference between vessel and parenchyma on unenhanced scans occurs in the presence of diffuse steatosis (see below).

Congenital anomalies of the liver can involve an entire lobe or only a segment of the liver. These include (a) absence (agenesis) (Fig. 6.3); (b) underdevelopment, either small size with normal architecture (hypoplasia) or small size with abnormal architecture (aplasia); and (c) overdevelopment. Underdevelopment may involve an entire lobe or only a segment (14,15,16). The most common anomaly is overdevelopment of the right lobe, referred to as Riedel lobe.

A common anatomic variant is the presence of an incomplete accessory hepatic fissure, occurring most commonly in the right lobe superiorly (17). Another common variant is leftward extension of the lateral segment of the left hepatic lobe, causing it to wrap around the spleen. Recognition of this variant is important so that it is not mistaken for a subcapsular fluid collection.

There are three phases of hepatic enhancement—vascular, redistribution, and equilibrium—reflecting different parts of the aortic-hepatic time–attenuation curve (Fig. 6.4) and the redistribution of the contrast agent from the vascular to the extravascular (interstitial) space (13). During the vascular or arterial phase, intravenous contrast agent is predominantly in the central blood compartment (aorta and hepatic artery). This phase is characterized by a rapid rise in aortic enhancement, with a peak soon after the end of the contrast
medium injection. During the redistribution phase, also known as the portal venous or hepatic parenchymal phase, contrast agent shifts from the vascular to the extravascular compartment of the liver, resulting in decreased aortic enhancement and increased hepatic enhancement. The equilibrium phase is the time period where both aortic and hepatic enhancement gradually decline as contrast agent in the liver shifts back into the central vascular compartment. The declining aortic and hepatic enhancement curves become parallel (13), and the difference between aortic and inferior vena caval attenuation values decreases to <10 HU. Hepatic lesions become indiscernible during this phase if the amount of interstitial contrast agent is similar in the lesion and in the adjacent normal parenchyma.

During the hepatic arterial phase, hypervascular tumors are maximally enhanced, appearing as hyperattenuating lesions. In the later portal venous phase, these lesions may become indiscernible as the liver parenchyma enhances maximally (Fig. 6.5). In contradistinction, hypovascular tumors may be barely visible or imperceptible during the hepatic arterial phase of enhancement because they receive relatively less blood from the hepatic artery, but they can be seen as hypoattenuating lesions during the portal venous phase of enhancement (Fig. 6.6) (18,19,20,21,22).

The magnitude of hepatic enhancement is dependent on total iodine dose, which is based on contrast medium
volume and concentration, and the rate of injection (13,23,24). For a given injection rate, maximum hepatic enhancement increases linearly with the dose of iodine administered. The magnitude of hepatic tissue enhancement also increases with faster injection rates, but this occurs in a nonlinear fashion.

The timing of peak hepatic arterial and venous enhancement is dependent on the injection duration. A shorter injection duration (either rapid rate or low-volume injection) results in earlier peak hepatic enhancement, whereas a longer injection duration (slow rate or high-volume injection) produces later peak hepatic enhancement (23,24). Peak hepatic arterial enhancement usually occurs within 10 seconds after the completion of the contrast medium injection, and peak hepatic parenchymal enhancement within 30 seconds (13).

Collimation and pitch will vary depending on the type of scanner used. For a 16-row detector, 0.75- to 1.5-mm collimation with a pitch of 1 to 1.5 suffices. For a 64-row detector, 0.6- to 1.25-mm collimation and a pitch of 1 to 1.5 suffice. A 5-mm section thickness and interval are usually adequate for routine viewing, with thinner reconstructions needed if multiplanar and 3D reconstructions are planned (18). (See Chapter 1.)

CT scans without contrast medium administration are not routinely needed, but they may be helpful in selected instances. Indications for noncontrast studies include identification of calcifications or acute hemorrhage and determination of the attenuation value of a parenchymal lesion, which may be useful in its characterization.

Single-phase contrast-enhanced CT during the peak time of parenchymal enhancement (portal venous phase) suffices for evaluation of most hepatic diseases in children (abscess, diffuse hepatic disease, venous vascular disorders). Intravenous contrast agent is administered at a dose of 2 mL/Kg (not to exceed 125 mL total). Hand injection of contrast medium is recommended if intravenous access is through a peripheral access line. With a hand injection, the contrast medium is injected as rapidly as possible. A power injector is used in children who have antecubital cannulas of 22 gauge or larger in place. The rate of contrast injection is determined by the size of the catheter (see Chapter 1). The scan delay time for hepatic imaging is 50 to 60 seconds after the start of the contrast administration.

Multiphase imaging in both arterial and portal venous dominant phases of enhancement is warranted for detection of hypervascular tumors, characterization of a lesion seen on screening sonography, and preoperative vascular mapping in patients who are considered for hepatic resection. In a typical dual phase hepatic CT study, the arterial phase begins 15 to 25 seconds after the start of contrast injection and the portal venous phase begins 45 to 50 seconds after the start of the contrast bolus. The scan delay for arterial phase imaging also can be timed using a bolus tracking device, rather than an empirical delay, with the region of interest placed on the abdominal aorta. A threshold value of 50 to 100 HU is used to trigger the diagnostic CT study. The liver alone is scanned during the arterial phase. The entire abdomen and pelvis are imaged in the portal venous phase. The contrast agent volume and flow rate are the same as described above.

Primary angiosarcoma is a rare tumor arising from endothelial cells. It affects children 3 to 10 years of age (25). The pathologic appearance is that of multiple small nodules or a large mass with or without satellite nodules. On unenhanced CT, the tumor is hypoattenuating, al-though areas of recent hemorrhage may be hyperattenuating or isoattenuating (13). After intravenous contrast administration, angiosarcoma may demonstrate nodular, irregular, or ring-shaped enhancement (Fig. 6.14). The areas of enhancement are often central in location and have an attenuation lower than aorta, unlike hemangioendothelioma and hemangioma, which are isoattenuating to aorta.

The malignant tumors of childhood that most frequently metastasize to the liver are Wilms tumor, neuroblastoma, rhabdomyosarcoma, and lymphoma. Neuroblastoma may affect the liver in either stage IV or IV-S disease. Stage IV disease is characterized by the presence of a retroperitoneal mass and distant metastases to skeleton, liver, or nodes. Stage IV-S neuroblastoma occurs in patients under 1 year of age who have small ipsilateral tumors (not crossing the midline) and metastases to liver, skin, and bone marrow but not to cortical bone. Clinically, patients with hepatic metastases present with hepatomegaly, jaundice, abdominal pain or mass, or abnormal hepatic function tests.

Hepatic metastases are typically multiple and well circumscribed. Less commonly they are solitary or diffusely infiltrating. On precontrast images, they may be isoattenuating or hypoattenuating to liver parenchyma. Most metastases are hypovascular and appear hypoattenuating relative to liver parenchyma on portal venous phase imaging (Fig. 6.15). On arterial phase images, metastases may demonstrate peripheral rim enhancement.

Hypervascular metastases are rare in children. Such tumors include sarcomas, renal cell carcinoma, pancreatic
islet cell tumors, and thyroid cancers. Metastases from these tumors are more easily seen during arterial phase imaging when they appear hyperattenuating relative to normal parenchyma. These metastases may become isoattenuating with surrounding parenchyma during the portal venous phase and difficult to recognize.

Diffuse replacement is usually secondary to stage IV-S neuroblastoma. CT shows widespread heterogeneity and innumerable hypoattenuating lesions (Fig. 6.15B). The differential diagnostic considerations for this appearance include hepatic fibrosis, cirrhosis, patchy steatosis, and diffuse infiltrating hepatoblastoma or hepatocellular carcinoma.

Involvement of the liver has been reported in ≤50% of all children with posttransplant lymphoproliferative disease (PTLD) (47,48,49). Three patterns of hepatic PTLD have been reported: discrete hypoattenuating nodular lesions (Fig. 6.16), poorly defined infiltrative lesions (Fig. 6.17), and periportal infiltration. This third pattern may result in biliary obstruction.

Benign tumors account for about one third of all primary liver tumors in children (26,27,28). These tumors can be classified by cell of origin as mesenchymal (infantile hemangioendothelioma, hemangioma, and mesenchymal hamartoma) or epithelial (focal nodular hyperplasia, adenoma, and cysts). Mesenchymal tumors are more common than epithelial tumors in children.

Pyogenic abscesses can result from trauma owing to penetrating injuries; contiguous spread of infection from adjacent organs, such as lung or bowel; arterial or portal venous spread related to septicemia; or less commonly from an ascending cholangitis (79). Immunosuppressed children, especially those with chronic granulomatous disease of childhood (X-linked recessive disorder characterized by failure of the leukocytes to lyse phagocytized bacteria), are particularly at risk of developing hepatic abscesses. Patients usually are symptomatic, presenting with fever, upper abdominal pain or discomfort, and occasionally hepatomegaly.

Hepatic abscesses have a predilection for the superior portion of the right lobe. The characteristic CT appearance is that of a well-circumscribed, hypoattenuating (10 to 20 HU), unilocular or multilocular mass with a contrast-enhancing rim (Fig. 6.29). Another pattern is the double-target sign, characterized by a hypoattenuating central area, a surrounding hyperattenuating ring, and an outer hypoattenuating zone (80). In some cases, an aggregate of multiple small abscesses, <2 cm (the cluster sign), may be seen (81). The CT appearance of hepatic abscess is not specific, and it also can be seen with complicated cysts, hematomas, and necrotic tumors. The most specific sign of abscess is intracavitary gas, in the form of either multiple bubbles or an air–fluid level (Fig. 6.30). Unfortunately, central gas occurs in only a few cases. Pyogenic hepatic abscesses, even multiloculated ones, usually respond to a combination of antibiotics and percutaneous catheter drainage.

Cat-scratch disease is characterized pathologically by a granulomatous or suppurative reaction to infection by
Bartonella henselae. It occurs in immunocompetent children and adolescents who have been scratched by a domestic cat. Patients present with fever and unilateral lymph node enlargement proximal to the site of inoculation. Contrast-enhanced CT shows multiple small nodular lesions, ranging between 3 mm and 2 cm in diameter (Fig. 6.31) (82). The appearance is nonspecific, and the differential diagnosis includes other infectious diseases, such as other pyogenic or fungal abscesses; neoplastic diseases, including metastatic disease and lymphoma; and other granulomatous diseases, such as tuberculosis and sarcoidosis.

Amebiasis is caused by the parasite Entamoeba histolytica. Infection is common in tropical or subtropical climates, with cases also reported in the southwestern United States. The most frequent site of involvement by amebiasis is the intestine. In the bowel, cysts that have been ingested in contaminated water or food dissolve, and the trophozoites colonize the colon, usually the cecum and ascending colon. The parasites may then penetrate the colonic mucosa and reach the liver via the portal system. The frequency of hepatic disease is 3% to 9% (83). Approximately 75% of hepatic amebic abscesses occur in the right lobe. Affected patients commonly present with right upper quadrant pain or tenderness. CT findings of amebic abscess are a solitary, unilocular, hypoattenuating (10 to 20 HU) mass with an enhancing wall (84) (Fig. 6.32). The wall may be smooth, nodular, or irregular, and in some cases, it may be surrounded by an incomplete hypoattenuating zone of edema (84). Imaging features of amebic and pyogenic abscess are similar, and aspiration of the fluid contents is needed for diagnosis. Medical therapy is effective in most patients with amebic abscesses, but percutaneous drainage may be required in some cases (83).

Fungal microabscesses are found almost exclusively in immunocompromised patients (85,86). The common inciting agent is Candida albicans and less frequently Aspergillus sp. or Cryptococcus sp. At CT, fungal microabscesses appear as multiple, small, hypoattenuating masses scattered throughout both lobes of the liver (Fig. 6.33)
(85,86,87,88). Occasionally, the abscesses contain a central high attenuation focus, believed to represent hyphae. Some lesions also may show rim enhancement (85). Concomitant abscesses in the spleen and kidney are frequent (86). The CT appearance of fungal abscesses is not specific; metastases, lymphoma, pyogenic or mycobacterial infection, cat-scratch disease, and multifocal steatosis can have similar appearances.

Medical therapy with antifungal drugs is the treatment for fungal abscesses. The lesions usually regress completely with antifungal treatment. Occasionally, sterilized lesions may persist and in some instances may calcify (86,87,89).

Hydatid disease is a parasitic infestation caused by the larval stage of the tapeworm Echinococcus granulosus or by Echinococcus alveolaris, the former being the more common in the United States. The liver is the most common site of involvement. Hepatic involvement by E. granulosus produces unilocular or multilocular cysts with thick or thin walls (90,91,92,93) (Fig. 6.34). Wall calcification is common (90,91,92,93). Usually, there is one dominant cyst (the mother cyst), which is surrounded by a number of smaller cysts (daughter cysts) (Fig. 6.34). Less commonly, the daughter cysts occupy almost the entire volume of the mother cyst. Surgical excision or percutaneous drainage combined with albendazole therapy has been the standard treatment of hepatic hydatid cysts (94,95). Dense calcification of the cyst may be noted during the healing process (Fig. 6.35).

In contradistinction to the CT appearance produced by E. granulosus, hepatic involvement by E. alveolaris produces solid ill-defined infiltrating masses, which are hypoattenuating to adjacent normal parenchyma and do not undergo contrast enhancement (96,97). Calcification is common, present in approximately 90% of cases. The CT appearance is not specific, and it may simulate a primary malignancy or hepatic metastases. Surgical excision in combination with antiparasitic therapy is the treatment of choice (98).

Bacillary angiomatosis is an infection occurring in immunosuppressed patients caused by Bartonella henselae (99). It is a vascular proliferative lesion that predominantly affects the skin, but it can involve the liver. In the liver, it produces cystic, blood-filled spaces (termed peliosis hepatis, see below). On CT, it appears as multiple hypoattenuating or hyperattenuating lesions, usually <1 cm, scattered throughout the liver.

There are two histologic types of hepatic steatosis: microvacuolar and macrovacuolar steatosis (100). In microvacuolar steatosis, tiny fat droplets or vacuoles fill the hepatocytes without displacing the nucleus; in the macrovacuolar type, one or more large droplets of fat fill the hepatocyte without expanding it and push the nucleus against the cell wall. Causes of microvacuolar steatosis include acute fatty liver of pregnancy, Reye syndrome, cystic fibrosis, and massive tetracycline therapy. Patients are acutely ill and present with a painful liver, vomiting, jaundice, and coma. Microvacuolar steatosis is rarely reversible.

Macrovacuolar steatosis is indolent and usually asymptomatic, although substantial fatty infiltration may produce smooth hepatomegaly and elevated serum liver function tests. Causes include nutritional abnormalities (Kwashiorkor, obesity, intestinal bypass); metabolic diseases; drug-related diseases (corticosteroids); viral infections; and cryptogenic disorders. Macrovacuolar steatosis is reversible if the underlying abnormality can be corrected.

Steatosis may be diffuse or focal. The attenuation of the normal liver measures 40 to 65 HU and is generally greater than that of the spleen. In patients with diffuse fatty replacement, the attenuation is decreased, measuring less than the spleen on unenhanced CT scans. With marked steatosis, the attenuation of the liver becomes less than that of hepatic venous structures, and as a result, the hepatic vessels stand out as high-attenuation structures against the background of low-attenuation fat (Fig. 6.36). Hepatic steatosis is more difficult to diagnose on contrast-enhanced CT scans because the spleen becomes higher in attenuation than the liver.

Focal hepatic steatosis can be patchy, segmental, or lobar with margins that are well circumscribed or ill defined (Fig. 6.37). Hence, the appearance may simulate that of hepatic neoplasm or abscess. However, focal steatosis has several characteristic features that can be helpful in differentiating it from hepatic mass lesions: (a) typical periportal or periligamentous location (e.g., anterior to the portal vein and adjacent to the fissure for the ligamentum teres); (b) absence of vascular displacement or distortion; (c) absence of mass effect and bulging of the hepatic contour; (d) sharply angulated borders; and (e) a nonspherical configuration (101).

Hemochromatosis refers to the presence of increased iron storage in the liver, leading to hepatic dysfunction.
There are three major types of hemochromatosis: (a) primary or genetic, (b) secondary, and (c) transfusional iron overload (102). Primary hemochromatosis is an HLA-linked inherited disorder in which a mucosal defect in the intestinal wall leads to increased absorption of ingested iron, which is deposited in parenchymal cells of the liver. Most patients eventually develop hepatic fibrosis or cirrhosis. Secondary or erythropoietic hemochromatosis results from increased absorption of iron secondary to excessive red cell breakdown, usually related to an underlying hemolytic anemia or excessive iron ingestion. Transfusional iron overload is found in patients who undergo multiple blood transfusions. The iron from the transfused erythrocytes is deposited in the liver and also in the spleen and bone marrow.

The characteristic CT finding of hemochromatosis is an increase in hepatic attenuation on unenhanced CT scans (Fig. 6.38). By comparison with normal unenhanced parenchyma, which has mean attenuation values of 40 to 65 HU, the liver in hemochromatosis has attenuation values of ≥70 HU (103,104). The sensitivity of single-energy CT scanning at 120 kVp for the diagnosis of iron overload is approximately 63% (104). Thus, normal attenuation values do not exclude the diagnosis of hemochromatosis. In some cases, reducing the scanning energy to 80 kVp may help make the diagnosis of iron overload if scanning at 120 kVp is equivocal (103). Increased attenuation values are not specific for iron overload and can be seen in other disorders, such as glycogen storage disease and Wilson disease, and also in patients who are receiving hyperalimentation or antineoplastic drugs, such as cis-diamminedichloro platinum (105).

Glycogen storage disease type I (von Gierke disease) results in the deposition of glycogen in multiple areas of the body, including the liver. The most common finding of hepatic involvement is hepatomegaly. Hepatic attenuation is normal or hypoattenuating owing to the associated steatosis (106,107). Less often, the liver is hyperattenuating, owing to the increased x-ray attenuation of the deposited glycogen. Hepatic adenomas and hepatocellular carcinomas have been reported in association with glycogen storage disease (68,108,109,110,111) (Fig. 6.39).

Wilson disease is an autosomal recessive abnormality of copper metabolism characterized by excessive copper deposition in the liver, brain, and cornea. A hyperattenuating liver may be seen on unenhanced CT (112,113). Portal hypertension and cirrhosis, hepatic adenoma, and hepatocellular carcinoma are complications of long-standing disease.

Hepatitis is a generalized inflammatory process of the hepatic parenchyma, and the leading cause of hepatitis in children is viral infection. Common pathogens are hepatitis A, B, C, non-A, and non-B viruses, but cytomegalovirus, herpes, and Epstein–Barr virus also can result in hepatitis.
Less often, injury from inhalation, ingestion, or parenteral administration of drugs or toxins is a cause of hepatic inflammation. Histologically, acute hepatitis is characterized by cellular necrosis and inflammatory cellular infiltration, usually sparing the portal tracts. Chronic hepatitis is classified as persistent or active. In chronic persistent hepatitis, periportal inflammation is present without architectural disruption; in chronic active hepatitis, there is extensive inflammation, necrosis, fibrosis, and architectural distortion (114,115).

Diagnostic imaging is not required in patients with clinical evidence supporting the diagnosis of uncomplicated acute hepatitis, but imaging can be useful to identify cirrhosis in patients with chronic hepatitis. The CT appearance of hepatitis varies with the severity and chronicity of the disease process. CT findings in acute hepatitis include hepatomegaly, diffuse fatty infiltration, enlarged nodes in the porta hepatis or gastrohe-patic ligament, gallbladder wall thickening, and periportal lucency (Fig. 6.40) (116). Findings of chronic hepatitis are similar to those seen in cirrhosis (see below).

Cirrhosis is the result of cellular destruction, which leads to regeneration and fibrosis (115). Common causes of cirrhosis in the pediatric population are bile stasis (e.g., as a result of biliary atresia or cystic fibrosis), hereditary diseases (e.g., Wilson disease, glycogen storage disease, tyrosinemia, galactosemia), and hepatitis (117,118,119,120). The role of CT in patients with cirrhosis is to evaluate liver size and the magnitude of portal hypertension prior to transplantation. In early cirrhosis, the liver often appears normal.

CT features of advanced cirrhosis include decreased hepatic size, heterogeneous parenchyma, irregular or nodular hepatic margins owing to the presence of regenerating nodules, and atrophy of the right hepatic lobe and medial segment of the left lobe with relative enlargement of the caudate lobe and left lateral segment (Fig. 6.41) (121,122,123). The ratio of the transverse width of the caudate lobe to the transverse width of the right hepatic lobe has been used for the diagnosis of cirrhosis (124). Although a caudate-to–right lobe ratio of ≥0.65 has been reported to be highly specific (100%) for cirrhosis, the sensitivity of this ratio can be <50% (124). In practice, measured ratios are rarely, if ever, needed for the diagnosis of cirrhosis, as the characteristic morphologic changes associated with cirrhosis are easily seen on visual inspection.

Regenerating nodules can be difficult to identify on unenhanced, arterial phase, and portal venous CT because they often are isoattenuating with liver. Some regenerating nodules, however, may become hypoattenuating during the arterial and portal venous phases of enhancement (Fig. 6.42). If they contain substantial hemosiderin, they may be recognized as high-attenuation nodules on unenhanced CT (Fig. 6.43) (125).

Secondary signs of cirrhosis include ascites, splenomegaly, and portosystemic collateral vessels. Portosystemic collateral vessels are a finding of portal hypertension and are described below.

Portal hypertension results when there is increased intrahepatic resistance to hepatopetal blood flow. Rarely, it is the consequence of increased blood flow owing to an arteriovenous fistula. Clinical features of portal hypertension include splenomegaly, ascites, prominent abdominal veins (caput medusae), hematemesis, and hypersplenism.

Obstruction to portal venous flow can occur at three levels: (a) prehepatic, caused by portal or splenic vein thrombosis; (b) intrahepatic, secondary to cirrhosis; and (c) post-hepatic, secondary to obstruction of hepatic venous outflow (126).

With increasing intrahepatic resistance and portal hypertension, portosystemic collaterals develop and blood flow is directed away from the liver to lower- pressure systemic vessels (hepatofugal flow). There are two main types of portosystemic shunts—tributary and developed collaterals (Fig. 6.44) (127). The former normally drain the portal venous system and include left gastric, short gastric, and superior and inferior mesenteric veins; the latter collaterals are normally not functional and include paraumbilical and splenorenal vessels.

At CT, portosystemic collaterals appear as well-defined, round, tubular, or serpentine structures that usually enhance to the same extent as adjacent vessels (128). The most common portosystemic shunts are the left gastric vein (coronary vein) to inferior esophageal vein, the paraumbilical to left portal vein, and the splenic vein to left renal vein. The left gastric (coronary) vein arises near the portosplenic confluence and ascends to the gastroesophageal junction to drain into the esophageal veins (Fig. 6.45). The recanalized paraumbilical vein connects with the left portal vein, which drains to the inferior epigastric veins in the anterior abdominal wall near the umbilicus (Fig. 6.46). The splenorenal collateral pathway shunts blood from the splenic vein via the splenorenal ligament into the left renal vein (Fig. 6.46).

Other portosystemic collaterals include gastrorenal, intestinal (mesentericocaval), and hemorrhoidal varices. Gastrorenal collaterals divert blood from the left gastric vein and short gastric veins to the left adrenal vein, which empties into the renal vein. Intestinal collaterals shunt blood from the superior mesenteric vein to the inferior vena cava through the root of the small bowel mesentery. The inferior mesenteric vein shunts splanchnic blood to the inferior hemorrhoidal vein.

Portal vein thrombosis is a cause of presinusoidal portal hypertension. In the pediatric population, common causes include sepsis, dehydration, shock, umbilical vein catheterization, and portal vein invasion by tumor. Less frequent causes include compression of the portal vein by enlarged periportal nodes or an enlarged pancreas, omphalitis, pylephlebitis owing to intra-abdominal
infection or inflammation (129), and coagulopathy disorders. Patients usually present with acute or subacute abdominal pain and occasionally with splenomegaly.

CT findings of acute portal vein thrombosis are a filling defect that completely or incompletely fills the portal vein lumen. The involved portal vein usually is enlarged (130). On unenhanced scans, acute thrombus may be hyperattenuating to liver. Following the administration of contrast medium, acute thrombus is hypoattenuating to surrounding tissue (Fig. 6.47). Streaky or diffuse enhancement of the obstructed portal vein indicates the presence of tumor clot rather than bland thrombus. With chronic portal vein thrombosis, small periportal collateral veins (portal vein cavernoma) can be seen (Fig. 6.48). These collaterals shunt blood around the obstructed vein into the liver, and they form only in patients who have normal intrahepatic circulation. Hepatic atrophy is also a late finding of portal venous thrombosis.

Two-flow related findings can be seen on contrast-enhanced scans. Decreased lobar or segmental parenchymal enhancement can occur during the portal venous
phase owing to locally diminished portal venous perfusion. Increased hepatic parenchymal enhancement can be seen during the late arterial phase owing to increased hepatic arterial flow to an area in which portal venous flow is decreased (131). The latter phenomenon is referred to as transient hepatic attenuation difference (THAD).

The Budd–Chiari syndrome refers to the clinical and pathologic abnormalities seen in the setting of obstruction to hepatic venous outflow, which can be at the level of the large hepatic veins or the inferior vena cava. Patients present with ascites, abdominal pain, jaundice, tender hepatomegaly, and hepatic failure. Most cases are idiopathic, but several specific causes have been described including hepatotoxins, radiation, chemotherapy, coagulation disorders, tumor thrombus (particularly Wilms tumor), and congenital webs or membranes involving the hepatic veins or inferior vena cava.

In acute thrombosis, noncontrast CT findings include diffuse hepatic hypoattenuation owing to parenchymal congestion, hepatomegaly, ascites, and hyperattenuating thrombi in the inferior vena cava or hepatic veins (132,133). Contrast-enhanced CT findings include homogeneous enhancement of the caudate lobe, a patchy or reticulated pattern of parenchymal enhancement in other hepatic segments, nonvisualized or small hepatic veins, and hypoattenuating thrombus within the hepatic veins or the inferior vena cava (Fig. 6.49) (132,133,134,135). Less common CT findings include hepatic infarcts, appearing as nonenhancing peripheral wedge-shaped areas; narrowing or occlusion of the inferior vena cava caused by pressure from an enlarged caudate lobe; and concomitant portal vein thrombosis. Extrahepatic findings include ascites, pleural fluid, and gallbladder wall edema.

CT findings of chronic venous occlusion include caudate lobe hypertrophy, compressed or poorly visualized hepatic veins, heterogeneous parenchymal enhancement (132), portosystemic collaterals, indicating portal hypertension, and splenomegaly. The collateral vessels are
usually intrahepatic (hepatic vein–to–hepatic vein and hepatic vein–to–portal vein) rather than extrahepatic shunts.

Veno-occlusive disease of the liver is the result of obstruction of small sublobular or postsinusoidal veins. It has been associated with hepatotoxins, and it also occurs in patients receiving radiation, chemotherapy, or bone marrow transplantation (136). Hepatotoxins cause hepatic edema, which slows blood flow within portal and hepatic venules, which in turn leads to venous stasis and subsequent thrombosis. Chemotherapy and irradiation produce inflammation of the endothelium of small venules and ultimately vessel occlusion. Clinical features include hepato-megaly, right upper quadrant pain, jaundice, and ascites. The diagnosis is difficult on CT, since sublobular veins cannot be directly visualized. Biopsy is generally required for definitive diagnosis.

Passive venous congestion in children is usually the result of severe right-sided heart failure and less commonly a complication of constrictive pericarditis. Acute venous congestion increases hepatic venous pressure, which in turn leads to decreased hepatic arterial flow and hypoxemia (137). Patients with hepatic congestion have tender hepatomegaly, abnormal liver function tests, or both findings.

The primary CT findings of passive congestion are heterogeneous mosaiclike parenchymal enhancement and dilatation of the hepatic veins and inferior cava owing to reflux of contrast-enhanced blood from the right atrium (Fig. 6.50) (137,138,139). Secondary findings include periportal hypoattenuation (140), cardiomegaly, pleural effusions, and ascites. The reticulated-mosaic enhancement pattern in passive venous congestion is indistinguishable from that seen in Budd–Chiari syndrome. However, in contrast to the Budd–Chiari syndrome, the hepatic veins in passive congestion are distended or enlarged rather than being small or poorly visualized.

Hepatic infarction is relatively uncommon because the liver has a dual blood supply. It usually is a complication of trauma or surgery. Acute infarction can be diffuse (Fig. 6.51) or focal. Focal infarction can have three patterns on CT: wedge shaped, rounded, or irregular areas of low attenuation paralleling bile ducts (141). Wedge-shaped infarcts are usually peripheral, whereas rounded lesions may have a peripheral or central location. With subsequent tissue necrosis, gas collections may be identified (142). Chronic changes include lobar or segmental atrophy and formation of hypoattenuating bile lakes, owing to biliary ductal necrosis.

Peliosis hepatis is a rare condition characterized by hepatic sinusoidal dilatation. The disease has been associated with hematologic disorders, human immunodeficiency virus (HIV), bacillary angiomatosis, and chronic
steroid therapy (143,144). Contrast-enhanced CT findings include multiple, small hypoattenuating lesions with delayed enhancement, large hypoattenuating areas with mild rim enhancement, and small round lesions with early enhancement (143,144,145,146). The CT apprearance mimics that of hemangioma, focal nodular hyperplasia, and nodular regenerative hyperplasia.

In the cadaveric procedure, the portal vein, vena cava, common hepatic artery, and common bile duct of the donor are anastomosed to the recipient. The donor bile duct is anastomosed to the recipient common duct or if the duct is too small, it is anastomosed to a loop of small bowel. In living donor transplants, the left hepatic artery, left portal vein, and left and middle hepatic veins are anastomosed to the recipient vessels; the recipient's vena cava is preserved. Biliary drainage is via a Roux-en-Y hepaticojejunostomy.

Preoperative CT is useful for assessing the patency of the intrahepatic vessels and for calculating hepatic volume. Postoperative CT is valuable for detecting transplant-related complications (150,151,152,153,154,155,156). The complications are similar in cadaveric and living-related donor grafting and include arterial and venous stenoses (Fig. 6.52A) and thromboses, hepatic infarction, biliary leak and stricture, bilomas (Fig. 6.52B), abscesses, and posttransplant lymphoproliferative disorder (Figs. 6.16 and 6.17).

A common expected finding on posttransplant CT is an irregular, hypoattenuating, nonenhancing area at the margin of the liver (157,158), representing subcapsular hepatic necrosis (Fig. 6.53). A second common finding is a hypoattenuating rim around peripheral branches of the portal vein (termed periportal lucency), thought to be owing to impaired lymphatic drainage caused by surgical disruption of lymphatic vessels (159). Both of these findings are usually minor complications of little clinical significance, although cases of posttransplant periportal low attenuation secondary to rejection or hepatic necrosis have been described (160,161).

The biliary tree is composed of the intrahepatic ducts, the common hepatic duct, and the common bile duct (Fig. 6.54). The intrahepatic ducts course from the periphery of the liver to the hepatic hilum, where they join to form the left and right hepatic ducts. These ducts, in turn, join to form the common hepatic duct. The cystic duct inserts into the common hepatic duct, which continues as the common bile duct. The common bile duct descends from the porta hepatitis to join the pancreatic duct at the ampulla of Vater. In approximately 60% to 70% of individuals, the common bile duct and the pancreatic duct unite to drain through a common orifice in the ampulla of Vater; in the remainder of individuals, they have separate orifices.

The common bile duct adjacent to the descending duodenum and the common hepatic duct in the porta hepatis are almost always seen in older children and adolescents. Their small size and paucity of intra-abdominal fat may preclude their visualization in infants and very young small children. Intrahepatic ducts are only occasionally seen on CT; the cystic duct is not seen. The use of thinly collimated sections and intravenous contrast medium improves identification of both normal and dilated bile ducts (163). Although absolute measurements of the extrahepatic common bile duct in children have not been documented on CT, they have been established by sonography. In general, the upper limits of size are 1 mm in neonates, 2 mm in infants ≤1 year of age, 4 mm in children 1 to 10 years of age, and 6 mm in adolescents and young adults (164).

The normal gallbladder is seen as a near-water-attenuation, oval or round structure in the interlobar fissure on the inferior surface of the liver. Although the position of the fundus (the bulbous distal end of the gallbladder) varies, the body and neck of the gallbladder are nearly always located in the porta hepatis and interlobar fissure. The wall of the normal gallbladder measures between 1 and 3 mm in thickness and may enhance after intravenous contrast administration. Bile has an attenuation similar to water and does not enhance. The normal gallbladder is <1 cm in diameter and 1.5 to 3 cm in length in infants and children younger than 2 years of age and 3.5 cm in diameter and 8 cm in length in children 2 to 16 years of age (165,166).

Oral and intravenous contrast agents are routinely used in CT examinations of the abdomen. However, oral contrast in the duodenum may cause streak artifacts that obscure stones in the common bile duct. Therefore, when common bile duct calculi are suspected, water is preferred as an oral contrast agent. Unenhanced CT scans of the upper abdomen are performed when gallbladder or ductal stones are suspected. Subsequently, contrast-enhanced scans are acquired through the upper abdomen from the liver through the pancreas. Single-phase scanning nearly always suffices for evaluation of biliary tract and gallbladder disease. Multiplanar reformatting and volume-rendered reconstructions can help in tracing the course of obstructed bile ducts or the vascular structures surrounding the biliary tree. Clinical applications for helical CT cholangiography with intravenous contrast agents have not been addressed fully in children.

Although CT is not widely used for evaluating children with suspected or known gallbladder disease, it can be useful to confirm or further delineate an abnormality detected with sonography. In addition, CT may reveal incidental congenital anomalies, cholelithiasis, or gallbladder wall thickening or edema during studies performed for other reasons.