Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

LIVER

enhancement than in those with slow enhancement. Most hemangiomas increase in lesion- to-liver contrast on ferumoxides-enhanced T2-weighted MR images.

Scintigraphy

Both planar and SPECT Tc-99m–red blood cell imaging have high sensitivity and specificity in differentiating cavernous hemangiomas from other liver tumors and are often used as gold standards in comparison studies. Scintigraphy is especially useful with an atypical imaging appearance of a suspected hemangioma; SPECT detects smaller hemangiomas than planar images. Reported false positives include hepatocellular carcinoma, sarcoma, metastatic small cell lung carcinoma, and intrahepatic extramedullary hematopoiesis (11). Occasional false negatives are reported, often due to a lesion’s small size.

Early perfusion phase Tc-99m–red blood cell scintigraphy shows a focal photopenic defect that gradually fills in in a centripetal manner during the blood-pool phase. A perfusion/ blood-pool mismatch is the hallmark for hemangiomas. Technetium-99m–sulfur colloid and hepatobiliary scintigraphy simply reveal a filling defect. The increased activity seen on blood-pool images is absent if a thrombus has developed within a hemangioma.

If early liver images are obtained during prostate immunoscintigraphy with indium- 111–labeled antibodies, a hemangioma has an appearance similar to that seen with Tc- 99m–red blood cell imaging.

A marked increase in perfusion detected by scintigraphy should suggest arterioportal shunting.

Biopsy

Some radiologists hesitate to biopsy suspected hemangiomas for fear of complications, yet many have been biopsied without undue complication. This does not suggest that suspected hemangiomas should be biopsied; noninvasive protocols generally suffice for diagnosis. Rather, in an atypical and otherwise nondiagnostic suspected or even unsuspected hemangioma a biopsy can be a safe and diagnostic procedure. A bigger problem is in placing in proper perspective a biopsy that simply reveals blood cells.

Therapy

Many liver cavernous hemangiomas are managed simply by observation. Indications for surgery include an uncertain diagnosis, imaging showing enlargement, or a symptomatic tumor. An occasional one manifests acutely by bleeding or rupture, thus necessitating emergency surgery. They have been resected using a laparoscopic approach.

Palliation of symptomatic hemangiomas by transcatheter arterial embolization is an option. A rare symptomatic cavernous hemangioma is treated by radiofrequency ablation (75).

Orthotopic liver transplantation is an extreme option in patients with unresectable giant hemangiomas.

Infantile Hemangioma

Previously some hemangioendotheliomas, especially in infants, were lumped together with hemangiomas (hemangioendotheliomas are discussed later). In a setting of a vascular tumor in an infant, especially with clinical evidence of Kasabach-Merritt syndrome, a hemangioendothelioma rather than a hemangioma should be considered.

An infantile hemangioma is more common in girls and tends to be multiple. With age, these tumors tend to involute with few sequelae.

Large hemangiomas in infants are identified on T2-weighted MRI as hyperintense nodules containing fast flow and are seen as flow voids on SE images and hyperintense tumors on GRE images. One in an infant revealed an early rim enhancement after gadolinium, with progressive fill-in on delayed imaging (76). These tumors are fed by enlarged hepatic vessels; the proximal abdominal aorta is prominent, while distal to the feeding vessel takeoff it had a decreased caliber. These nodules regress after interferon-a-2a therapy and are replaced by normal-appearing liver parenchyma with no evident fat or fibrosis.

If necessary, embolization of the feeding vessels is performed to control congestive failure or treat other complications. A lobectomy is appropriate if a solitary hemangioma is confined to an anatomic lobe.

Some infants believed to have lifeor func- tion-threatening hemangiomas are treated with

radiation therapy (77); some of these infants have Kasabach-Merritt syndrome, and in these infants the hemangiomas regress and platelet counts increase after radiation therapy.

ADVANCED IMAGING OF THE ABDOMEN

A hypervascular metastasis, hemangioma and even an abscess are occasionally included in the differential diagnosis.

Peliosis Hepatis

Clinical

Peliosis hepatis is a rare benign disorder of unknown etiology characterized by numerous blood-filled cavities in the liver. An association exists between peliosis hepatis and some drugs, including hormones. It has regressed in extent after cessation of oral contraceptive use. It can be congenital. Other associated conditions include malignant histiocytosis, myeloid metaplasia, and hemangiosarcoma. It has developed after liver transplantation. In AIDS patients, peliosis hepatis is associated with bacillary angiomatosis. It can be subclinical and be detected incidentally, when it is a source of diagnostic confusion.

These cavities range from single to diffuse, vary in size, and are lined by endothelial cells. Hepatic insufficiency develops if enough parenchyma is replaced by these vascular spaces. Rarely, peliosis hepatis ruptures spontaneously, and an intraperitoneal hemorrhage ensues.

Imaging

Rare liver calcifications are identified in patients with peliosis hepatis.

Postcontrast CT reveals well-marginated contrast collections in the liver. Some show delayed enhancement. Occasionally some of these cystic spaces do not opacify with contrast, presumably due to thrombosis. Some tumors have gradual centripetal CT contrast enhancement, thus mimicking a hemangioma (78). These tumors are hypoechoic or even anechoic with US. They are hyperintense with all MRI sequences and enhance postcontrast.

Angiography reveals numerous vascular spaces. Typically, contrast accumulates in the late arterial phase, which becomes more distinct during the venous phase. No communication exists with the portal venous system. Early feeding arteries and draining veins are not present in peliosis hepatis.

Hepatic scintigraphy with Tc-99m–sulfur colloid or gallium is normal.

Solitary (Fibrosing) Necrotic Nodule

Solitary nodules, initially described on the anterior aspect of the liver, consist of a necrotic center surrounded by a hyalinized fibrotic capsule containing elastic fibers. They are considered nonneoplastic, yet a number have been reported in patients with underlying neoplastic disease. Many are subcapsular and solitary; currently they are believed to be the final common pathway of a number of benign disorders.

They develop after microwave tissue coagulation and after various liver infections.

Focal Nodular Hyperplasia

Clinical

Focal nodular hyperplasia (FNH) is a development abnormality of unknown etiology and pathogenesis. Current thought is that it is a regenerative process; nodules appear related to a focal increase in arterial blood flow and resultant hyperplastic tissue response, and in that sense differ from adenomas, which are neoplastic. They are the second most common benign liver tumor in adults, after hemangioma. In distinction to a hepatic adenoma, FNH is not premalignant and is generally an incidental finding, but, similarly to adenomas, some of these tumors are associated with contraceptive use, and some regress after drug discontinuation. A high prevalence is found in women of childbearing age, with an occasional one encountered in infants and children.

Single nodules predominate. These are discrete tumors often having a characteristic central stellate scar containing blood vessels. Feeding vessels are of hepatic artery origin. Although vessels are prominent and the lesion contains typical hepatic structures, including hepatocytes, Kupffer cells, and proliferating bile ducts, the usual arrangement of portal tract vessels and bile ducts is not present. Histologically, anomalous arteries in fibrous septa connect to capillaries, which drain into sinusoids adjacent to fibrous septa, which in turn drain into veins located in the parenchyma.

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These findings suggest a focal arteriovenous malformation with growth of surrounding parenchyma as etiology for their formation. About 80% of FNHs have a typical histologic appearance; a minority reveal prominent vessels or, interestingly, show cellular atypia to the point of suggesting an adenoma.

A core biopsy reveals deranged architecture, a finding not appreciated with cytology.

Clinically, these lesions are usually an incidental finding and rarely present with bleeding, thus distinguishing them from adenomas. Their primary significance in most patients is that they mimic a more ominous tumor. With time they can increase, decrease, or remain constant in size.

Imaging

An occasional report describes fat within or adjacent to FNH, but fat is an uncharacteristic finding. Likewise, calcifications are uncommon. These calcifications are similar in appearance to those seen in some fibrolamellar hepatocellular carcinomas.

Although the specific pattern is different, FNH has a similar appearance with various imaging modalities. Most are solitary, subcapsular in location, are often lobulated and tend to be sharply marginated. Their usual homogeneous appearance reflects a lack of hemorrhage and necrosis, except in some larger ones. Majority are <5cm in diameter, with only a rare one being >10cm. An occasional one is exophytic. A central hypodense region, representing a central scar with radiating fibrosis, is found in larger tumors, although a scar is generally not visible in smaller ones. Presence of a central scar is not specific for FNH and a scar is occasionally found in other tumors, such as hepatocellular carcinomas, adenomas, intrahepatic cholangiocarcinomas, and some larger hemangiomas.

With unenhanced CT these nodules tend to be homogeneous and hypoto isodense. Precontrast FNHs tend to blend into surrounding parenchyma. Especially when small, they are identified mostly on immediate postcontrast images.

These lesions are supplied primarily by hepatic artery branches and thus arterial-phase CT reveals a transient but marked initial enhancement, which progresses to isodensity

during the parenchymal (portal) phase. Multiphasic CT in patients with FNH found all tumors to be hypervascular and hyperattenuating to the liver during arterial phase, and 92% were isodense on delayed scans (79); most enhanced homogeneously, and were smooth in outline and subcapsular in location. Single-level dynamic CT during hepatic arteriography of small FNH reveals centrifugal blood flow through a fibrous scar, drainage into adjacent dilated veins and then into a hepatic sinusoid in adjacent liver tissue (80).

Ultrasonography reveals FNHs tending toward a homogeneous and isoechoic appearance and often blending into normal adjacent parenchyma, although considerable variability exists. They tend to be well marginated. A central scar is either hypoechoic or not visualized. In some, color Doppler identifies vessels radiating in a central stellate pattern; the feeding artery shows a high diastolic flow rate and has a low pulsatility index. It has a high frequency and low resistance. Contrastenhanced US reveals intralesional flow, peripheral vascularity, and at times also a centripetal afferent vessel.

Magnetic resonance imaging appears more sensitive and specific than CT in detecting and characterizing FNH. On T1-weighted unenhanced MRI sequences, FNHs appear isoto hypointense compared to normal liver and isoto hyperintense on T2-weighted images (Fig. 7.22). Marked hyperintensity on T2-weighted

Figure 7.22. Extensive focal nodular hyperplasia. T1-weighted MRI identifies a hypointense tumor (arrows). (Courtesy of Algidas Basevicius, M.D., Kaunas Medical University, Kaunas, Lithuania.)

images is distinctly unusual and should suggest another diagnosis. A central scar, identified in over half of these tumors, tends to be hypointense on T1and hyperintense on T2weighted images, presumably due to prominent central vascularity. Although not encapsulated, compression of surrounding liver parenchyma often results in a hyperintense rim being evident on T2-weighted images,in distinction to the fibrotic hepatocellular carcinoma capsule which tends to be hypointense on both T1and T2-weighted images. Here also considerable variability exists and heterogeneity and a hyperintense signal is identified in some FNH on T1weighted images.

Postgadolinium MRI reflects their rich vascularity radiating in a stellate pattern. Similar to adenomas, an intense, uniform contrast enhancement is common during the arterial phase, becoming isodense during portal venous phase. A central scar is initially hypointense but gradually enhances on delayed images.

Because Kupffer cells are present in most FNH nodules, these tumors take up iron oxide contrast. Six FNH nodules showed a mean of 43% signal intensity loss on ferumoxidesenhanced T2-weighted images (81). A SPIOenhanced MR detects more tumors than a precontrast MR, but little difference in signal loss exists between FNH and adenomas.

Similar to adenomas, the presence of hepatocytes within the FNH results in Mn-DPDP (mangafodipir) uptake.

ADVANCED IMAGING OF THE ABDOMEN

Due to their Kupffer cell content, these tumors take up Tc-99m–sulfur colloid. Clearance of tracer from the tumor is delayed. A positive colloid scan is thus useful in differentiating these lesions from most hepatocellular carcinomas. A finding of decreased colloid uptake, on the other hand, is nonspecific. It should be kept in mind that some regenerating nodules, focal steatosis, and adenomas also take up sulfur colloid.

Iminodiacetic acid (IDA) hepatobiliary tracers accumulate within FNH, and most larger nodules appear as hot spots. Thus Tc-99m-IDA hepatobiliary scintigraphy shows increased tracer uptake on delayed scans in most larger tumors.

Using the hepatocyte receptor Tc-99m- neoglycoalbumin (NGA), scintigraphy showed normal to increased uptake in FNH, thus differentiating these tumors from most malignancies, which have very low to no uptake.

A characteristic arteriographic finding is a single artery feeding a hypervascular tumor that contains intense venous staining (Fig. 7.23). Although arteriography readily detects FNH, it lacks specificity in differentiating it from other focal hypervascular tumors, and angiography is rarely employed in the workup of these tumors.

The use of imaging techniques is illustrated by a report of a 17-year-old girl with a 2-cm- diameter left lobe tumor detected incidentally by US (82); CT and MRI confirmed the tumor,

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and postcontrast CT and CO2-enhanced US demonstrated early central enhancement with gradual enhancement spreading to the periphery. Color Doppler flow imaging detected a central color spot, shown by pulsed Doppler spectrum analysis to represent an artery. The lesion was not detected by hepatic arteriography. Biopsy confirmed the FNH.

In summary, no pathognomonic imaging finding exists for FNH. A fibrolamellar hepatocellular carcinoma has a very similar CT and US appearance. Likewise, overlap exists with some large hemangiomas, adenomas, and metastases. In a cancer patient findings suggesting a FNH should be confirmed by biopsy.

Nodular Regenerative Hyperplasia

Nodular regenerative hyperplasia (at times simply called regenerative nodules) is a specific nonneoplastic entity consisting of multiple nodules surrounded by liver atrophy. It differs from regenerating nodules seen in cirrhosis (discussed in a later section). This uncommon condition occurs in noncirrhotic livers. About half the patients have portal hypertension. The etiology is unknown, and pathogenesis appears to be multifactorial, with focal ischemia postulated by some. It is associated with collagen vascular diseases, rheumatoid arthritis, Felty’s syndrome, some myeloproliferative disorders, and some drug therapy.A rare association exists with CREST syndrome (calcinosis cutis, Raynaud’s phenomenon, esophageal dysfunction, sclerodactyly, and telangiectasia) and primary biliary cirrhosis.

Histologically, numerous hyperplastic nodules varying in size are diffusely scattered throughout the liver. No significant fibrosis is evident, thus distinguishing this condition from cirrhosis and focal nodular hyperplasia; the exception is if long-standing portal hypertension has developed, when fibrosis with portal venous obliteration develop; whether portal venous obliteration precedes portal hypertension or vice versa is unknown. Histologically,this condition mimics an adenoma,but the latter is usually single, while nodular regenerative hyperplasia is diffuse throughout the liver.

Computed tomography reveals multiple hypodense nodules that do not enhance postcontrast. Associated hemorrhage modifies the

imaging appearance. Nodule rupture is a rare complication, with the resultant appearance mimicking an adenoma.

Ultrasonography is generally noncontributory.

The T1-weighted MR appearance is variable; most nodules are hypoto isointense on T2weighted images.

These regenerating nodules contain Kupffer cells and take up Tc-99m–sulfur colloid.

Inflammatory Tumor

Inflammatory tumors (pseudotumors) are rare in the liver. Some are discovered incidentally. Their etiology is unknown; although some probably represent an atypical inflammatory reaction to infection, others appear to be a variant of Castleman’s disease (These tumors are discussed in more detail in Chapter 14). Some progress in size and are multifocal. Resection establishes the diagnosis, although at times even then a diagnosis is not definitive. At times immunohistochemistry is useful to exclude a malignancy.

Computed tomography and US usually reveal a poorly defined heterogeneous hypovascular tumor mimicking a neoplasm. Magnetic resonance contrast enhancement is variable, at times with rapid wash-out of contrast. Gadolinium enhanced gradient-echo MR of one tumor revealed an early, intense peripheral enhancement followed by homogeneous enhancement (83); no uptake was evident with ferumoxides nor mangafodipir. Some of these tumors are surrounded by a hyperintense capsule. In general, imaging suggests a malignancy.

Regenerating Nodules in Cirrhosis

Cirrhotic livers develop small nodules, called regenerating or regenerative nodules and siderotic nodules, histologically consisting of either foci of regeneration or dysplasia (or both), together with supporting stroma. They contain hepatocytes, bile ducts, Kupffer cells, and fibrosis, and often contain increased iron. The reason for their formation is unknown, but they appear to represent a response to growth factors. These regenerating nodules tend to be small and scattered throughout the liver.

In a broader context, regeneration is a common feature after hepatocyte damage,