This chapter focuses on the imaging of masses and mass-like entities of the upper abdomen arising from the liver, bile ducts, and pancreas. Radiologists play the crucial role in first discovering these masses, determining their site of origin, and providing the differential diagnosis. Masses in adults are different than those in children, and some masses arising from the biliary tree, liver, and pancreas even may be diagnosed prenatally.

The claw sign is a universal sign that helps show the organ of origin of a mass, and that organ should form a “claw” around or encircle the mass. Occasionally, a mass can be so large and abut multiple organs that the site of origin is difficult to determine. In children, ultrasound (US) is the initial imaging modality for abdominal masses and can be used to depict clearly masses from all three organs. If further evaluation is warranted, contrast-enhanced magnetic resonance imaging (MRI) is useful but in some children may require sedation. With faster sequences, proper patient preparation, and distraction techniques such as those provided by certified Child Life specialists, there is increasingly less need for sedation, even in young children [1]. Computed tomography (CT) is faster than MRI and rarely requires sedation, but it carries the risk of ionizing radiation. However, in the past decade, enormous improvements have been made in decreasing radiation dose by altering protocols and utilizing postprocessing software [2]. Nuclear medicine studies rarely play a role in the initial imaging of these masses.

In children, the histology of the liver mass can be predicted based on patient age at presentation, mass appearance, and elevated alpha-fetoprotein (AFP) [3] . Tumors that occur in the newborn period, some of which are discovered prenatally, include benign (infantile hemangioendothelioma/ infantile hepatic hemangioma, mesenchymal hamartoma) and malignant (hepatoblastoma) tumors. The most common primary hepatic malignancy in toddlers is hepatoblastoma. In older children, hepatocellular carcinoma (HCC) is the most common primary malignancy and is typically associated with underlying liver disease. Hepatoblastoma and HCC can be distinguished from benign liver masses such as focal nodular hyperplasia (FNH) and hepatic adenoma by elevations in alpha-fetoprotein. A rare tumor, fibrolamellar HCC is a low-grade malignancy and does not cause AFP elevation. Many masses occurring in childhood have classic appearances, which are described below. In children, just as in adults, the most common liver tumors are not primary but secondary to metastases [4].

Imaging is extremely important in helping to diagnose a hepatic mass. Typically, US is the initial imaging modality and discovers the mass because US is the main screening modality of the abdomen in children. After it is determined that the mass arises from the liver, assessing size, location within the liver, vessel involvement and patency, and other anomalies in the abdomen are important. US contrast agents also help make liver masses more conspicuous, but this has mainly been studied in adults [5]. Elastography, often used in determining the character of the liver, specifically in distinguishing cirrhotic from normal livers, is not routinely used to better define liver masses in children [6].

In the past decade, advances in MRI techniques and new contrast agents have allowed for more specific diagnosis of liver masses. Contrast-enhanced MRI is fundamental for evaluating liver masses. The most widely available contrast agents are extracellular gadolinium chelates. The enhancement pattern using this contrast has been widely studied. Newer hepatocyte-specific contrast agents are taken up in variable quantities by functioning hepatocytes and are excreted by the bile. These agents give an increased contrast to noise ratio (CNR) for nonhepatocellular lesions compared with that of the background liver, which increases conspicuity on delayed T1-weighted images. Tumors that contain hepatocytes, such as adenomas, FNH, and nodular regenerative hyperplasia, demonstrate enhancement on delayed imaging. Some hepatocyte-specific agents are first distributed into extracellular spaces and then taken up by hepatocytes, giving the benefit of dynamic imaging and delayed hepatobiliary-phase imaging [3, 7, 8]. Pediatric tumors have been recently described with such agents.

CT also beautifully depicts hepatic masses, with its main downside being ionizing radiation. However, CT rarely requires sedation and may be more readily available than MRI. Ionizing radiation can be kept to a minimum by adhering to low-dose principles [9] and eliminating unnecessary phases, such as scanning before administration of contrast.

Nuclear medicine rarely plays a role in the initial evaluation of liver masses, unless the mass originates from another organ, such as neuroblastoma or lymphoma, and if metastases are discovered in the liver using isotopes specific to the primary mass. Occasionally, FNH is distinguished from other tumors because Kupffer cells present within the mass take up radiotracer on sulfur colloid scintigraphy.

Benign and malignant hepatic masses can be distinguished by AFP elevation. This glycoprotein is normally synthesized by the fetal yolk sac, liver, and intestine. Thus, the value is normally elevated in the newborn period and gradually declines [10]. AFP is elevated in epithelial liver tumors (hepatoblastoma and HCC); however, its elevation is also associated with yolk-sac tumors (not arising from the liver), nonneoplastic conditions, including acute liver disease, and hereditary disorders [11]. The following discussion of hepatic masses is divided by patient age, with masses listed in order of frequency from common to uncommon.

This section on hepatic masses will primarily discuss tumors of the liver, with very little on infection.