Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

neuroendocrine liver metastases detected by other imaging and often discovers new sites. Octreotide scintigraphy also detects those tumors potentially responsive to radiolabeled octreotide therapy, but even here caution is necessary—liver, spleen, kidneys, and bladder are routinely visualized, while neuroendocrine tumors (and lymphomas) show variable uptake; therapy using somatostatin analogues may not be appropriate for all tumors accumulating In-111-DTPA-D-Phe-octreotide (In-111- pentetreotide).

Schwannoma

A primary schwannoma of the liver without associated neurofibromatosis is rare. Initially these are solid tumors, but with growth they tend to necrose and invade adjacent structures, including stomach.

Carcinoid

Most carcinoids originate in the gastrointestinal tract, with an occasional primary in some other organ, such as breast; even these metastasize to the liver. Primary liver carcinoids are rare; of interest is that the carcinoid syndrome tends not to develop in these patients. Carcinoid syndrome develops if tumor blood flow drains into the systemic venous circulation. Thus, in general, in a setting of a known primary carcinoid, the presence of carcinoid syndrome usually implies hepatic metastases, although exceptions occur and occasionally carcinoid syndrome is not evident even with widespread liver involvement.

Multiple liver foci are more common than solitary ones. Being metastatic, these carcinoids are malignant, yet specific imaging findings do not aid in differentiating between benign and malignant ones. Precontrast CT identifies carcinoid metastases as hypodense tumors, although small ones tend to be isodense. Central necrosis in larger tumors is hypodense. They enhanced during the arterial phase, with contrast enhancement then decreasing; superficially, they can mimic a hemangioma. A minority of focal carcinoids are best seen precontrast. Some are identified only on one phase.

Ultrasonography shows these tumors to range from hypoto hyperechoic; central necrosis is anechoic. Some tumors are surrounded by a hypoechoic halo.

ADVANCED IMAGING OF THE ABDOMEN

These tumors are hypointense on T1and hyperintense on T2-weighted MRI; they enhance markedly during the arterial phase, less so during portal phase. A peripheral hypointense ring is seen in some of these tumors during delayed imaging, a finding also detected with some other hypervascular metastases.

Digital subtraction angiography identifies neovascularity. Larger tumors are fed by tortuous and elongated arteries. Carcinoids tend to displace and compress adjacent portal veins but not invade.

Most carcinoids contain somatostatin receptors, and octreotide scintigraphy is useful in localizing them. They are amenable to octreotide therapy.

Indium-111 pentetreotide and iodine- 123–vasoactive intestinal peptide receptor imaging should suggest a carcinoid. Indium- 111-pentetreotide SPECT detects more tumors than planar scans or conventional imaging.

The radiopharmaceutical metaiodobenzylguanidine (MIBG), used in the imaging and therapy of pheochromocytomas and neuroblastomas, has a limited role in imaging metastatic liver carcinoids because other imaging is more sensitive. Iodine-131-MIBG is useful, however, in treating carcinoids.

Interferon therapy has led to a clinical response, but symptoms recur after end of therapy. Embolization has had a limited application.

Gastrinoma

A rare primary liver gastrinoma has been reported.

T2-weighted sequences reveal a wellmarginated, homogeneous, hyperintense tumor. Arterial phase MR sequences show larger tumors to have either homogeneous or peripheral enhancement, while smaller ones tend to be homogeneous and mimic hemangiomas. Delayed sequences show faster contrast washout than with most hemangiomas, and these sequences are most useful in differentiating metastatic gastrinomas from hemangiomas.

The intraarterial secretin test consists of sampling venous blood after secretin injection. In patients with Zollinger-Ellison syndrome, the secretin test is positive in only half or so of patients. This detection rate is less than with

women, but children are not spared. The underlying liver is usually normal.

Imaging defines the size and extent of these tumors, but their appearance often suggests a hepatocellular carcinoma. Liver biopsy is needed to establish a diagnosis. Ultrasonography is generally used to follow these lesions.

Varying degrees of mottled calcifications develop eventually.

Computed tomography reveals a hypodense tumors (Figs. 7.48 and 7.49). They tend to be hypervascular in their periphery. Contrast CT shows early peripheral enhancement followed by delayed central enhancement. Central enhancement is absent if sufficient fibrosis or thrombosis evolve. Even with extensive tumors in both lobes, the underlying hepatic vascularity tends to be normal.

These tumors are hypointense on T1and hyperintense on T2-weighted images, although considerable variability exists.

Patients with epithelioid hemangioendotheliomas have undergone successful liver transplantation.

Other Hemangioendotheliomas

Retiform, polymorphous, and composite hemangioendotheliomas and the malignant endovascular papillary angioendothelioma

usually infiltrate locally and have a high rate of local recurrence, and some will metastasize. They should be considered low-grade malignancies.

Figure 7.49. Epithelioid hemangioendothelioma in an 18-year- old woman. Contrast-enhanced CT reveals a large heterogeneous, lobulated tumor involving both right and left lobes. Some of the vessels coursing into it are attenuated. She underwent liver transplantation. (Courtesy of Patrick Fultz, M.D., University of Rochester.)

LIVER

Figure 7.50. Metastatic hemangiopericytoma. She had surgery years ago for a meningeal hemangiopericytoma. A large right lobe tumor is evident. (Source: Cancer 1999;85(10):2245–2248. Copyright 1999 American Cancer Society. Reprinted by permission of Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc. Courtesy of Dr. F. Grunenberger, Hôpital de Hautepierre, Strasbourg.)

Hemangiopericytoma

The rare primary liver hemangiopericytoma occurs in both children and adults. Clinically and radiologically these tumors suggest a hemangioma or a hemangioendothelioma. Some express angiogenic factors such as fibroblast growth factor and vascular endothelial growth factor.

Occasionally detected is a metastatic hemangiopericytoma (Fig. 7.50). Some of these patients present with hypoglycemia; a history of a hemangiopericytoma resected years ago is not uncommon.

Calcification

Numerous disorders lead to increased liver parenchymal density and most are discussed in their respective sections. Not all increased density is due to calcium deposition. Increased iron stores and thorotrastosis are two such noncalcium conditions.

Whether calcifications are diffuse or focal, linear or curved, dense or barely visible allows one to narrow the differential diagnosis (Table 7.16). The patient’s age, likewise, aids in differentiating diagnoses.

Table 7.16. Conditions associated with liver calcifications

Diffuse parenchymal calcifications

In childhood

Toxoplasmosis

Cytomegalovirus

Rubella

Herpes simplex virus

Neonatal syphilitic hepatitis

Granulomatous hepatitis

Tuberculosis

Histoplasmosis

Schistosomiasis (Schistosoma japonicum)

Pneumocystis carinii (usually in AIDS)

Associated with hemoor peritoneal dialysis

Amyloidosis

Silicosis

Lymphoma

After liver ischemia

Post-transplant hepatic artery thrombosis

Focal calcifications:

Prior pyogenic abscess

Prior amebic abscess

Hydatid cyst

Primary liver tumors

Cavernous hemangioma

Focal nodular hyperplasia

Fibrolamellar hepatocellular carcinoma

Epithelioid hemangioendothelioma

Metastatic neoplasm

Mucin-producing adenocarcinoma

Osteogenic or chondrosarcoma

Some neuroendocrine tumors

Aneurysms

Intrahepatic biliary stones

Vascular Disorders

Vascular Congestion

Systemic venous engorgement, such as with congestive heart failure, constrictive pericarditis, or an episode of hypotension, leads to liver congestion, hepatomegaly, and, if severe enough, eventual liver ischemia and central lobular necrosis. Underlying liver damage is often unexpected. Shock has led to acute liver necrosis. Cirrhosis is an end point if congestion is chronic. Associated renal failure is common. These patients present with a marked increase in transaminase levels that tend to normalize rapidly. A coagulopathy with a prolonged prothrombin time is common.

Contrast-enhanced CT reveals a mottled pattern throughout the liver and delayed hepatic vein enhancement. Periportal lowattenuation nonenhancing fluid is a manifestation of venous stasis. The inferior vena cava becomes engorged and enlarges.

Enlarged hepatic veins and, at times, an enlarged inferior vena cava are gray-scale US findings of liver vascular congestion. Doppler US shows prominent portal vein pulsatility. In patients with heart failure, duplex Doppler US reveals that those with more severe left ventricular failure have a reduced portal vein pulsatility ratio; in fact, portal vein pulsatility correlates better with worsening cardiac function than vena caval or hepatic vein diameters. In chronic congestive heart failure the hepatic veins lose their triphasic contractility.

Infarction

The liver’s dual blood supply makes major hepatic infarcts rare in the absence of prior surgical or radiologic intervention. Most major infarcts are encountered after liver transplantation. After a pancreaticoduodenectomy, liver infarction is more common in patients undergoing combined portal and superior mesenteric vein resection but not in those without a combined vascular resection. Liver infarcts develop in patients with vasculitides, polycythemia vera, and disseminated intravascular hypercoagulation. Sickle cell disease is associated with liver infarcts, although many of these are not detected with imaging. Blunt trauma is a rare cause of a major liver infarct. Liver infarction is a rare complication of transjugular intrahepatic portosystemic shunt (TIPS). Focal liver infarcts occur in association with diffuse or focal liver disease, often after obstruction of an intrahepatic branch of the hepatic artery. Occlusion of a portal vein branch does not result in an infarct (except in neonates). Etiology of some biopsyconfirmed focal necrosis can be identified, but in others differentiation among infection, tumor, and ischemia is not possible. Some focal infarcts evolve into focal necrosis and an abscess; others presumably heal with few sequelae.

A typical infarct appears as a wellmarginated, wedge-shaped, peripheral defect; more centrally located infarcts are harder to identify. Within several days of necrosis, con-

ADVANCED IMAGING OF THE ABDOMEN

ventional radiography and CT detect gas bubbles in the liver, a transient finding. With a sufficient insult eventual parenchymal atrophy and scarring ensue. Some patients eventually develop diffuse liver calcifications.

Contrast-enhanced CT of an infarct reveals a hypodense region due to hypoperfusion, generally in the periphery. With hepatic artery obstruction, Doppler US identifies the lack of flow in the porta hepatis, although intrahepatic arterial flow may still exist due to collateral vessels. Occluded feeding arteries are identified in some patients.

Following abdominopelvic surgery not related to the liver, some patients develop wedge-shaped liver defects. These defects range from hypoto hyperdense on precontrast CT. Postcontrast, these defects appear homogeneous and of higher attenuation than the surrounding liver. Of interest is that in some patients CT also detects portal vein branch clots within these defects. Follow-up CT reveals most of these defects diminishing with time, although some can persist. These postoperative defects probably represent a portal venous system thromboembolic phenomenon, and CT contrast enhancement is due to compensatory increase in hepatic arterial flow.

Postcontrast MRI reveals an infarct as a hypointense region.

Veno-Occlusive Disease

Hepatic vein obstruction is secondary to thrombosis or to such etiologies as chemotherapy or toxin ingestion. It is a rare complication of therapeutic liver radiation.

Jamaican herbal tea (Senecio vulgaris) drinkers are at increased risk of hepatic venoocclusive disease. Clinically, these patients have findings similar to Budd-Chiari syndrome but imaging reveals patent major hepatic veins and inferior vena cava. Portal vein blood flow is decreased in this condition.

Budd-Chiari syndrome is discussed in Chapter 17.

Shunts

Arteriovenous

Intrahepatic arteriovenous shunting, if significant enough, leads to hepatic artery

LIVER

enlargement and early venous contrast enhancement through the shunt. Early venous enhancement is also seen with some hypervascular neoplasms such as hepatocellular carcinomas. These findings are identified with postcontrast CT and arteriography.

Arterioportal

Small arterioportal venous shunts (not related to neoplasms), identified by hepatic angiography, appear as perfusion defects at CT arterial portography.

Arterial-phase dynamic MRI of small nontumorous arterioportal shunts reveal wedgeshaped, nodular, or irregular-shaped regions of contrast enhancement (156); they are not identified by unenhanced MRI. These nontumor arterioportal shunts are a cause of focal liver arterial MR hyperperfusion in a setting of normal precontrast MRI.

Liver in Idiopathic

Portal Hypertension

Whether idiopathic noncirrhotic, chronic portal hypertension is one condition or a final pathway of several disorders is speculation. Histology reveals dense portal fibrosis, portal venous obliteration, and intralobular fibrosis; the appearance is similar to that found in nodular regenerative hyperplasia.

Some patients with idiopathic portal hypertension develop decreased portal venous peripheral perfusion. These regions enhance during arterial phase dynamic and are hypointense on T1and hyperintense on T2weighted MRI. Technetium-99m-GSA scintigraphy shows decreased accumulation, indicative of dysfunction.

Other Vascular Disorders

In patients with portal vein obstruction (in either the right or left lobe portal veins), immediate CT and MR contrast-enhanced images reveal an initial transient increase in segmental hepatic enhancement distal to the obstruction; this enhancement is secondary to increased hepatic arterial blood flow to the obstructed segment.

Hepatic sinusoidal dilatation, a rare interesting vascular condition, is associated with focal hepatocyte necrosis and intrasinusoidal fibrosis. It occurs in a setting of oral contraceptive use or pregnancy. CT reveals a poorly marginated heterogeneous hypodense region during the portal phase which gradually becomes isodense (157); T2-weighted MR identifies vessels within the lesion, an uncommon finding with a neoplasm. Delayed contrast enhancement separates this condition from peliosis hepatis, which tends to enhance early.

Vasculitides are prone to bleed, usually intrahepatic, less often intrabiliary or intraperitoneally. Some of these bleeds can be treated with transcatheter arterial embolization.

HIV/AIDS

Clinical

It is common to see hepatomegaly and splenomegaly in a setting of HIV infection. In general, low attenuation lesions in the liver should not be ascribed directly to HIV; most are secondary to another infection or a neoplasm.

Liver histology is abnormal in most autopsied HIV patients; steatosis is most common. Steatosis developing in HIV infected patients receiving antiretroviral therapy leads to hepatomegaly and is a severe and potentially fatal complication.

Imaging

Ultrasonography findings in HIV-positive patients are less specific than in the general population. In almost half of these patients US shows a diffuse hyperechoic liver, presumably secondary to steatosis.

Some opportunistic liver and splenic infections result in an US snowstorm appearance. Fibrosis or a fibrinous exudate probably accounts for this appearance.

Infection

Bacillary angiomatosis, a liver infection complicating AIDS, is caused by infection with

Bartonella henselae or B. quintana. It mimics Kaposi’s sarcoma. Computed tomography