Книги по МРТ КТ на английском языке / Advanced Imaging of the Abdomen - Jovitas Skucas

Figure 7.17. Iron overload in a 10-year-old with sickle cell disease and multiple transfusions. MR shows a markedly hypointense liver and spleen. (Source: Burgener FA, Meyers SP, Tan RK, Zaunbauer W. Differential Diagnosis in Magnetic Resonance Imaging. Stuttgart: Thieme, 2002, with permission.)

Liver iron deposition introduces MR interpretation problems when imaging other diseases, because iron reduces parenchymal signal intensity, and some malignant foci appear more bright than usual and tend to simulate benign lesions. The reverse is also true with inflammatory conditions that increase parenchymal signal intensity.

Therapy

Hereditary hemochromatosis is often not considered in patients with end-stage liver disease because elevated iron levels are common in many other diseases. In fact, at times hemochromatosis is not even diagnosed prior to transplantation. An incidental hepatocellular carcinoma is not an uncommon finding at the time of transplantation.

These patients appear to have a higher posttransplantation mortality than patients transplanted for other causes. Also, they have increased infectious and cardiac complications.

exists between iron concentrations obtained from percutaneous needle biopsies and a ratio of signal intensity of the liver to background noise obtained from MR sequences. Skeletal muscle T2 values are relatively constant over a wide range of iron stores and can thus be used as a reference standard. The liver-to-muscle proton density ratio correlates with hepatic iron, yet the low signal-to-noise ratios seen with high iron levels are difficult to quantitate. Although in practice MRI appears sufficiently accurate to measure liver iron concentrations, in general, qualitative results aid in establishing a diagnosis but quantitative correlation is imprecise.

Thalassemic patients without blood transfusions have an MR appearance similar to those with hereditary hemochromatosis, but after blood transfusions the picture alters, with excess iron in organs rich in reticuloendothelial cells. A multicenter MR study of thalassemia major patients suggests that the signal intensity ratio of liver-to-muscle is related to liver iron concentration (53). Hepatic 1/T2 values correlate with liver iron concentration. In general, a close correlation exists between biopsy liver iron levels and MR signal intensity.

Silicosis

Hepatosplenic silicosis also develops in patients with pulmonary silicosis. Liver biopsy in these patients reveals birefringent crystals in hyalinized nodules.

Calcified splenic nodules are detected with conventional radiography, CT, or US. Faint intrahepatic calcifications or “egg-shell” lymph node calcifications are best evaluated by CT.

Amyloidosis

Amyloidosis can be primary, secondary, or familial. In primary systemic amyloidosis, amyloid is deposited in the liver, spleen, and other structures; at times primarily liver deposits occur. A rare patient progresses to cholestasis, severe jaundice, and hepatic failure. Sinusoidal portal hypertension and spontaneous liver rupture are rare complications of primary amyloidosis.

Liver enlargement is a prominent feature in some patients. The liver is diffusely involved in both primary and secondary amyloidosis, but at times involvement is nonuniform. Hepatic and splenic calcifications are rare in primary amyloidosis. When amyloidosis is well established, noncontrast CT reveals decreased liver

LIVER

parenchymal attenuation. Heterogeneous enhancement is evident postcontrast. Ultrasonography reveals a heterogeneous echo pattern.

Liver transplantation is the only effective treatment of familial amyloidosis; patients in a good nutritional state have a significantly better survival rate.

Sarcoidosis

Sarcoidosis, a granulomatous disorder of unknown etiology, is commonly diagnosed on clinical and imaging ground. The previously used Kveim test is considered too nonspecific. The angiotensin-converting enzyme (ACE) level correlates with disease activity. The lungs are primarily involved; in the abdomen, liver, spleen, and lymph nodes are most often involved, although hepatic sarcoidosis is usually asymptomatic and cholestasis infrequent.

Biopsy reveals noncaseating epithelial granulomas, often when liver function tests are still normal, but other causes of liver granulomas need be excluded, including a secondary sarcoid reaction to a neoplasm. At times histologic differentiation from sclerosing cholangitis is difficult.

Occasionally hepatic sarcoidosis progresses to cirrhosis and portal hypertension. Resultant esophageal variceal bleeding that some of these patients develop is amenable to sclerotherapy, although occasionally a shunt is necessary.

The most common CT findings of abdominal sarcoidosis are hepatosplenomegaly and diffuse adenopathy. In a minority of patients multiple hypodense nodules secondary to coalescing granulomas are found in the liver and spleen; these nodules are better identified on contrast CT. Periportal adenopathy is common and at times sufficiently extensive to result in obstructive jaundice.

Sarcoid nodules are hypointense both on T1and T2-weighted images. They show mild, delayed contrast enhancement. The surrounding vasculature tends to be normal, thus differentiating these nodules from a malignancy. Nevertheless, the overall imaging appearance often suggests metastases, lymphoma, or even hepatic tuberculosis, especially in the uncommon patient with a normal chest radiograph.

Reticuloendothelial Failure

Hepatic reticuloendothelial failure is a poorly understood condition associated with decreased Kupffer cell function. It tends to be underdiagnosed. These patients are prone to develop infections.

Radiocolloid Tc-99m-phytate scintigraphy suggests the diagnosis; the liver is not visualized with this radiocolloid agent, but is imaged with conventional hepatobiliary agents such as Tc- 99m-DTPA.

Liver in Pregnancy

Middle hepatic vein Doppler US waveform early in pregnancy has normal pulsatility, but with progression of pregnancy hepatic vein pulsatility becomes more and more flat.

Acute fatty liver occurs during pregnancy, occasionally even as early as 26 weeks, can progress to acute hepatic failure. Any transaminase elevation during pregnancy should be viewed with suspicion. A high fetal death rate is associated with acute fatty liver, but some pregnancies are managed successfully; following delivery liver function returns to normal, with only an occasionally preeclampsia persisting postpartum, liver enzymes remaining elevated, and hemolytic-uremic syndrome developing. Liver rupture is a rare complication of severe preeclampsia.

A rare complication of preeclampsia is the HELLP syndrome (hemolysis, elevated liver enzymes, and low platelets). It occurs during both pregnancy and the puerperium. Gross liver involvement ranges from subcapsular hematoma, to infarction, to small vessel occlusion, to spontaneous liver rupture.

Upper abdominal pain, nausea, and vomiting, by themselves nonspecific findings in pregnancy, are common in the HELLP syndrome. Initial clinical findings tend to suggest gallbladder disease, and US is often first performed. Although US will detect a hematoma, CT not only identifies hematomas but should also detect liver infarction or rupture, and identify any active bleeding site. Once HELLP is established, thrombocytopenia and fibrinolysis become evident and suggest the diagnosis. Liver MR spectroscopy of seven women with HELLP syndrome found relative hepatic concentrations

of phosphorus-containing metabolites and absolute concentrations of adenosine triphosphate to be similar to controls, but severe involvement results in a relative increase in phosphomonoester and an absolute decrease in hepatic adenosine triphosphate (54).

Several intrahepatic pregnancies have been described. At times CT and US identify a living fetus. Massive hemorrhage and hypovolemic shock are not uncommon presentations during the first trimester.

Tumors

A list of focal liver tumors is legion. Even such entities as focal fat and focal sparing, generally not considered as tumors, are in the imaging differential diagnosis. The simplest classification of primarily hepatocellular-origin tumors is to subdivide them into regenerative nodules (solitary necrotic nodule, focal nodular hyperplasia, nodular regenerative hyperplasia, and regenerative nodule of cirrhosis) versus dysplastic-neoplastic nodules (dysplastic nodule, adenoma, and hepatocellular carcinoma). Benign liver tumors can be divided into

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nonneoplastic and neoplastic (Table 7.8). With some neoplastic tumors the boundary between benign and malignant is blurred to the point that even a pathologist is hard-pressed to differentiate them. Some of the tumors developing in infants and children are unique to the pediatric patient, while others are similar to those seen in adults (Table 7.9). The most common liver malignancy in children under 3 years of age is a hepatoblastoma, while in older children a hepatocellular carcinoma is more common.

Discussion of hepatobiliary tumors is divided between this chapter and Chapter 8, but such a distinction is arbitrary. Those tumors manifesting primarily by bile duct involvement, both intrahepatic and extrahepatic, are discussed in Chapter 8. The outline adopted here is based on a broad pathologic classification modified by imaging findings. Most specialists would subdivide liver tumors based on their own viewpoint.

Detection of Focal Tumors

From an imaging viewpoint, a useful differentiation is between hypervascular and hypovascu-

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Table 7.9. Focal Lesions in Infants and Children

Nonneoplastic

Benign cysts

Focal nodular hyperplasia

Regenerating nodules

Focal fatty infiltration

Hemangioma

Hamartoma

Teratoma

Neoplasms

Benign

Adenoma

Hemangioendothelioma

Malignant

Hepatoblastoma

Hepatocellular carcinoma

Metastases

Neuroblastoma

Lymphoma

Leukemia

Sarcoma

Rhabdomyosarcoma

Angiosarcoma

Fibrosarcoma

Leiomyosarcoma

Teratocarcinoma

lar tumors (Table 7.10) and between cystic and solid tumors. Most hepatic neoplasms are fed primarily by the hepatic artery rather than the portal vein. However, many neoplasms are better identified on portal venous phase images when normal liver parenchyma achieves maximum enhancement than during the arterial phase. A minority of tumors are hypervascular, and these are most conspicuous during the arterial phase, when greater tumor enhancement than liver parenchymal enhancement is achieved. In fact, some of these hypervascular tumors become isodense and merge into normal liver parenchyma during the portal phase. Incidentally, no universal definition exists for what constitutes a hypervascular tumor; one definition used is any tumor showing an enhancement >10HU above that of liver parenchyma during arterial phase CT.

Arterial landmarks are better seen during the arterial phase. Likewise, parenchymal perfusion abnormalities are more often detected during the arterial phase.

Computed Tomography

The most common indication for biphasic (postcontrast arterial and portal phases) CT is

to detect the usually hypervascular hepatocellular carcinomas and hypervascular metastases. Common hypervascular metastases include breast, renal cell, and thyroid carcinomas, various neuroendocrine tumors, melanoma, and most sarcomas. Diagnostic confusion stems from benign hypervascular lesions, such as focal nodular hyperplasia, adenoma, and hemangioma.

Although a majority of focal liver lesions enhance on arterial phase CT images, the pattern of enhancement often suggests a specific diagnosis (55); thus abnormal internal vessels are seen with a hepatocellular carcinoma, peripheral contrast puddles with hemangiomas, and ring-like enhancement with metastases.

Portal-phase CT images are often used when evaluating suspected hypovascular lesions, although biphasic acquisition does reveal more focal liver lesions and aids in anatomic visualization of liver vasculature.

Ultrasonography

About two thirds of liver nodules < or = 1cm in diameter are homogeneous in appearance, and for these US cannot differentiate benign from malignant; with larger nodules, however, a heterogeneous US pattern suggests a malignancy.

Introduction of microbubble contrast agents increased both sensitivity and specificity of detecting focal liver tumors. These agents not only result in blood pool enhancement, but some also have a hepatosplenic specific late phase. Harmonic US during this late phase or stimulated acoustic emission improve detection of focal liver tumors and in some studies approach results achieved with CT (56). Thus in select patients with suspected metastases, conventional US detected 195 focal tumors, CT detected 231, and pulse inversion harmonic US using Levovist as a contrast agent detected 287 tumors, differences that are statistically significant (57); the latter technique detected the smallest lesions, which were undetected by CT and conventional US. In characterizing focal liver tumors, harmonic gray-scale US using the microbubble contrast agent perfluorocarbon is superior to conventional Doppler US (58).

Mostly hypervascular

Nonneoplastic conditions Peliosis hepatis Hemangioma Arteriovenous malformation Focal nodular hyperplasia

Hereditary hemorrhage telangiectasia

Cystic structures

Arterial aneurysm

Angiodysplasia

Neoplasms

Angiomyolipoma

Adenoma

Hepatocellular carcinoma

Hypervascular metastases

Some sarcomas

Neuroendocrine tumors

Hemangioendothelioma

Hemangiopericytoma

Adult hepatoblastoma

Mostly hypovascular

Nonneoplastic conditions Abscess1

Granulomas

Tuberculoma

Focal fatty infiltration Inflammatory (pseudo)tumor Nodular regenerative hyperplasia Solitary necrotic nodule

Mesenchymal and biliary hamartoma

Cystic structures Abscess1

Simple cysts and related structures Polycystic disease

Caroli’s disease Hydatid cyst

Intrahepatic choledochal cyst Biloma

Hematoma Endometrioma

Neoplasms

Necrotic neoplasm

Intrahepatic cholangiocarcinoma Some sarcomas

Hypovascular metastases Lymphoma

Undifferentiated embryonal cell carcinoma2

Magnetic Resonance

When discussing MR findings, malignant tumors are best subdivided into hypervascular and hypovascular. Different imaging parameters are best suited for each category. Thus hypovascular malignancies are best detected on portal venous phase images than on other phases, and arterial phase is best for hypervascular malignancies. Only a rare focal liver tumor is detected on unenhanced images and is not visible on one of the postcontrast phases. Differential diagnosis of a detected tumor is a different topic and is discussed in the next section.

Ultrasonography reveals fat-containing tumors to be hyperechoic; T1-weighted chemical shift MRI separates these tumors from other hyperechoic tumors without fat.

Overall, postgadolinium MR detects more focal liver tumors and is markedly superior in characterizing them than postcontrast CT. A

multicenter clinical study evaluating focal liver tumors concluded that significantly more and smaller tumors are detected on combined pre– and post–Gd-BOPTA images compared with precontrast images alone (59). In a cirrhotic liver, T2-weighted MRI does not provide additional diagnostic information than gadolinium enhanced MRI in detecting and characterizing tumors (60). The hepatobiliary-specific agent Gd-EOB-DTPA also detects more focal tumors than unenhanced or Gd-DTPA-enhanced images.

In tumors with low or absent reticuloendothelial tissue, the use of SPIO agents increases signal intensity between tumor and liver, and, overall, more focal liver tumors are detected than with postcontrast CT or precontrast MR. On the other hand, both normal liver and some well-differentiated tumors take up these iron oxide particles, and after contrast these tumors appear isointense and difficult to detect. Overall,

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combining pre– and post–contrast-enhanced images is more accurate than the use of postcontrast MR alone.

Biopsy

In a prospective study of consecutive patients with discrete focal tumors <1cm in diameter, found at US, biopsy achieved a sensitivity of 98% and specificity of 100% in detecting malignant tumors (61).

Differential Diagnosis of

Focal Tumors

A common problem facing the radiologist is distinguishing a hemangioma or some other benign tumor from a malignancy. The problem is especially acute in a setting of a known malignancy in some other structure where the presence of liver metastases affects future patient management. The difficulty of differentiating a hemangioma from other liver tumors is evident from the extensive literature on this subject.

For incidental tumors <2cm in size in patients with no known malignancy, about 75% are benign (62); hepatocellular carcinomas predominate among malignancies. With a history of a malignant tumor, about 50% of tumors <2cm in size are benign.

If a hemangioma is suspected in a tumor larger than several centimeters, a useful approach is to perform Tc-99m–red blood cell SPECT scintigraphy; smaller lesions are best studied with MRI. Still, in spite of inherent limitations, CT is often employed because of ready availability. In some situations a biopsy is justified.

The following approach to evaluating focal liver tumors is adopted from the American College of Radiology Appropriateness Criteria (63):

1.Typical postcontrast CT or US benignappearing tumor, with no history of malignancy: These are usually classified as benign. If needed, inand out-of-phase MR should detect focal fat.

2.Typical postcontrast CT or US benignappearing tumor, with a known history of malignancy: Similar recommendation as above, except these should be followed-up using the same imaging modality.

3.Typical postcontrast CT or US malignant tumor: Biopsy confirmation if clinically appropriate.

4.Indeterminate postcontrast CT or US tumor in an otherwise normal liver: Postcontrast MRI is often helpful with these tumors. For some tumors scintigraphy or dynamic CT is useful. If still indeterminate, biopsy should be considered.

5.Indeterminate postcontrast CT or US tumor in a cirrhotic liver: Although MRI is useful to follow these tumors, biopsy is generally needed for diagnosis.

Some very small tumors earn the sobriquet of too small to characterize; depending on the clinical situation, some radiologists suggest imaging follow-up.

Computed Tomography

Hemangiomas vary in their rate of contrast enhancement. Most malignant neoplasms have considerably faster enhancement, but overlap does occur with hemangiomas. The slower enhancement seen with some hemangiomas is rather characteristic but is mimicked by posttherapy metastases, although the clinical scenario should differentiate these two entities.

Arterial phase CT densities are significantly higher in focal nodular hyperplasias (mean, 118

± 15HU) than in hepatocellular adenomas (80 ± 10H) (64); no significant attenuation differences were evident precontrast and during the portal venous phase.

Tumor appearance during biphasic and triphasic CT aids in differentiating between focal liver tumors. Small hemangiomas often tend to be isodense to aorta; the postcontrast CT appearance of enhancing nodules seen in larger hemangiomas is seldom found with hypervascular metastases. A homogeneous, hyperdense tumor seen during the arterial phase, combined with a hypodense tumor but continued hyperdense periphery during the parenchymal phase, suggests a carcinomas.

Inclusion of arterial and portal venous phase CT does not improve sensitivity but does improve specificity in differentiating hemangiomas from hypervascular malignancies (65).

Ultrasonography

The use of a US contrast agent aids in characterizing and differentiating some focal liver tumors. Tumor enhancement during arterial phase or reticular enhancement during parenchymal phase achieved a 92% sensitivity and 96% specificity for hepatocellular carcinoma. Ring enhancement during the arterial to portal phases or a parenchymal phase defect reached a 90% sensitivity and 95% specificity for cholangiocellular carcinoma or metastases, and puddled enhancement during the portal phase was 60% sensitive and 100% specific for a hemangioma (66). In general, postcontrast tumor flow signals (hypervascularity) favor a malignancy; lack of flow signals is more common in benign tumors but does not exclude a malignancy. A late phase hypoechoic signal favors a malignancy. Current evidence suggests that, overall, contrast enhanced US has a similar accuracy to CT in characterizing focal liver tumors.

Magnetic Resonance Imaging

T2 relaxation time is prolonged in a hemangioma because of blood stagnation. Tumor T2 relaxation times are rather specific in distinguishing a malignancy from a cavernous hemangioma or cyst. For instance, one study found T2 relaxation times for malignancies to be 92 (± 22), hemangiomas 136 (± 26) and cysts 284 (± 38) (67). In fact, if T2 relaxation times are calculated, gadolinium enhancement may not be necessary to distinguish hemangiomas from metastases (68). It should be kept in mind, however, that a rare hepatocellular carcinoma does have a prolonged T2 time. A sharp tumor margin and a tumor signal greater than or equal to that of the cerebrospinal fluid (CSF) predicts a hemangioma.

Early peripheral nodular contrast enhancement is a feature of most cavernous hemangiomas. Peak contrast enhancement occurs more than 120 seconds after contrast injection in a majority of hemangiomas, a finding uncommon with hepatocellular carcinomas, which have a much earlier peak contrast. Cysts have no contrast enhancement; most metastases show variable and moderate enhancement; peak enhancement for hypervascular metastases is generally during initial hepatic artery phase,

ADVANCED IMAGING OF THE ABDOMEN

with enhancement for hypovascular metastases peaking later.

Hypervascular tumors, similar to cavernous hemangiomas, are hyperintense on unenhanced T2-weighted MR images. A hypervascular tumor is best seen during the arterial phase. In the differential diagnosis of early enhancing tumors, however, are also some benign ones, such as focal nodular hyperplasia and adenomas. Delayed postcontrast T1-weighted SE sequences help differentiate hypervascular metastases, which range from hypoto isointense, from hemangiomas, which remained hyperintense.

The SPIO contrast agents aid in differentiating liver nodules; normal liver parenchyma shows greatest signal loss, less so by adenomas and hemangiomas and, as expected, malignant tumors have only minimal signal loss. SPIO also improves capsule and scar detection. A tumor classification of high, isoand low intensity on SPIO-enhanced T2-weighted and heavily T1weighted gradient-echo images achieved a 96% diagnostic accuracy in differentiating hemangiomas, metastatic and cysts (69).

Ultrasmall superparamagnetic iron oxide particles, consisting of blood pool contrast agents, aid in differentiating highly vascular lesions, such as hemangiomas, from more solid neoplasms. The degree of enhancement on T1weighted images and the signal intensity drop on T2-weighted images when using these contrast agents is significantly lower with malignant tumors than with hemangiomas.

Use of MnDPDP improves accuracy of differentiating a hepatocellular carcinoma from focal nodular hyperplasia and differentiating hepatocellular carcinoma from a metastasis (70), but study of this contrast agent is still in its infancy.

Nonneoplastic Tumors

The classification of nonneoplastic liver nodules is generally based on their histopathologic characteristics rather than on their etiology or imaging findings. Biliary and mesenchymal hamartomas are discussed in Chapter 8.

Some of these tumors occur singly, others tend to be multiple. Some develop preferentially in a diseased liver. Their pathogenesis is diverse and includes a vascular or hormonal imbalance,

LIVER

metabolic abnormality, prior infection, and hamartomatous transformation.

Adult Cavernous Hemangioma

Clinical

Aside from congenital disorders, capillary hemangiomas (telangiectasias) are uncommon in the liver (hereditary hemorrhagic telangiectasia was discussed above; see Congenital Abnormalities).

A nonneoplastic cavernous hemangioma occurs at all ages. It is the most common liver tumor in adults. While some hemangiomas produce considerable mischief on their own, their primary importance, especially with smaller ones, lies in their imaging mimicry of metastases and hepatocellular carcinomas. They are considerably more common in women. For unknown reasons hemangiomas tend to predominate in the posterior segment of the right lobe. An occasional patient develops multiple hemangiomas, most often identified as discrete tumors. Diffuse hemangiomatosis is rare and is more often encountered in infants, where the tumor’s extensive vascular shunting induces cardiac failure. Histologically, these are bloodfilled, well-marginated cavities varying considerably in size and are lined by a single layer of endothelial cells.Associated fibrosis is common, and the cavities are surrounded by fibrous septa. Whether extensive fibrosis is a natural evolutionary process and represents an end-stage development is conjecture.

Most hemangiomas are asymptomatic and discovered incidentally. A minority manifest with pain. An occasional one bleeds internally or ruptures spontaneously, and the patient presents with a hemoperitoneum. Some undergo traumatic rupture, especially larger ones. Risk of malignant degeneration appears negligible.

What is the growth pattern of a liver hemangioma? Unresected hemangiomas do not always grow and do not necessarily require surgery. An US follow-up reveals that only a minority change in size: some increase, an occasional one decreases and even regresses spontaneously. A reasonable approach appears to be that if a suspected hemangioma increases in size in a setting of chronic liver disease, aspiration biopsy should be performed to exclude a malignancy.

In a setting of cirrhosis, hemangiomas tend to fibrose, and imaging diagnosis becomes more difficult. A tendency toward decreasing size is evident with progressive cirrhosis.

Giant liver cavernous hemangiomas can have unusual findings, such as portal vein obstruction due to pressure by the hemangioma, intrahepatic bile duct obstruction, or simply an elevated erythrocyte sedimentation rate (Fig. 7.18). Giant hemangiomas tend to contain a central scar.

An association between hemangiomas and Kasabach-Merritt syndrome has been described. An interesting suggestion is that this syndrome does not occur with hemangiomas and, if present, a Kaposiform hemangioendothelioma or similar tumor rather than a hemangioma should be considered.

Some hemangiomas are associated with focal nodular hyperplasia. The primary clinical significance of this association is that biopsy from such a region adjacent to a hemangioma will suggest a wrong diagnosis. A rare association of hemangiomatosis and arterioportal venous shunting appears to exist.

A rare subcapsular hemangioma is exophytic and becomes pedunculated; even a volvulus of such a pedunculated hemangioma has developed.

Figure 7.18. Angiographic appearance of large cavernous hemangioma (arrows). (Courtesy of Oscar Gutierrez, M.D., University of Chile, Santiago, Chile.)

Imaging

In a 1995 study of patients with proven cavernous hemangiomas, diagnostic sensitivity of CT angiography was 77%, US 61%, MRI 92%, and angiography 85% (71). Other studies confirm that MR detects more liver hemangiomas than other imaging modalities. Tumor detection and especially lesion characterization with T2-weighted sequences are the primary MRI advantages compared to other imaging modalities.

Iodized oil is used to characterize and follow certain focal liver lesions, especially hepatocellular carcinomas. After intraarterial injection of iodized oil, some is retained in hepatic hemangiomas, albeit inadvertently, and can be followed with subsequent CT. Serial CT shows predominantly peripheral iodized oil retention within a hemangioma; central retention is seen after injection of large amounts of oil, but retention here tends to be spotty or nodular. The oil washes out slowly but persists for at least 3 months and in some hemangiomas up to several years.

Most hemangiomas throughout the body tend to contain phleboliths, a finding not seen in liver hemangiomas. Calcifications do develop in an occasional liver hemangioma, especially in a setting of a thrombus, and tend to be most common at the tumor margin.

Postcontrast, typical liver hemangiomas reveal an irregular, nodular peripheral enhancement pattern of varying intensity and thickness during the arterial phase, contrastenhancement from the periphery toward the center, a prolonged tumor blush, vascular lakes, and delayed contrast washout. They have welldefined borders, and smaller ones are round while larger ones tend toward an oval or lobular shape. Early more peripheral parenchymal enhancement correlates with arterioportal shunting; one study found shunting in 21% of small hemangiomas (72). Shunting is not pathognomonic of hemangiomas but is also found in a number of other entities, including hepatocellular carcinomas.

An atypical hemangioma appearance is sufficiently common that the diagnosis is not always straightforward, and considerable diagnostic uncertainty ensues. Imaging findings of some hemangiomas mimic a metastatic carcinoma, focal nodular hyperplasia, hepatic

ADVANCED IMAGING OF THE ABDOMEN

angiosarcoma, metastatic neuroendocrine tumor, or even focal intrahepatic extramedullary hematopoiesis. In particular, small hemangiomas and hypervascular metastases often have a similar imaging appearance.

Some hemangioma contains extensive fibrosis or hyalinization to the point that their vascularity is decreased considerably (compared to a more typical hemangioma). These extensively fibrotic hemangiomas tend to have an atypical imaging appearance and mimic a malignancy; typical of fibrosis, they have lower signal intensity than cerebro-spinal fluid on T2-weighted images and lack early enhancement on postcontrast MRI (Fig. 7.19). Some even have contrast enhancement from the center toward the periphery (73).

Precontrast CT and MR of some hemangiomas detect a fluid–fluid level, presumably representing blood sedimentation. Fluid–fluid levels are not specific and are seen with other lesions.

Although an imaging appearance of progressive postcontrast opacification toward the center is a characteristic hallmark of hemangiomas, this finding is not pathognomonic; an occasional hepatocellular carcinoma, cholangiocarcinoma, and metastasis have a similar finding. Likewise, early peripheral nodular enhancement is found with some focal nodular hyperplasias and vascular neoplasms.

Computed Tomography

With unenhanced CT most typical hemangiomas are hypodense to normal liver parenchyma and isodense to blood. A fibrotic component or thrombosis lowers CT density.

A typical postcontrast appearance for a hemangioma has already been described and in these CT is often very suggestive of the diagnosis. Yet CT evidence of peripheral contrast enhancement followed by progressive fill-in is found only in about two thirds; in others enhancement is either diffuse or evident only at the tumor periphery, with most of the tumor consisting of cystic cavities or scar tissue. Central scarring modifies enhancement and results in incomplete central opacification. An extensive region of hemangiomatosis reveals a honeycomb pattern. Some larger hemangiomas mimic a hypovascular metastasis, but more

vascular ones suggest a hepatocellular carcinoma or a hypervascular metastasis.

Small hemangiomas, in particular, have gradual diffuse contrast enhancement.A minority of hemangiomas smaller than approximately 1cm in diameter, however, tend to opacify with contrast rather rapidly, appear homogeneous, and mimic hypervascular malignancies; they do remain hyperdense on delayed-phase images, a finding atypical for a malignancy. Some contain small enhancing dots which persist both during arterial and portal venous phases.

Some hemangiomas are associated with arterioportal shunts. These shunts are suggested when hepatic arterial-phase CT reveals wedgeshaped or irregular homogeneous parenchymal enhancement adjacent to a hemangioma; during the portal phase this region is isoor slightly hyperdense. These shunt are more common in rapidly enhancing hemangiomas.

Hemangiomas do develop in a fatty liver and they do enhance postcontrast, similar to those in a normal liver; the exception being that hemangiomas are hyperdense on precontrast scans compared to the hypodense fatty liver,and during the arterial phase they tend to be more isodense than usual.

Ultrasonography

The most common US appearance of a hemangioma is that of a homogeneous, wellmarginated, and hyperechoic tumor. With

growth, hemangiomas become heterogeneous. Less often encountered is a hyperechoic rim of varying thickness; these tend to have an isoechoic or even hypoechoic center. Some exhibit a bright hyperreflective pattern. An occasional one has increased through-transmission.

Operative or laparoscopic US detects hemangiomas as homogeneously hyperechoic tumors. Some are compressible by a probe, thus allowing differentiation from incompressible metastases.

Blood flow velocity in most hemangiomas is sufficiently slow that Doppler US is noncontributory, although occasionally a large peripheral feeding vessel is detected. Contrast-enhanced US reveals no or weak intratumoral signals with hemangiomas; a pulsatile signal or vessels within a tumor identified by Doppler US should suggest another diagnosis.

Contrast-enhanced US using pulse inversion harmonic imaging identifies a hypovascular tumor in the early arterial phase and peripheral globular enhancement in about half, especially evident in larger ones. Many hemangiomas <1cm show either a peripheral nodular or homogeneous enhancement. Centripetal fill-in is evident in some during the late phase. Delayed washout is common (Fig. 7.20).

Most sonographically suspected hemangiomas undergo further imaging workup, although some patients at low risk for malignancy and with a typical-appearing hemangioma at US are followed with US.