Hepatic cysts are common liver lesions. Pathogenesis of these cysts is not clear, and acquired cysts are thought to represent retention cysts of bile ductule derivation [3]. The cyst is typically lined with a single layer of cuboidal to columnar epithelial cells. On imaging, cysts are homogeneous and well circumscribed and possess a sharp margin with the liver, with no visible wall. Although slight variations are common, cysts are usually rounded or oval shaped [4].

Simple cysts are fluid attenuation on CT (demonstrating an attenuation <20 HU) with a thin wall and no significant postcontrast enhancement. On MRI, these demonstrate low signal intensity on T1-weighted images and high signal intensity on T2-weighted images, with retention of the high signal intensity on the longer echo time (e.g., TE ≥120 ms) T2-weighted images. Simple cysts should not enhance following intravenous contrast administration (Fig. 2.4). The presence of thick walls or enhancing internal components suggests the diagnosis of hepatic abscess or neoplasm rather than a simple cyst. In addition, simple cyst can be differentiated from cystic metastases using the delayed phase of contrast enhancement where metastases can show enhancement, while cysts do not [5]. Rarely, hemorrhage or infection can complicate cysts, and these account for difficulty in differentiation from other complex cystic lesions including abscesses and neoplasms.

Liver involvement is the most frequent extrarenal manifestation in autosomal dominant polycystic kidney disease [6]. Liver cysts are the most frequently encountered manifestation, but other pathologies can occasionally be encountered, including congenital hepatic fibrosis and segmental dilation of the biliary tract. Cysts are usually multiple, with variable sizes (Fig. 2.5), and associated cysts in the kidneys or pancreas and positive family history are helpful in reaching the diagnosis.

Patients with autosomal dominant polycystic liver disease are usually asymptomatic, and severe symptoms can occur in 20 % of patients who develop massive hepatomegaly with compression of the surrounding organs. Rarely, acute complications can be caused by torsion, intracystic hemorrhage, and infections, with liver dysfunction occurring only sporadically [7]. However, advanced disease can result in liver failure, or Budd–Chiari syndrome.

Cysts demonstrate typical features of fluid-containing well-circumscribed lesions with fluid attenuation on CT (manifested by attenuation <20 HU) and MRI (manifested by low signal intensity of T1-weighted and high signal intensity on T2-weighted images) with no significant enhancement on postcontrast series. Because of the larger number of cysts, intracystic hemorrhage is more frequently encountered than in cases of simple hepatic cysts [8]. Intracystic hemorrhage is usually seen as areas of higher attenuation on CT and areas of altered MRI signal intensities due to blood degradation products [9].

Biliary hamartoma (also known as von Meyenburg complexes) are benign biliary malformations which are categorized under the spectrum of fibro-polycystic diseases of the liver. They arise from bile duct embryological remnant due to anomalous development of the ductal plate [10]. Biliary hamartomas is not uncommon and occurs in approximately 3 % of patients. These can be solitary or more commonly multiple, and multiple lesions can be extensive.

On CT and MR images, lesions are uniformly small (usually <1.5 cm) and well defined. On CT, the lesions may not be well characterized due to the too small size which may preclude accurate characterization [11]. The fluid content renders these lesions high signal intensity on T2-weighted images and low signal intensity on T1- weighted images. On heavily T2-weighted images, the signal intensity increases further, similar to that of fluid on T2-weighted images.

On MR cholangiography, bile duct hamartomas appear as multiple tiny cystic lesions that do not communicate with the biliary tree [10].

Although this appearance resembles simple cysts, biliary hamartomas may demonstrate a subtle thin rim of enhancement on postcontrast images (Fig. 2.6) due to enhancing fibrous capsule and/or compressed surrounding parenchyma. This rim of subtle enhancement should not be mistaken for other diagnoses such as abscesses and metastases. Capsule can also demonstrate low signal intensity on T2-weighted images.

Caroli disease is a rare autosomal recessive condition characterized by congenital cystic dilation of the intrahepatic bile ducts [12]. Although Caroli disease generally involves the entire liver, it may be segmental or lobar. Two forms of Caroli disease have been described: (a) a less common “pure” form (type 1) and (b) a more complex form (type 2) that is associated with other ductal plate abnormalities such as hepatic fibrosis [13].

Often, patients clinically present with bouts of recurrent fever and right upper quadrant pain. Jaundice may occur if the calculi obstruct the common bile duct. On cross-sectional CT and MRI, Caroli disease appears as multiple variable-sized, non-enhancing cystic lesions. MR cholangiopancreatography is a noninvasive imaging technique that has become the first-choice modality for diagnosing Caroli disease [14]. MRCP demonstrates the connection of these cysts to the biliary ducts (Fig. 2.7), which is the key feature for the diagnosis.

The central dot sign is described on contrast-enhanced CT and MRI as well as color Doppler ultrasound as a tiny enhancing focus in the center of cystic-appearing morphology and is due to the contrast-enhanced small portal venous branch protruding into the dilated intrahepatic bile ducts. This sign has been used to make a diagnosis of Caroli disease [15].

Complications may occur secondary to bile stagnation which leads to cholangitis and formation of calculi which may result in biliary obstruction and jaundice. Cholangiocarcinoma has been reported to complicate Caroli disease in 7 % of cases [16].

Biliary cystadenoma/cystadenocarcinoma is a group of rare biliary neoplasms occurring most commonly in middle-aged women. Biliary cystadenomas are considered premalignant neoplasms with the majority located in the liver with a significantly less incidence in the extrahepatic biliary tree. These are often incidental, but symptoms can arise due to mass effect of large lesions.

Typically, these tumors appear as multiloculated lobulated cystic lesions with several septations and occasional solid nodules. On CT and MR imaging, these appear as cystic (typical fluid characteristics on CT and MRI), with enhancing septations and solid components [17]. Wall or septal calcifications can be also seen. Altered characteristics of the fluid can be seen due to mucin/protein contents or hemorrhage. These can be demonstrated as higher CT attenuation and higher signal intensity on T1-weighted images (Fig. 2.8) [18].

Although malignant tumors tend to have thicker nodular septations, there is no definite reliable criterion for differentiating cystadenoma from cystadenocarcinoma as nodular soft tissue projections can be seen in both tumors [19].

Hypoenhancing lesions show mild enhancement after intravenous contrast injection. This enhancement is, however, less than the background liver. For this reason, these lesions appear hypodense on contrast-enhanced CT and hypointense on contrast-enhanced T1-weighted images. Other lesions such as focal fatty infiltration may appear similarly hypodense on CT and hypointense on contrast-enhanced T1-weighted mimicking hypoenhancing mass lesions (this can be differentiated by drop of signal intensity on out of phase compared to in phase and will be discussed under lesions containing intracellular lipid).

Hepatocellular carcinoma will be discussed under the category of arterially enhancing lesions as HCC most commonly manifests as arterially enhancing mass. However, hepatocellular can rarely appear as a hypovascular mass (Fig. 2.24).

These lesions demonstrate increased enhancement compared to the liver parenchyma on arterial phase after intravenous contrast administration:

Other less common causes of arterially enhancing lesions/pseudolesions:

HCC is considered the most common primary hepatic malignancy. The incidence is higher in cirrhotic liver as compared to the non-cirrhotic liver with hepatitis B is the major risk factor in the non-cirrhotic liver [52].

HCC is three times more common in men than women [53]. The difference in the 5-year survival rate for untreated symptomatic HCC in contrast to the survival rate in cirrhotic patients with small (<2 cm) HCC who have undergone liver transplantation is very impressive (5 and 80 %, respectively). This highlights the importance of early detection and treatment of HCC [52].

The pathogenesis of HCC begins with the development of benign regenerative nodules which in turn transform to dysplastic nodule. Dysplastic nodule is a premalignant lesion that finally progress to HCC [54].

MRI is better than CT in detection and characterization of HCC due to its superior contrast resolution, multiplanar imaging, and the ability to obtain serial dynamic imaging after IV administration of contrast using various pulse sequences. The combined use of all these features helps in superior characterization of hepatic nodules/masses and reaches a definitive diagnosis [55].

Larger HCCs >2 cm are more likely to follow the typical features of HCC, which have high signal intensity on T2-weighted images and low signal intensity on T1-weighted images with hyperenhancement on arterial phase and washout on delayed phase. The appearance of smaller HCCs <2 cm in MRI may vary as follows: on T2-weighted images, lesions may appear iso- to hyperintense to the surrounding liver, and on T1-weighted images, it is usually hypointense to the liver [55], and rarely HCC can contain high-protein content or less likely fat, which results in a high signal intensity on T1-weighted images.

The process of neoangiogenesis of HCC causes the specific features of HCC and renders the arterial phase of contrast-enhanced images to be the most sensitive sequence for detecting HCC [56], where the HCC shows moderate to intense enhancement in the early arterial postcontrast phase with gadolinium combined with washout on the delayed phase. These features are considered the only noninvasive radiological features in diagnosing HCC by the organization responsible for liver transplantation in the USA by the “United Network for Organ Sharing” {UNOS} [57]. Few HCC lesions are hypovascular and appear isointense with the liver parenchyma in the arterial postgadolinium phase, but show delayed washout in interstitial postgadolinium phase [58] (Fig. 2.26). This feature of delayed washout of the HCC can help in differentiating HCCs from other arterial enhancing lesions such s atypical hemangiomas (flash-filling hemangiomas), focal nodular hyperplasia, and hepatic adenomas.

Due to the development of the HCC from high-grade dysplastic nodules, there is an overlap between both entities. But there are some features that can be helpful in raising the suspicion of HCC over dysplastic nodules, such as development of new foci, larger size >2 cm, the presence of delayed enhancement washout, presence of a coexistent large HCC, and rapid interval growth of lesion (most reliable feature); while regression or disappearance of lesion on follow-up MR studies, small size of nodule <1.5 cm (in combination with the absence of a large coexistent HCC) is more consistent with dysplastic nodule.

Diffuse HCC occurs in 13 % of cases of HCC and appears as diffuse extensive heterogeneous permeative tumor often associated with portal vein thrombosis. These tumors appear as heterogeneous mottled moderate low signal intensity on T1-weighted images and moderate high signal intensity on T2-weighted images. These features are nonspecific and limit the ability of noncontrast sequences in detecting rather diagnosing the diffuse disease due to the similarity with the hepatic background. These tumors have patchy or nodular early enhancement pattern with enhancement washout and pseudocapsule enhancement on delayed phase [59].

Venous thrombosis associated with HCC is almost always affecting the portal veins. However, it can involve the hepatic veins. Tumor thrombosis is commonly high signal in T2-weighted images and enhances with gadolinium, in contrast to bland thrombus which appears hypointense in T2-weighted images and does not enhance with gadolinium [59]. On diffusion-weighted image, tumor thrombus demonstrates restricted diffusion, in contrast to bland thrombus which does not restrict diffusion.

Hepatocellular adenomas (HCAs) are monoclonal hepatocellular benign epithelial neoplasms. Typically, 90 % of adenomas are described in young women with history of oral contraceptive [60]. Tumors typically involute spontaneously after patients withdraw from birth control pills. Other much less frequent associations include the use of anabolic steroids and disorders associated with abnormal carbohydrate metabolism [61]. Patients may present with a variety of symptoms, most commonly right upper quadrant abdominal pain, which is mostly related to hemorrhage into the tumor in 30 % of cases, and 10 % of adenomas transform into malignancy [62].

The histological hallmark consists of clusters of benign hepatocytes arranged in slender plates of cells. Neoplastic hepatocytes are separated by slit-like sinusoids and numerous thin-walled veins. Bile ducts are absent in internal structure of hepatocellular adenoma [63]. The typical MR appearance of adenoma is homogeneously mild hyperintensity on T2-weighted images. On T1-weighted images, homogeneously mild hypointensity, isointensity, or hyperintensity are described (Fig. 2.27). On postcontrast CT and MR images, homogeneous blush immediately after contrast has been described which uniformly fades to isodensity/isointensity relative to the liver parenchyma in the delayed phase. Some adenomas contain intracellular lipid yielding a drop in signal intensity on out-of-phase compared to in-phase pulse sequences.

Recently, hepatocellular adenomas have been classified into four subtypes (Table 2.2): hepatocyte nuclear factor-1α-mutated HCAs (HNF-HCAs), HCAs characterized by β-catenin mutations, inflammatory HCAs (I-HCAs), and miscellaneous category, which are due to different gene mutations. They have different pathological features which impact their features on imaging, the I-HCAs are the most common subtype and account for about 40–50 % of all hepatocellular adenomas, this typically appears hyperintense on T2-weighted images with intense arterial enhancement that persist in the venous and delayed phases, while the HNF-HCAs represent the second most common type of hepatocellular adenoma and constitute about 30–35 % of all hepatocellular adenomas and show diffuse steatosis which appears as homogeneous high signal in T1-weighted images with moderate arterial enhancement that does not persist into the venous phase, and lastly the HCAs with β-catenin have no specific features and generally shows arterial enhancement with portal venous washout, which make it very difficult to distinguish from HCC [64, 65] (Table 2.3).

Dysplastic nodules represent intermediate step in the pathway of carcinogenesis of hepatocytes in cirrhotic liver. These are considered as premalignant nodules and are found in about 20 % of cirrhotic livers. Some studies have documented the development of HCC within a dysplastic nodule in few months. Dysplastic nodules appear most commonly as isointense or hypointense on T2-weighted images and isointense to hyperintense on T1-weighted images [66]. Unlike regenerative nodules, dysplastic nodules have been found to contain isolated arteries, with documented correlation between the extent of enhancement on arterial phase and the grade of dysplastic nodules; the greater the degree of arterial enhancement of the nodule, the more likely the potential of malignant transformation of HCC [67]. However, dysplastic nodules have the tendency to fade toward background signal of the liver in the interstitial and delayed phase of enhancement, which help in its differentiation from the small HCC which are most likely to exhibit enhancement washout with delayed capsule enhancement. Other features that can help differentiate dysplastic nodules from HCC include: high signal intensity of HCC on T2-weighted images. Foci of small HCC, which develop in the high-grade dysplastic nodules, appear as high signal intensity foci within low signal intensity nodule on T2-weighted images (nodule within a nodule sign) [68].