Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

regardless of etiology. Regenerative activity among liver cells and their nuclei is mild in a setting of primary biliary cirrhosis, more so in alcoholic cirrhosis, and most prominent in posthepatitic cirrhosis (HBV related); thus patients with posthepatitic cirrhosis are at greater risk for carcinoma than those with other types of cirrhosis.

Histologic differentiation of a dysplastic nodule from a regenerating nodule in a cirrhotic liver tends to be difficult from small biopsy specimens.

Neither CT nor US detects smaller nodules. Precontrast, detectable nodules are isoto hyperdense, depending on their iron content and, in fact, are better identified precontrast than postcontrast. In distinction to most liver tumors, the blood supply to regenerating nodules is mostly portal venous, and they are thus isodense during portal phase imaging. Because of their mostly portal blood supply, these nodules do not enhance much on postcontrast arterial phase CT. In distinction to hepatocellular carcinomas,which derive most of their blood from an arterial supply, a regenerating or dysplastic nodule should be suspected if dynamic CT suggests a mostly portal venous blood supply to a nodule. One should keep in mind, however, that a dual blood supply is normal, and in actual practice this blood supply differentiation is difficult to apply to small tumors.

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Computed tomography arterial portography performed prior to partial liver resection for hepatocellular carcinoma in cirrhotic patients revealed regenerative nodules as enhancing 3- to 10-mm tumors surrounded by lower density fibrous septa (84) (Fig. 7.24); CT hepatic arteriography identified regenerative nodules as nonenhancing tumors, similar in size to those seen with arterial portography, surrounded by enhancing fibrous septa. Tumor conspicuity is determined to a large degree by surrounding fibrosis. Nevertheless, CT hepatic arteriography detects more nodules and is more sensitive than CT arterial portography.

With US, regenerating nodules range from hypoechoic to hyperechoic compared to normal liver parenchyma.

Magnetic resonance imaging is more sensitive in detecting regenerating nodules than CT or US. Most regenerating nodules are isoto hypointense on T1and hypointense on T2weighted images. Surrounding inflammation and fibrous septa tend to be hypointense on T1and isoor hyperintense on T2-weighted images. Their hypointensity on T2-weighted sequences is secondary to a paramagnetic effect from their increased iron and, in part, to surrounding fibrous septa. More complex nodules are inhomogeneous and contain discrete foci of iron (siderotic nodules),and T2-weighted MR in these reveals a hypointense focus due to the iron within a larger hyperintense iron-poor nodule.

LIVER

Regenerating nodules enhance with MR contrast similar to liver parenchyma and thus are difficult to identify postcontrast.

An occasional regenerating nodule infarcts and resembles a hypovascular hepatocellular carcinoma or a metastasis on CT and MR imaging.

Dysplastic Nodules

The currently accepted term dysplastic nodule was preceded by a varied nomenclature, including adenomatous hyperplasia, adenomatoid hyperplasia, macroregenerative nodule, hepatocellular pseudotumor, and regenerative and dysplastic nodules. Whether dysplastic nodules are neoplastic or hyperplastic in origin is not settled, although a number of authors believe they represent a transition in eventual hepatocellular carcinoma development and thus should be considered premalignant, keeping in mind that some cancers appear to bypass a dysplastic stage.

A distinct nodule containing dysplastic cells without histologic evidence for malignancy is the hallmark of dysplastic nodules, although not uncommonly a pathologist identifies a small focus of hepatocellular carcinoma. From a clinical and imaging viewpoint, dysplastic nodules are often considered to represent one end of a continuous spectrum from benign to malignant tumors. Nevertheless, whether carcinogenesis involves a stepwise progression from a regener-

ative nodule to a dysplastic nodule and an eventual carcinoma is conjecture; some cancers appear to bypass a dysplastic stage.

Dysplastic nodules develop earlier in the course of cirrhosis in patients with chronic hepatitis than in those with an alcohol cause. Also, these nodules are more common in a setting of several congenital conditions such as a1- antitrypsin deficiency and Wilson’s disease.

Pathologists distinguish low-grade from high-grade dysplasia by the degree of cytologic atypia. Lack of invasion distinguishes highgrade dysplasia from malignancy. In distinction to hepatocellular carcinoma, immunohistochemical staining reveals no a-fetoprotein expression in dysplastic nodules. A confident diagnosis of dysplasia is problematic from a small needle biopsy, and often a resected specimen is necessary to exclude malignancy. Also, a biopsy finding of dysplasia does not exclude an adjacent malignancy.

Dynamic CT achieves a low sensitivity in detecting dysplastic nodules when subsequent orthotopic liver transplantation is used as a gold standard. Computed tomography arterial portography and CT hepatic arteriography confirm a variable and inconsistent portal and arterial blood supply (85) (Fig. 7.25).

Dysplastic nodules vary in their MR appearance, although they are rarely hyperintense on T2-weighted images, and such a finding helps differentiate them from mostly hyperintense hepatocellular carcinomas. Dysplastic nodules

tend to have a mostly portal venous blood supply and, in distinction to hepatocellular carcinomas, do not enhance as intensely during the arterial phase. Nevertheless, dysplastic nodules are believed to be capable of inducing angioneogenesis, and some do contain enough of an arterial blood supply to make arterial-phase differentiation difficult. A hepatocellular carcinoma developing in a dysplastic nodule tends to be hyperintense within a hypointense tumor on T2-weighted images, although the presence of cirrhosis makes MR differentiation difficult. Pretransplantation MRI in cirrhotic patients detects only a minority of dysplastic nodules.

Adding confusion to this issue, some nondysplastic nodules in cirrhotic livers are hyperintense on T1-weighted gradient-echo MRI and do not lose signal intensity on opposed-phase imaging (86); they also do not enhance during the arterial phase.

Because dysplastic nodules contain Kupffer cells, on post-SPIO contrast these nodules range from isoto hypointense. The contrast ratios between dysplastic nodules and their surrounding liver parenchyma during SPIO-enhanced T2-weighted fast spin echo (FSE) MRI was zero or nearly zero (87); histopathologically, essentially no difference was evident in the number of Kupffer cells in these dysplastic nodules and surrounding parenchyma.

The risk of cancer in a cirrhotic liver containing dysplastic nodules makes nodule resection a rarely viable option. Liver transplantation is considered in these patients.

Extramedullary Hematopoiesis

Intrahepatic extramedullary hematopoiesis develops in some congenital hematologic disorders and hematologic malignancies. Hematopoiesis ranges from focal tumors to a rare extensive diffuse involvement. It can mimic fatty infiltration.

Intrahepatic Spleen

Splenic tissue located outside Glisson’s capsule usually represents splenosis after implantation of splenic tissue due to prior splenic trauma.

Intrahepatic spleens are uncommon. Most present as a focal tumor. Splenic tissue partly embedded into liver should represent hyperpla-

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sia of congenitally ectopic splenic tissue rather than splenosis as long as it is beneath a common capsule, even in a patient postsplenectomy.

Teratoma

A teratoma is a developmental anomaly, arising from embryonal totipotential cells and containing ectoderm, mesoderm, and endoderm tissue. It is rare in the liver. It occurs primarily in infants. Some are associated with an elevated serum a-fetoprotein level and thus are confused with a hepatoblastoma.

Cystic Lesions

The differential diagnosis for an intrahepatic cyst is rather extensive. Rare cystic metastases to the liver include neuroendocrine and ovarian neoplasms. Primary cystic neoplasms are mostly of bile duct origin and are discussed in Chapter 8.

Liver and renal cysts develop in a setting of Ehlers-Danlos syndrome. The origin of some intrahepatic cysts is difficult to determine and histologic examination not only of the cyst content but also of the cyst wall is often helpful. Pathologists distinguish between true cysts (i.e., those containing an epithelial lining) and other cystic structures. A need to establish a specific diagnosis also influences therapy. Thus while percutaneous aspiration and ethanol sclerotherapy is therapeutic for some cysts, cystectomy, cyst unroofing, or fenestration provide access to a larger cyst wall specimen.

Uncomplicated cysts are hypointense on T1and hyperintense on T2-weighted images. They do not enhance on postgadolinium arterialphase T1-weighted SGE images; they appear hypointense during portal venous phase, and continue to maintain their lack of enhancement during the even later hepatic venous phase.

Peribiliary Cysts

Peribiliary cysts are believed to be secondary to obstruction and dilation of peribiliary glands; these cysts are distinct from congenital simple cysts. They are more common in a cirrhotic liver.

These cysts tend to be multiple, tubular, <1.5cm in diameter, and are located either close to the hilum or along larger portal tracts. Some

LIVER

enlarging cysts obstruct adjacent bile ducts. If extensive, a string-of-beads appearance is evident. A peribiliary cyst is also suggested if CT or US reveals water-density cysts around secondto fourth-order intrahepatic portal vein branches.

On T2-weighted MR images these cysts are similar in appearance to dilated intrahepatic bile ducts.

Simple Cysts

Simple liver cysts are common entities. They are lined by epithelium similar to that in bile ducts but they do not communicate with bile ducts. They are more common in women and increase in number with age. Some are congenital and probably originate from dilated aberrant bile ducts that did not connect with the biliary tree.

Most simple cysts are asymptomatic and are discovered as an incidental finding. An occasional one becomes huge and compresses adjacent structures. Some are complicated by bleeding or infection. Hemorrhage often leads to pain. Spontaneous rupture also occurs, at times into the peritoneal cavity.

Most simple cysts are unilocular with no internal structures. Occasionally cyst wall calcifications develop.

These cysts range in CT density between 0 and 10HU. The cyst wall is thin or not visible, a finding not seen with most intrahepatic abscesses. A thin wall is seen, however, with cystadenomas and cystocarcinomas. A simple cyst reveals no contrast enhancement. Computed tomography of most simple cysts is diagnostic. Hemorrhage into a cyst increases the CT attenuation by 20HU or so, seen with MRI as increased signal intensity.

Ultrasonography of a simple cyst reveals an anechoic lumen, increased throughtransmission, and a thin, well-defined wall. Increased echogenicity generally is secondary to hemorrhage.

Simple cysts are very hypointense on T1and markedly hyperintense on T2-weighted MR images, an appearance similar to a hemangioma and an occasional metastasis. Contrastenhanced MRI distinguishes an enhancing hemangioma from a nonenhancing cyst.

Cytology, a-fetoprotein, CA 19.9, CEA, fluid culture, and lack of communication between the

cyst and intrahepatic bile ducts shown by cholangiography aid in excluding other, more ominous diagnoses. Especially with hemorrhage into these cysts, the differential diagnosis includes a cystic or necrotic metastasis and a primary hepatobiliary cystic neoplasm.

Because these cysts are lined by epithelium, they recur after simple aspiration. They are amenable to laparoscopic fenestration and percutaneous ablation therapy. The simplest initial therapy for patients with symptomatic cysts is US-guided percutaneous aspiration and ethanol sclerotherapy. Few complications are encountered.

Ciliated (Foregut) Cysts

The presence of ciliated epithelium within a liver cyst should suggest a hepatic foregut cyst. These rare, congenital lesions are usually less than several centimeters in size, well defined, unilocular, isolated, and subcapsular in location. Some bulge beyond the liver outline and a thin cyst wall is evident. They tend to be hypodense on precontrast CT and hypoto anechoic on US. They have a variable signal intensity on T1and are strongly hyperintense on T2weighted MR images. These cysts do not enhance postcontrast, but at times the cyst wall will enhance. They tend to be filled with bloody mucinous fluid and their hyperintensity on T1weighted images depends on the amount of mucin and blood within the cyst.

Although a foregut cyst can be strongly suspected from its MR appearance, occasionally a cystic metastasis has a similar appearance.

Lymphangioma

The classification of lymphangiomas varies. They are related to hemangiomas in their histologic appearance and, when generalized, the term systemic cystic angiomatosis is often used. In the liver they range from solitary to multiple, but are rare. These cystic nonepithelial tumors are lined by endothelial cells and are filled with serous fluid or chyle. Most are solitary.

Imaging reveals complex cysts containing thin septa.

Intrahepatic Pseudocysts

Intrahepatic pseudocysts in a setting of pancreatitis are rare. Computed tomography reveals

hypodense homogeneous fluid collections, at times multiple; some obstruct a bile duct. An elevated amylase level in aspirated fluid confirms the diagnosis. If necessary, percutaneous drainage is performed.

Hepatic Artery Aneurysm

With gray-scale US, some aneurysms simulate a cyst; the two are distinguishable with Doppler US unless the aneurysm contains a thrombus.

Benign Neoplasms

The vast majority of intrahepatic tumors found to be isoor hyperintense on T1-weighted MR sequences are of liver parenchymal origin (this finding applies only to high field strength magnets).

A common feature of benign liver neoplasms is the presence of fat within the tumor, either within hepatocytes or as discrete lipomatous tissue.

Adenoma

Clinical

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enlarges, and she presents with an acute abdomen due to tumor rupture and hemoperitoneum.

An adenoma has a potential for malignant transformation, although the risk of malignancy is unknown. a-Fetoprotein levels are not elevated with an adenoma; an elevated level in a setting of a known adenoma suggests carcinomatous transformation.

These tumors are supplied mostly from hepatic artery branches and an acute hemorrhage is amenable to therapeutic arterial embolization. Suspected adenomas are usually resected rather than biopsied because of possible hemorrhage and their potential for carcinomatous transformation. In addition, as discussed below, in a number of these tumors a clear-cut differentiation from hepatocellular carcinoma is not feasible by imaging alone.

Pathology

Adenomas are well-marginated, hypervascular benign neoplasms, at times containing regions of hemorrhage or necrosis. These tumors consist of hepatocytes containing increased glycogen and lipid interspersed by dilated sinusoids, but a normal acinar arrangement is missing, and supporting bile ductule and portal

circumscribed, hypervascular, and usually solitary neoplasm. Most occur in women between the ages of 20 and 40 years who are on oral contraceptive therapy. In some, an adenoma regresses after cessation of therapy. Less often adenomas develop in women taking estrogen or in the presence of estrogen-producing neoplasms and in men on anabolic steroids. Adenomas do develop in infants. They are more prevalent than expected in patients with glycogen storage diseases types I and III, and in these patients multiple adenomas (adenomatosis) are encountered. An occasional one becomes pedunculated.

Some adenomas are discovered as incidental findings. Symptoms, if present, range from dull pain, to repeated episodes of severe pain due to hemorrhage into the tumor or adjacent parenchyma, to sudden onset of pain and shock due to spontaneous intraperitoneal hemorrhage. A not atypical scenario is a woman taking oral contraceptives, imaging and biopsy reveal an incidental hepatic adenoma, the tumor

Kupffer cells are present. Only an inconstant capsule is evident, and thus hemorrhage, common with these tumors, readily spreads to surrounding liver parenchyma.

Imaging

Adenomas tend to contain fat and glycogen, with the fat being relatively uniform in distribution. Focal calcifications are identified in some. An occasional adenoma is pedunculated. In distinction to focal nodular hyperplasia, adenomas do not have a central stellate scar. The presence of a scar implies prior necrosis.

Precontrast CT reveals an isoto hypodense tumor, the latter presumably secondary to increased fat or prior necrosis. Acute hemorrhage appears hyperdense on noncontrast CT, a condition usually found in patients with acute symptoms. Adenomas enhance during the arterial phase. Small feeding arteries accentuate an arterial phase peripheral enhancement, followed by a centripetal pattern. The lesion